Name: graphs, curve fit, and standard deviation
Uploaded: 2020-03-26T07:22:24.000Z
Duration: 4 min 34 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry

CHEM.1

CHEM.1.D

Scientific Measurement and Lab Skills

Tradurre i risultati sperimentali da punti dati a rappresentazioni visive, come i grafici, è essenziale per determinare la relazione tra due o più proprietà. Queste proprietà sono denominate variabili. Quando ci sono due variabili, un grafico creato dai dati è chiamato bidimensionale. Il grafico ha due assi. La variabile indipendente viene tracciata sull'asse x e la variabile dipendente viene tracciata sull'asse y.
Prendiamo, ad esempio, questi dati campione per la temperatura e il volume di un gas. Il volume del gas dipende dalla temperatura. Pertanto, traccheremmo la temperatura misurata sull'asse x e il volume sull'asse y.
Quando ci sono diversi punti dati con lo stesso valore x, come se misurassimo il volume più volte a una temperatura, calcoliamo anche la deviazione standard di tali misurazioni. La deviazione standard è un valore statistico, che indica la quantità di variazione presente in un insieme di valori.
La deviazione standard viene calcolata utilizzando questa formula, dove n è il numero di punti dati, x bar è il valore medio dei punti dati e xi rappresenta ogni singolo punto dati. È possibile calcolare la deviazione standard a mano oppure un programma per fogli di calcolo può calcolarla automaticamente. Più la deviazione standard è vicina a 0, più i punti dati sono vicini al valore medio. Se la deviazione standard è uguale a 0, tutti i punti dati immessi hanno lo stesso valore.
Diamo un'occhiata ai valori medi e alle deviazioni standard delle misurazioni del volume a ciascuna temperatura nel nostro set di dati. Possiamo riassumere ogni insieme di punti dati come la media più o meno la deviazione standard. Determiniamo le cifre significative per ogni media dalle misurazioni corrispondenti e arrotondiamo i valori medi di conseguenza.
La deviazione standard di ogni gruppo deve avere lo stesso numero di cifre decimali della media, quindi arrotondiamo ogni deviazione standard alla centesima. Per determinare graficamente la relazione tra due variabili, possiamo adattare i dati con una funzione best-fit.
La funzione viene generata automaticamente da un software per fogli di calcolo e può assumere la forma di una linea di tendenza lineare, di una funzione polinomiale o di una funzione esponenziale o logaritmica. Nel caso dei nostri dati di temperatura e volume, la relazione è lineare. Quindi, i punti dati vengono adattati mediante regressione lineare dei minimi quadrati. Il tuo programma di foglio di calcolo restituirà l'equazione per la linea di adattamento migliore e un valore r al quadrato. Più il valore r al quadrato è vicino a 1, migliore è l'adattamento dei dati.
Successivamente, è possibile utilizzare il software per fogli di calcolo per trovare le deviazioni standard della pendenza, l'intercetta y e il valore y calcolato. Per determinare le cifre significative per i valori nell'equazione, seguiamo una semplice regola. L'ultima cifra significativa di ciascun valore corrisponde alla prima cifra decimale significativa della sua deviazione standard.
Quindi, arrotondiamo la pendenza al millesimo e l'intercetta y al decimo, e arrotondiamo le deviazioni standard in modo che corrispondano. La nostra pendenza è di 0,167 +/- 0,003 litri per Kelvin e la nostra intercetta y è di -40,6 +/- 1,2 litri. Qualsiasi valore y calcolato sarà arrotondato al decimo e sarà di +/- 0,8 litri. Questa equazione descrive la relazione tra la temperatura e il volume di un gas.
In questo laboratorio, creerai un set di dati di variabili dipendenti e indipendenti misurando il diametro e la circonferenza di bicchieri di varie dimensioni. Utilizzerai quindi questi dati per creare un grafico a dispersione ed eseguire una regressione lineare, tenendo presente l'importanza delle cifre significative. Potrai anche esercitarti nelle abilità di laboratorio, come filtrare e misurare il volume utilizzando pipette, prestando attenzione all'incertezza nelle misurazioni e nell'analisi.

