Name: measuring with accuracy in the lab
Uploaded: 2020-03-26T07:22:24.000Z
Duration: 4 min 14 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Specialised Topic in Science

112.51.c.1.

112.51.c.1.D.

Lab Techniques

في البحث العلمي ، تعتبر قابلية تكرار التجربة مهمة للغاية. وبالتالي ، فإن الاحتفاظ بدفتر ملاحظات مختبر يحتوي على تسجيلات إجرائية مفصلة جنبا إلى جنب مع تقنيات المختبر المناسبة يساعد في إعادة إنتاج النتائج التجريبية. 
على سبيل المثال ، يجب إجراء قياسات بسيطة ، مثل حجم السائل ، باستخدام الأواني الزجاجية المناسبة لضمان الدقة. قم دائما بقياس الحجم باستخدام الأواني الزجاجية الحجمية، مثل القارورة الحجمية أو الأسطوانة المتدرجة أو الماصة الحجمية. يتم قياس الحجم في الجزء السفلي من الغضروف المفصلي. 
علامات الحجم على الأكواب وقوارير Erlenmeyer ليست دقيقة وتعمل كإرشادات فقط. عند اختيار الأواني الزجاجية الحجمية ، حدد أصغر حاوية ممكنة لوحدة التخزين المطلوبة. تتم معايرة الأواني الزجاجية الحجمية إما لتسليمها أو احتواءها. 
تم تصميم الحاويات التي تمت معايرتها للتسليم لتوفير الحجم المذكور مع فهم أن كمية صغيرة من السائل ستبقى في الأواني الزجاجية بعد إفراغها. في هذه الحالة ، ليست هناك حاجة لإزالة السائل المتبقي أو سيتم إفراغ أكثر من الحجم المطلوب. الأواني الزجاجية التي تمت معايرتها لاحتوائها ستحتفظ بالحجم المذكور وتسليمه ولكنها تتطلب سكب السائل المتبقي حتى يتم استلام الحجم الكامل. 
نقيس للحصول على قيمة حقيقية. ومع ذلك ، سيكون هناك دائما مستوى معين من عدم اليقين والخطأ. القيمة المقاسة هي أفضل تقدير لدينا للقيمة الفعلية ، والتي غالبا ما تكون غير معروفة لنا. الخطأ هو الفرق بين القيم المقاسة والفعلية. يصف عدم اليقين في القياس النطاق الذي نعتقد أنه من المحتمل أن تكمن فيه القيمة الفعلية. عند تسجيل القياسات ، من المهم الحفاظ على العدد المناسب من الأرقام المهمة. 
الأرقام المهمة هي الأرقام في القياس التي تحمل معنى. يحدد الرقم الأخير المسجل مستوى عدم اليقين. جميع الأرقام بخلاف الأصفار البادئة واللاحقة مهمة. والأصفار اللاحقة مهمة عندما تكون هناك فاصلة عشرية تسبقها. 
على سبيل المثال ، في قياس الطول باستخدام المسطرة ، نرى أن الطول لا يقل عن بوصة واحدة ، ولكن بالتأكيد ليس 2 بوصة. إذن، الرقم الأول المهم هو واحد. تمثل علامة التجزئة التالية 0.1 بوصة وهي مهمة أيضا. 
يحتوي التسجيل البالغ 1.1 بوصة على رقمين مهمين ويشير إلى أن عدم اليقين يكمن في الخانة العاشرة. ومع ذلك ، فإن العرض الحقيقي يقع بين علامتي تجزئة. لذا ، فإن عدم اليقين يكمن هنا في خانة المئات حيث تم الإبلاغ عن الطول على أنه 1.15 بوصة. 
عند إجراء العمليات الحسابية باستخدام القيم المقاسة ، تذكر عدم إجراء العمليات الحسابية بدقة أعلى من القياس الأصلي. هذه الأرقام الإضافية ليست مهمة ولا ينبغي تضمينها. على سبيل المثال ، عند حساب مساحة المربع بطول ضلع 1.15 ، نرى أن الطول يحتوي على ثلاثة أرقام معنوية. لذلك ، يجب أن تحتوي الإجابة أيضا على ثلاثة أرقام مهمة. 
المساحة المحسوبة البالغة 1.3225 بوصة مربعة تتكون من خمسة أرقام وتقدم اليقين في الحساب الذي لم يكن موجودا في القياس الأصلي. وبالتالي ، فإن المساحة الصحيحة هي 1.32 بوصة مربعة. 
في هذا المختبر ، ستمارس مهارات المختبر المناسبة عن طريق قياس كثافة البويضة واستخدام الأرقام المهمة في حساباتك وتسجيلاتك. بالإضافة إلى ذلك ، سوف تتدرب على حفظ السجلات في دفتر المختبر الخاص بك وفحص دقة القياسات باستخدام الأواني الزجاجية الحجمية.