misure scientifiche e competenze di laboratorio

Grafici bidimensionali
Alcuni esperimenti di chimica prendono la forma di cambiare direttamente una proprietà del sistema che si sta studiando, altrimenti nota come variabile indipendente, come la temperatura, e di misurare gli effetti su un'altra proprietà, altrimenti nota come variabile dipendente, come il volume. Una volta raccolti i dati, l'interazione tra i due parametri deve essere quantificata o convertita in una forma che può essere valutata e confrontata con altre relazioni.
I grafi bidimensionali possono essere utilizzati per derivare certi tipi di relazioni matematiche tra due proprietà o per stabilire che tale relazione non esiste tra di loro. L'analisi determinerà in ultima analisi come cambia la variabile dipendente in risposta alla variabile indipendente. Nell'esempio della regolazione della temperatura di un liquido o di un gas e del monitoraggio delle variazioni del suo volume, la temperatura è la variabile indipendente e il volume è la variabile dipendente.
Per creare un grafico bidimensionale, ogni punto dati deve avere un valore noto come coordinata sia per le variabili dipendenti che per quelle indipendenti. La variabile indipendente viene tracciata sull'asse x e la variabile dipendente viene tracciata sull'asse y. Questi grafici sono facilmente realizzabili in un software per fogli di calcolo, che può essere utilizzato anche per analizzare i dati tracciati.
Raccordo curvo
Una volta che un set di dati è stato tracciato su un grafico bidimensionale, l'adattamento della curva può essere utilizzato per generare un'equazione, o funzione, per la variabile dipendente in termini di variabile indipendente. Le funzioni rappresentano un modello matematico che modella al meglio i dati da cui è derivato. L'adattamento della curva è la tecnica per trovare una funzione che produce una linea che corrisponde bene al modello di punti dati. Il software per fogli di calcolo dispone di vari strumenti per l'adattamento delle curve, che viene definito "best-fit". Di solito si tratta di un'analisi di regressione lineare dei minimi quadrati, sebbene la maggior parte dei software offra anche una regressione dei minimi quadrati non lineare.
La precisione dell'equazione lineare più adatta può essere verificata inserendo i valori x per i punti dati e confrontando i risultati "teorici" dell'equazione con i valori y effettivi dei punti dati. Il software per fogli di calcolo in genere calcola il valore del coefficiente di determinazione (R2) per la funzione, che mostra quanto bene la funzione corrisponde ai punti dati. Più il valore R2 è vicino a 1, migliore è l'adattamento per una regressione lineare. Altre funzioni dispongono di metodi più specializzati per determinare l'adattamento della funzione ai dati.
Determinare l'incertezza dei valori dipendenti calcolati dalla funzione best-fit richiederebbe complicate tecniche di "propagazione dell'errore". Tuttavia, è possibile calcolare l'incertezza all'interno dell'equazione sotto forma di deviazione standard sia per la pendenza che per l'intercetta y di una funzione di migliore adattamento. Questa operazione viene solitamente eseguita con uno strumento diverso da quello utilizzato per generare un grafico bidimensionale.
deviazione standard
La deviazione standard descrive la quantità di variazione presente in un insieme di valori. La deviazione standard della popolazione (σ) viene utilizzata quando sono presenti dati da ciascun membro di una popolazione finita, ad esempio la massa di ogni biglia in un sacchetto di biglie. La deviazione standard del campione viene utilizzata per tutti gli altri casi ed è il calcolo della deviazione standard predefinito nel software per fogli di calcolo. 1 Si può presumere che la "deviazione standard" si riferisca alla deviazione standard del campione.
Si presume che l'errore di misurazione casuale segua una distribuzione approssimativamente "normale", in cui circa il 68% di un insieme di valori si trova all'interno di un intervallo di una deviazione standard su entrambi i lati della media, il 95% dei valori si trova all'interno di due deviazioni standard su entrambi i lati della media e il 99,7% dei valori si trova all'interno di tre deviazioni standard su entrambi i lati della media. Pertanto, la deviazione standard è un modo utile per descrivere l'errore e l'incertezza.
L'equazione per la deviazione standard del campione è:
<img src="/CDNSource/lm/labs/42/42_Concepts_1.jpg">
In questa equazione, N è il numero di valori; <img src="/CDNSource/lm/labs/42/30034eq02.jpg"> è la media (o media) dei valori; e xi rappresenta ogni singolo valore. Quindi, per calcolare a mano la deviazione standard, calcolare la media dell'insieme di valori, sottrarre la media da ogni valore, elevare al quadrato ogni differenza, sommare le differenze al quadrato, dividere la somma totale per uno in meno del numero di valori e prendere la radice quadrata del quoziente. Più s è vicino a zero, minore è la variazione tra i valori. Se i valori vengono inseriti nel software per fogli di calcolo, la deviazione standard può essere calcolata all'interno del software.
Il numero di cifre significative in una deviazione standard dipende dai valori a cui si riferisce. Quando si riporta la deviazione standard per un gruppo di punti di dati presi nelle stesse condizioni, si deve prima determinare il numero appropriato di cifre significative nel valore medio. La deviazione standard viene quindi arrotondata allo stesso numero di cifre decimali della media. Per un insieme di volumi con quattro cifre significative, una media di 15,361 ml e una deviazione standard di 0,2313, la media e la deviazione standard sarebbero riportate come 15,36 ml ± 0,23 ml.
Quando si riporta la deviazione standard della media e dell'intercetta y per una funzione di adattamento determinata dall'analisi dei minimi quadrati, che è il metodo usuale per i software per fogli di calcolo, la prima cifra decimale della deviazione standard è l'ultima cifra significativa della media o dell'intercetta y. Pertanto, la deviazione standard deve essere arrotondata a una cifra decimale significativa e la pendenza o l'intercetta y deve essere arrotondata alla cifra decimale corrispondente. Ad esempio, se la pendenza è 0,1691 L·K-1 e ha una deviazione standard di 0,00512, la pendenza deve essere riportata come 0,169 L·K-1 ± 0,005 L·K-1.
Se la deviazione standard della pendenza o dell'intercetta y è molto più piccola del suo valore corrispondente che seguire questa regola darebbe cifre più significative alla pendenza o all'intercetta y di quanto consentissero i dati di misurazione originali, allora determina invece le cifre significative della pendenza o dell'intercetta y dai valori x e y e arrotonda la deviazione standard a una cifra significativa. Pertanto, per una pendenza di 0,1691 L·K-1, una deviazione standard di 0,0000512 e valori x e y con quattro cifre significative, la pendenza dovrebbe essere riportata come 0,1691 L·K-1 ± 0,00005 L·K-1. In questo caso, è possibile che il valore calcolato e la relativa deviazione standard abbiano un numero diverso di cifre decimali. Tieni presente che il metodo dei minimi quadrati utilizza sia i valori x che y per calcolare la pendenza e l'intercetta y.
Quando si riporta la deviazione standard come intervallo di incertezza, assicurarsi di notare in modo specifico che l'incertezza rappresenta una deviazione standard. Questo dice al lettore che c'è circa il 68% di probabilità che il vero valore di una misura rientri in quell'intervallo della media, assumendo una distribuzione normale. Spesso può essere più adatto riportare l'incertezza come due deviazioni standard dalla media, in quanto ciò aumenta la probabilità a circa il 95%. Per fare ciò, è sufficiente moltiplicare la deviazione standard per due prima di arrotondarla al numero appropriato di cifre significative.
referenze
<ol style="dimensione del carattere:17px">
	<li style="dimensione del carattere:17px">Harris, D.C. (2015). Analisi chimica quantitativa. New York, NY: W.H. Freeman e compagnia.</li>
</ol>