تقنيات المختبر

<h4>دقة القياس</h4>
تعد القدرة على تكرار التجربة والحصول على نفس النتائج ، أو قابلية التكرار ، أمرا ضروريا في البحث العلمي. ومع ذلك ، من المستحيل تكرار تجربة إذا كنت لا تعرف كيف فعلتها. وبالتالي ، يحتفظ العلماء بسجلات مفصلة لتجاربهم في دفاتر المختبر. تتضمن هذه السجلات معلومات مهمة مثل كمية كل مادة مستخدمة وأوصاف كل خطوة من خطوات الإجراء.
تستخدم المعدات ، مثل الموازين والأواني الزجاجية الحجمية أو المتدرجة ، لقياس المركبات الصلبة والسائلة للتجارب. دقة كل قياس محدودة بالمعدات المستخدمة. تعرض الموازين والمعدات الأخرى المزودة بقراءات رقمية القيم تلقائيا بأقصى دقة يمكنها دعمها. تعتمد دقة الأواني الزجاجية والمساطر المتدرجة على الزيادة ، وهي الكمية التي تمثلها أصغر مسافة بين علامتين.
ينحني سطح معظم السوائل لأعلى عند الحواف في العبوات الزجاجية. يسمى هذا النوع من السطح السائل المنحني الغضروف المفصلي المقعر. عندما تنحني حواف السائل لأسفل ، يطلق عليه الغضروف المفصلي المحدب. عند قياس أحجام السائل ، تكون النقطة المرجعية للجزء العلوي من السائل هي الجزء السفلي من الغضروف المفصلي المقعر أو الجزء العلوي من الغضروف المفصلي المحدب. لتحديد موضع الغضروف المفصلي بدقة ، تتم قراءة أحجام السائل من خلال النظر إلى سطح السائل من الجانب على مستوى العين. سيؤدي النظر إلى السائل من أعلى أو أسفل إلى ظهور مستوى سطح السائل أعلى أو أقل مما هو عليه في الواقع.
عندما تلامس النقطة المرجعية للغضروف المفصلي علامة على الأواني الزجاجية المتدرجة أو الحجمية ، فإن الأواني الزجاجية تحمل الحجم المحدد لتلك العلامة. تم تصميم الأواني الزجاجية الحجمية لقياس حجم معين ، لذلك لها علامة واحدة فقط. تم تصميم الأواني الزجاجية المتدرجة لقياس مجموعة من الأحجام ، لذلك تحتوي على العديد من العلامات ، أو التخرج ، والتي يتم تصنيفها على فترات منتظمة. قد تحتوي كل من الأواني الزجاجية المتدرجة والحجمية على معلومات إضافية مقدمة من الشركة المصنعة حول دقتها.
<h4>المعايرة وعدم اليقين</h4>
يقوم المصنعون بمعايرة المعدات للتأكد من دقة القياسات ضمن نطاق محدد من عدم اليقين. على سبيل المثال، قد يقرأ الميزان التحليلي عالي الدقة إلى أربعة منازل عشرية (0.0000 جم) مع عدم يقين ± 0.0001 جم. تشير قراءة 0.0345 جم إلى أن القيمة الحقيقية تتراوح بين 0.0344 جم و 0.0346 جم.
عادة ما يكون للأواني الزجاجية حالة من عدم اليقين في القياس مطبوعة عليها. لتسجيل قياسات الحجم بدقة ، يجب مراعاة كل من الزيادة وعدم اليقين. على سبيل المثال ، سيتم تسجيل قياس دقيق للحجم في أسطوانة متدرجة سعة 100 مل بزيادات قدرها 1 مل وعدم يقين قدره 0.5 مل ± إلى مركز الأعشار (000.0 مل ± 0.5 مل). إذا كانت النقطة المرجعية للغضروف المفصلي تقع بين علامتين ، فيجب عليك تقدير قيمة لخانة الأعشار. خلاف ذلك ، اتركه على أنه 0. نظرا لأن درجة الحرارة تؤثر على الحجم ، طباعة درجة الحرارة التي تتم معايرة الأواني الزجاجية من أجلها عليها.
تتم معايرة الأواني الزجاجية إما لاحتواء (TC) أو لتسليم (TD). تحتفظ الأواني الزجاجية TC بالحجم المحدد من السائل عند ملؤه بالعلامة ، ولكن سيتم ترك كمية صغيرة من السائل عند سكبها في وعاء آخر. غالبا ما يتم تحضير المحاليل في الأواني الزجاجية TC لأن دقة حجم المحلول تؤثر على دقة التركيز. عادة ما تتم معايرة القوارير الحجمية لاحتوائها.
تحتوي الأواني الزجاجية TD على أكثر بقليل من كمية السائل المحددة عند ملؤها بالعلامة ، لكنها توزع الحجم المحدد فقط. وبالتالي ، لا ينبغي إفراغ الأواني الزجاجية TD تماما عند توزيع السوائل. هذا النوع من الأواني الزجاجية مفيد لنقل حجم دقيق من السائل إلى حاوية أخرى. عادة ما تتم معايرة كل من الماصات المتدرجة والحجمية لتوصيلها.
ما لم ينص على خلاف ذلك ، يفترض أن يكون عدم اليقين في الرقم على مقياس الرقم الأخير. وبالتالي ، من المهم بشكل خاص الإبلاغ عن عدم اليقين في القياس إذا كان عدم اليقين على نطاق أوسع. بالنسبة للمساطر والأواني الزجاجية المتدرجة دون عدم يقين معروف ، يقدر عدم اليقين بنصف أصغر زيادة. يعتمد الرقم غير المؤكد على مكان وقوع الغضروف المفصلي بين علامتي علامة.
<h4>أرقام مهمة</h4>
الأرقام المعنوية هي الأرقام في القيمة ذات المغزى أو الضرورية للتعبير عن تلك القيمة بالدقة المناسبة. جميع الأرقام بخلاف الأصفار البادئة (0.001) أو الأصفار اللاحقة (1,000) أو مضاعفات التدوين العلمي (10x) تكون دائما مهمة.
الأصفار البادئة ليست ذات أهمية أبدا لأنه يمكن إزالتها عن طريق إعادة كتابة الرقم إما في التدوين العلمي أو ، إذا كانت وحدة SI ، في وحدات أصغر. على سبيل المثال ، لا يوجد سوى ثلاثة أرقام معنوية بالقيمة 0.00123 م لأنه يمكن إعادة كتابتها إما على أنها 1.23 × 10-3 م أو 1.23 مم دون فقد أي معلومات. لاحظ أن عامل القياس بين المضاعفات المختلفة لنفس وحدة SI لا يؤثر على عدد الأرقام المعنوية.