Misure scientifiche e competenze di laboratorio

Grafici bidimensionali
Alcuni esperimenti di chimica prendono la forma di modificare direttamente una proprietà del sistema che si sta studiando, altrimenti noto come ...

Education

Didattica

JoVE Lab Manual

Chimica

graphs, curve fit, and standard deviation

Two-dimensional graphs help students compare an independent variable with a dependent variable in science data. In a chemistry example, temperature is the independent variable and volume is the dependent variable. After data are collected, the relationship between the two measurements must be quantified so it can be evaluated and compared with other relationships. A graph can also show when no relationship exists between the two variables.

To make a two-dimensional graph, each data point needs a coordinate for both variables. The independent variable is plotted on the x-axis, and the dependent variable is plotted on the y-axis. These plots are often made in spreadsheet software, which can also be used to analyze the data. This helps show how the dependent variable changes in response to the independent variable.

Once the data are plotted, curve fitting can turn the pattern into an equation, or function. A function is a mathematical model that best matches the data it comes from. Curve fitting looks for a line or curve that fits the data points well. In many cases, spreadsheet tools use a best-fit method based on linear least-squares regression, although nonlinear least-squares regression is also available in most software.

The quality of a best-fit line can be checked by comparing the x values in the equation with the actual y values from the data. Spreadsheet software usually reports the coefficient of determination, or R squared, which shows how well the function matches the plotted points. A value closer to 1 means a better fit for linear regression. Finding uncertainty for values calculated from the equation can be more complicated, but standard deviation can be used to describe the uncertainty in the slope and y-intercept of a best-fit line.

Standard deviation shows how much values vary in a set. Population standard deviation, written as sigma, is used when data are available for every member of a finite population, such as the mass of each marble in a bag. Sample standard deviation, written as s, is used in all other cases and is the default in spreadsheet software. For that reason, standard deviation usually means sample standard deviation. Random measurement error is assumed to follow a roughly normal distribution, where about 68% of values fall within one standard deviation of the mean, 95% fall within two, and 99.7% fall within three.

The sample standard deviation can also be calculated by hand. First find the mean. Then subtract the mean from each value, square each difference, add the squared differences, divide by one less than the number of values, and take the square root. The closer s is to zero, the less variation there is in the data. Spreadsheet software can also calculate standard deviation directly.

When reporting standard deviation, the number of significant figures depends on what it describes. For a group of measurements taken under the same conditions, the standard deviation is rounded to the same number of decimal places as the mean. For example, 15.361 mL with a standard deviation of 0.2313 should be reported as 15.36 mL ± 0.23 mL. When reporting the standard deviation of a slope or y-intercept from least-squares analysis, the first decimal place of the standard deviation becomes the last significant figure of the value. So 0.1691 L·K-1 with a standard deviation of 0.00512 is reported as 0.169 L·K-1 ± 0.005 L·K-1.

If the standard deviation is much smaller than the slope or y-intercept, this rule can give more significant figures than the original x and y data allow. In that case, use the significant figures from the data and round the standard deviation to one significant figure. For example, 0.1691 L·K-1 with a standard deviation of 0.0000512 and x and y values with four significant figures is reported as 0.1691 L·K-1 ± 0.00005 L·K-1. It is important to state whether the uncertainty represents one standard deviation. One standard deviation gives about a 68% chance that the true value lies within that range. Two standard deviations may be a better choice when a wider uncertainty range is needed, since that raises the chance to about 95%.

Graphs, Curve Fit, and Standard Deviation

lab techniques

Measuring with Accuracy in the Lab

Measuring with accuracy in the lab starts with careful records, the right tools, and correct reading of data. Scientists use lab notebooks to record how an experiment was done. These notes include the amounts of materials used and each step of the procedure. Without that record, an experiment cannot be repeated in the same way. Balances and glassware are used to measure solids and liquids for experiments. The precision of a measurement depends on the equipment used. Digital balances show the...

scientific measurement and lab skills

Two-dimensional graphs help students compare an independent variable with a dependent variable in science data. In a chemistry example, temperature is the independent variable and volume is the dependent variable. After data are collected, the relationship between the two measurements must be quantified so it can be evaluated and compared with other relationships. A graph can also show when no relationship exists between the two variables. To make a two-dimensional graph, each data point needs...

stoichiometry, product yield, and limiting reactants

stoichiometry-product-yield-and-limiting-reactants

Balancing Equations and Reactant Limits

Chemical equations show how reactants change into products during a reaction. Stoichiometry is the study of the relative amounts of reactants and products. It is based on the Law of Conservation of Mass, which says matter is neither created nor destroyed. Because atoms are only rearranged, the number and identity of atoms must match on both sides of a reaction. That is why a chemical equation must be balanced. A balanced equation has the same number of atoms for each element in the reactants...

redox reactions

Oxidation States in Redox Reactions

Oxidation states help identify redox reactions. In a redox reaction, oxidation is the loss of one or more electrons, and reduction is the gain of one or more electrons. These reactions are often tracked by changes in oxidation state, also called oxidation number. Each atom in a molecule has an oxidation state. This number shows how oxidized an atom is compared with its free elemental form. It is written as the charge an atom would have if every bond were purely ionic. In that model, bonding...

ideal gas law

Gas Laws Behind the Ideal Gas Equation

The ideal gas equation brings together the main gas laws used in chemistry. Gases have molecules that are far apart, so their behavior is described with four measurable variables: pressure, volume, number of moles, and temperature. The ideal gas law links these variables with the gas constant, R, which equals 8.314 J·K−1 mol−1. Boyle’s law describes the link between pressure and volume. When temperature and the number of moles stay constant, pressure and volume change in opposite directions.