تكون الأصفار الزائدة مهمة عندما تكون قبل أو بعد الفاصلة العشرية. على سبيل المثال ، يحتوي القياس 100.110 مل على ستة أرقام مهمة و 100.0 مل يحتوي على أربعة. يفترض أن الأصفار اللاحقة في رقم مكتوب بدون فاصلة عشرية غير مهمة ما لم يتم وضع علامة عليها بخلاف ذلك. هناك العديد من الاصطلاحات المختلفة لتمييز الأصفار المهمة في هذه الحالات ، ولكن لتجنب الغموض ، ينصح بإعادة كتابة هذه القيم في التدوين العلمي. إذا كان القياس بوحدات SI ، فاستخدم وحدات أكبر.
دقة القياس تحد من دقة أي قيم محسوبة منه. تحتوي القيمة المحسوبة على نفس عدد الأرقام المعنوية مثل القياس أو القيمة الأقل دقة المستخدمة في حسابها. إذا كانت النتيجة تحتوي على عدد كبير جدا من الأرقام المهمة ، تقريبها بالدقة المناسبة. إذا كانت النتيجة تحتوي على عدد قليل جدا من الأرقام المعنوية ، تمديدها بأصفار زائدة كبيرة للحفاظ على الدقة. يجب أيضا مراعاة الأرقام المعنوية للقيم المشتقة تجريبيا ، مثل الكتلة المولية أو الكثافة أو بعض عوامل تحويل الوحدة ، عند استخدامها في العمليات الحسابية.
لا يقتصر عدد الأرقام المعنوية في القيمة المحسوبة على الثوابت الفيزيائية أو الرياضية في الصيغ أو الأرقام الدقيقة ، مثل عدد نقاط البيانات المكتسبة في التجربة. على سبيل المثال ، صيغة حساب حجم الكرة هي V = 4/3πr3. يتأثر عدد الأرقام المعنوية في الحجم المحسوب فقط بعدد الأرقام المهمة في قياس نصف القطر r. 4/3 و π ثوابت ، و 3 هو ببساطة تدوين لعملية رياضية.
يتم التعامل مع عوامل التحويل المحددة أيضا على أنها ثوابت. على سبيل المثال ، يتم تعريف البوصة على أنها 25.4 ملم بالضبط. لذلك ، يعتمد عدد الأرقام المهمة للقيمة المحولة فقط على القياس الأصلي. تحقق دائما مما إذا كان عامل التحويل محددا بدقة أو قيمة تجريبية عند التحويل بين الوحدات.
لجمع القياسات وطرحها البسيط ، نستخدم حساب الدلالة لتحديد عدد الأرقام المعنوية. في هذه الحالة ، تحتوي الإجابة على عدد المنازل العشرية مثل القياس بأقل عدد من المنازل العشرية بغض النظر عن عدد الأرقام المعنوية. وهكذا ، 15،643.7 مل + 0.613 مل = 15،644.3 مل.
بالنسبة للحسابات متعددة الخطوات أو المعادلات المعقدة التي تقسيمها إلى أجزاء أصغر لحلها ، يجب أن تحتفظ برقم واحد أو اثنين على الأقل من الأرقام غير المهمة على القيم الوسيطة. على سبيل المثال ، إذا قمت بحساب قيم البسط ومقام الكسر بشكل منفصل قبل تقسيمهما ، فيجب أن تحتفظ بالأرقام الضئيلة للبسط والمقام عند تقسيمتهما. هذا يقلل من خطأ التقريب ، وهو الفرق عن الرقم الذي ستحصل عليه إذا أدخلت الصيغة بأكملها في الآلة الحاسبة مرة واحدة. إذا قمت بتدوين هذه القيم الوسيطة ، فيجب أن تلاحظ الأرقام غير المهمة.
<h4>مراجع</h4>
هاريس ، العاصمة (2015). التحليل الكيميائي الكمي. نيويورك ، نيويورك: دبليو إتش فريمان وشركاه.