acid and base concentrations

Acids, Bases, and pH in Titration

Acids, bases, and pH explain how acidic or basic a solution is. An Arrhenius acid releases hydrogen ions when it dissolves in water, while an Arrhenius base releases hydroxide ions. These reactions also create conjugate partners: an acid forms a conjugate base after losing hydrogen, and a base forms a conjugate acid after losing hydroxyl. Every acid has a conjugate base, and every base has a conjugate acid. pH measures the amount of hydrogen ions in a solution. The pH scale runs from 0 to 14...

buffers

Buffer pH and the Common Ion Effect

Buffer solutions help keep pH steady when acids or bases are added. In water, an Arrhenius acid (HA) dissociates into hydrogen ions, H+, and a conjugate base, A-. More hydrogen ions make a solution more acidic and lower its pH. An Arrhenius base (BOH) dissociates into a conjugate acid, B+, and hydroxide ions, OH-. Strong acids and strong bases usually change pH a lot because they react completely in water and raise the amount of H+ or OH-. Buffers reduce those pH changes. A buffer usually...

enthalpy of reaction

Reaction Heat, Hess’s Law, and Calorimetry

Reaction heat is explained through enthalpy, Hess’s law, and calorimetry. These ideas show how heat moves during chemical reactions and how scientists measure it. Thermodynamics is the study of heat energy and other forms of energy, such as work. Its laws are used across science, from biology to physics. The first law says energy is conserved in an isolated system. Energy can change form, such as thermal energy becoming work, but it cannot be created or destroyed. The second law of...

solubility

Solubility in Water and Other Solvents

Solubility describes how much of a solute can dissolve in a specific solvent. It is usually reported as solute mass per solvent volume or as solute mass per solvent mass. For example, sodium chloride has a solubility of 36 g per 100 mL of water at room temperature. If solubility is given by solvent mass, that mass must be converted to volume for later calculations. Temperature changes solubility. Sodium carbonate dissolves at about 7 g per 100 mL of water at 0 °C, 22 g per 100 mL at room...

metal flame emission

Flame Test Metal Color Clues

Flame testing uses a metal sample and a flame to identify a substance by its color. Each element gives off a characteristic light emission in the flame, so the color can help distinguish one element from another. This same idea appears in fireworks, where different metal components create different colors. The color comes from energy absorbed by the metallic component. When a species absorbs energy, that energy can go into translation, vibration, rotation, or electron excitement. Electron...

balmer series

Hydrogen Emission Lines

Hydrogen emission lines show how electrons move between fixed energy levels in the Bohr model. Niels Bohr proposed this model for the hydrogen atom in 1913. In this model, an electron can only occupy certain orbits, and it cannot exist between them. These energy states are labeled with the quantum number n. The ground state is n = 1. Higher values, such as n = 2, 3, 4, and so on, are excited states. An electron changes state only by absorbing or emitting energy equal to the difference between...

beer's law

Absorbance Spectra and Beer-Lambert Analysis

Absorbance spectra and the Beer-Lambert Law show how light can be used to study molecules in solution. When light hits a substance, it may be absorbed, transmitted, or reflected. A substance often interacts with a range of wavelengths, and each wavelength can affect the molecules or atoms in a different way. When a molecule absorbs light, that energy can be used for translation, vibration, rotation, or electron excitement. Electron excitement is especially important in spectroscopy because it...

concentration dependence

Reaction Order and Rate Constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate. In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is...

temperature dependence

Reaction Rate, Arrhenius, and Catalysts

Chemical kinetics explains how fast a reaction happens and why that speed changes. The reaction rate is the speed of a chemical reaction. It is defined as the change in concentration of a reactant or product over time. Reaction speed depends on several factors, including the concentration of the reactants and the temperature of the reaction. Each reactant affects the rate in a specific way. That relationship is described by the rate law. The rate law is an equation that connects the...

galvanic cells

Battery Power from Redox Reactions

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electrolytic cells

Electroplating with Electrolytic Cells

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proper lab notebook keeping

Recording a Reproducible Lab Experiment

Accurate lab notebook records are essential in science and organic chemistry labs. Without clear notes, descriptions, quantities, and observations, an experiment cannot be repeated or checked. Good record keeping supports reproducibility, which means other people can follow the same work and get similar results. A laboratory notebook is useful in school labs, research settings, and industry. It can also become a legal record when an idea, process, substance, or technique is patented and...

basic organic chemistry techniques

Organic Lab Glassware and Safe Handling

Organic chemistry labs use specific glassware, careful weighing steps, and controlled heating to keep reactions safe and accurate. Beakers and Erlenmeyer flasks are often used for mixing or holding solvents, but they are not meant for measuring volume unless only an approximate amount is needed. Volumetric glassware is used when a precise volume matters. This group includes graduated cylinders, volumetric pipettes, and volumetric flasks. A graduated cylinder is commonly used in the organic lab...

melting points

How Impurities Change Melting Points

Melting point helps identify an organic compound by showing the temperature where a solid turns into a liquid at 1 atmosphere of pressure. It is a physical property, like solubility, density, color, and electronegativity. Because the exact start of melting is hard to measure, the result is reported as a range. The lower end is when the first drops of liquid appear. The upper end is when the last solid has melted. Scientists compare this range with literature values to help identify a compound. <br...

boiling points

Boiling Point and Vapor Pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value. Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas...

recrystallization

Purifying Crystals by Solubility

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction. Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary...

extraction

Separating Mixtures with Solvent Extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point. There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase...

simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Video completi e incentrati sul curriculum sia per gli istruttori che per gli studenti nei corsi di laboratorio di Chimica introduttiva.

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Learn how to plot independent and dependent variables, use best-fit lines, and report standard deviation to describe uncertainty in science data.

Graphs, Curve Fit, and Standard Deviation | JoVE K12

Preparati a insegnare Misure scientifiche e competenze di laboratorio con questa guida per istruttori di 3 minuti. Copre i passaggi chiave di preparazione per il laboratorio Chemistry su JoVE Lab Manual.

Preparazione dell'istruttore per Misure scientifiche e competenze di laboratorio in Chemistry - Oltre 5.7K visualizzazioni | JoVE Lab Manual

Segui le procedure passo dopo passo di Misure scientifiche e competenze di laboratorio in questo video di protocollo studente di 15 minuti. Parte della collezione Chemistry su JoVE Lab Manual.

Protocollo studente per Misure scientifiche e competenze di laboratorio in Chemistry - Oltre 20K visualizzazioni | JoVE Lab Manual

Scientific Laws and Theories

Scientific Laws

In science, a law is defined as a concise, verbal or mathematical, statement that summarizes a vast number of experimental observations. ...

The Scientific Method

Chemistry is an empirical science. Scientists often pose questions to understand the chemistry in everyday life and seek answers to these questions. To ...

CHEM.1.A

What is an Experiment?

An experiment is a planned activity carried out under controlled conditions. The purpose of an experiment is to investigate the relationship between two ...

Study Design in Statistics

A study design is a set of techniques that allow a researcher to collect and analyze data from different variables defined for a specific research ...

Observational Studies

Observational studies are a type of analytical study where researchers observe events without any interventions. In other words, the researcher does not ...

Experimental Designs

An experimental design is a systematic process that allows researchers to evaluate the relationship between dependent and independent variables. There are ...

Randomized Experiments

The randomization process involves assigning study participants randomly to experimental or control groups based on their probability of being equally ...

Crossover Experiments

Crossover experiments, also called the repeated-measurements design, is a study design in which all experimental units are exposed to all treatments in ...

Controls in Experiments

When conducting an experiment, it is crucial to have control to reduce bias and accurately measure the dependent variables. It also marks the results more ...

Bias

Bias refers to any tendency that prevents a question from being considered unprejudiced. In research, bias occurs when one outcome or answer is selected ...

Blinding

Blinding is a commonly used method of not telling participants which treatment a subject is receiving. Blinding is a critical part of a randomized control ...

CHEM.1.B

Proper Personal Protective Equipment

Robert M. Rioux &#38; William A Elliott, Pennsylvania State University, University Park, PA
Hazards are many and varied in the laboratory, but the right ...

Emergency Eyewash and Shower Stations

Robert M. Rioux, Pennsylvania State University, University Park, PA
The Occupational Safety and Health Administration (OSHA) mandates that an emergency ...

Electrical Safety

Robert M. Rioux &amp; Suprita Jharimune, Pennsylvania State University, University Park, PA
Among the many hazards present in the laboratory, electrical ...

Working with Centrifuges

Source: Robert M. Rioux &amp; Suprita Jharimune, Pennsylvania State University, University Park, PA
Earth's gravitational force is capable of separating ...

Working with Hot and Cold Sources

Source: Robert M. Rioux &amp; Suprita Jharimune, Pennsylvania State University, University Park, PA
Working with extreme temperatures, both high and low, ...

guidelines-in-case-of-a-laboratory-emergency

Guidelines in Case of a Laboratory Emergency

Robert M. Rioux &amp; Zhifeng Chen, Pennsylvania State University, University Park, PA
The most common laboratory emergencies include chemical spills, ...

chemical-storage-categories-hazards-and-compatibilities

Chemical Storage: Categories, Hazards And Compatibilities

Source: Robert M. Rioux &amp; Taslima A. Zaman, Pennsylvania State University, University Park, PA
While the use of various chemicals in experimental ...

Safe Handling of Mineral Acids

Source: Robert M. Rioux &#38; Taslima A. Zaman, Pennsylvania State University, University Park, PA
A mineral acid (or inorganic acid) is defined as a ...

Handling Chemical Spills

Source: Robert M. Rioux &#38; Taslima Zaman, Pennsylvania State University, University Park, PA
Since chemicals are commonly used for laboratory research ...

Proper Use of Autoclaves

Robert M. Rioux &amp; Zhifeng Chen, Pennsylvania State University, University Park, PA
Autoclaving is one of the most commonly used methods in the ...

Fume Hoods and Laminar Flow Cabinets

Robert M. Rioux &amp; William A. Elliott, Pennsylvania State University, University Park, PA
Fume hoods and laminar flow cabinets are engineering controls ...

handling-air-and-water-sensitive-chemicals-using-a-schlenk-line

Handling Air- and Water-Sensitive Chemicals Using a Schlenk Line

Robert M Rioux, Ajay Sathe, Zhifeng Chen, Pennsylvania State University, University Park, PA
The use of reagents sensitive to oxidation or moisture ...

proper-operation-of-vacuum-based-equipment

Proper Operation of Vacuum Based Equipment

Source: Robert M. Rioux, Ajay Sathe &#38; Zhifeng Chen, Pennsylvania State University, University Park, PA
Vacuum is required for a number of laboratory ...

Operating the Glovebox

Robert M Rioux, Ajay Sathe, Zhifeng Chen, Pennsylvania State University, University Park, PA 
A glovebox is an isolated enclosure that is designed to ...

operation-of-high-pressure-reactor-vessels

Operation of High-pressure Reactor Vessels

Robert M Rioux, Pennsylvania State University, University Park, PA
The use of gases in a synthetic chemistry laboratory is essential for carrying out a ...

CHEM.1.C

Lab Techniques

Measurement Accuracy
The ability to repeat an experiment and get the same results, or reproducibility, is essential in scientific research. However, ...

Two-Dimensional Graphs
Some chemistry experiments take the form of directly changing one property of the system that you are studying otherwise known as ...

Proper Lab Notebook Keeping

Organic Lab Notebook Keeping
Record keeping is a vital skill in science and organic chemistry labs. Without proper notes, descriptions, quantities, or ...

Basic Organic Chemistry Techniques

Glassware in Organic Chemistry
There are standard pieces of glassware that are used in the organic chemistry lab. Beakers and Erlenmeyer flasks are ...

Measurement: Standard Units

Every measurement provides three kinds of information: the size or magnitude of the measurement (a number), a standard of comparison for the measurement ...

Measurement: Derived Units

The International System of Units or SI system, by international agreement, has fixed measurement units for seven fundamental properties: length, mass, ...

uncertainty-in-measurement-accuracy-and-precision

Uncertainty in Measurement: Accuracy and Precision

Scientists typically make repeated measurements of a quantity to ensure the quality of their findings and to evaluate both the precision and the accuracy ...

uncertainty-in-measurement-reading-instruments

Uncertainty in Measurement: Reading Instruments

Counting is the type of measurement that is free from uncertainty, provided the number of objects being counted does not change during the process. Such ...

CHEM.1.E

Scientific and engineering practices. The student, for at least 40% of instructional time, asks questions, identifies problems, and plans and safely conducts classroom, laboratory, and field investigations to answer questions, explain phenomena, or design solutions using appropriate tools and models. The student is expected to:

Graphs, Curve Fit, and Standard Deviation

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