دقة القياس
تعد القدرة على تكرار التجربة والحصول على نفس النتائج ، أو قابلية التكرار ، أمرا ضروريا في البحث العلمي. لكن...

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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 maximum precision they can support. For rulers and graduated glassware, precision depends on the increment, which is the smallest distance between two marks.

Liquid levels must also be read correctly. Most liquids curve upward at the edges in glass containers. This curved surface is called a concave meniscus. When the edges curve downward, it is called a convex meniscus. To measure volume, the reference point is the bottom of a concave meniscus or the top of a convex meniscus. The reading should be taken from the side at eye level. Looking from above or below can make the level seem higher or lower than it really is.

Graduated and volumetric glassware are designed for different jobs. Volumetric glassware measures one specific volume and has only one mark. Graduated glassware measures a range of volumes and has many marks, or graduations, at regular intervals. When the meniscus reference point touches the mark, the glassware is holding the volume defined by that mark. Both types may also include manufacturer information about precision.

Manufacturers calibrate equipment so measurements stay within a stated range of uncertainty. A high-precision analytical balance may read to four decimal places, such as 0.0000 g, with an uncertainty of ±0.0001 g. That means a reading of 0.0345 g could represent a true value between 0.0344 g and 0.0346 g. Glassware usually has its uncertainty printed on it. For a 100 mL graduated cylinder with 1 mL increments and an uncertainty of ±0.5 mL, the volume should be recorded to the tenths place.

Temperature also affects volume, so calibrated glassware lists the temperature it is made for. Glassware can be calibrated to contain, or TC, or to deliver, or TD. TC glassware holds the stated volume when filled to the mark, but some liquid remains after pouring. This is why volumetric flasks are usually TC and are often used to prepare solutions, since solution concentration depends on volume accuracy. TD glassware dispenses the stated volume and should not be emptied completely. Graduated and volumetric pipettes are typically TD and are useful for transferring an exact volume to another container.

Recording numbers correctly also requires understanding uncertainty and significant figures. Unless stated otherwise, uncertainty is assumed to be on the scale of the last digit. If a ruler or glassware has no known uncertainty, estimate it as half of the smallest increment. Significant figures are the digits that are meaningful for the precision of a value. Leading zeroes are not significant, while trailing zeroes are significant when they appear before or after a decimal point. A number written without a decimal point may need scientific notation to avoid ambiguity.

Calculated values must match the precision of the measurements used to find them. The result should have the same number of significant figures as the least precise value in the calculation. If the answer has too many significant figures, round it. If it has too few, add significant trailing zeroes to show the correct precision. Constants such as 4/3 and π in the sphere volume formula do not limit the number of significant figures, and exact conversion factors like 1 inch = 25.4 millimeters also do not. For addition and subtraction, the answer keeps the same number of decimal places as the least precise value. For multi-step problems, keep one or two extra digits during intermediate steps to reduce rounding error.

Measuring with Accuracy in the Lab

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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...

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Acids, Bases, and pH in Titration

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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

Electrochemistry studies how chemical change and electrical energy are linked. In this branch of chemistry, electrons move from one species to another. That movement can either produce current or be driven by an applied current. The main reaction in electrochemistry is the oxidation-reduction, or redox, reaction. A redox reaction has two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. These changes also shift the oxidation state of each substance. The...

electrolytic cells

Electroplating with Electrolytic Cells

Electrochemistry links electrical energy with chemical change, and electrolytic cells show how electricity can drive a nonspontaneous reaction. In these reactions, electrons move from one species to another. The key chemistry is a redox reaction, or oxidation-reduction reaction, which happens in two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. The substance that loses electrons is the reducing agent, and the substance that gains electrons is the...

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

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Learn how students measure liquids and solids, read meniscus levels, use TC and TD glassware, and apply uncertainty and significant figures.

Measuring with Accuracy in the Lab | JoVE K12

CHEM.1

CHEM.1.D

Biology

BIO.1

BIO.1.E

استعد لتدريس تقنيات المختبر من خلال هذا الدليل الإرشادي للمدرسين لمدة 1 دقيقة. يغطي خطوات التحضير الأساسية لمختبر Chemistry على JoVE Lab Manual.

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The Scientific Method

The scientific method is a detailed, empirical problem-solving process used by biologists and other scientists. This iterative approach involves ...

Inductive Reasoning

Inductive reasoning is a form of logical thinking that uses related observations to arrive at a general conclusion. It is uncertain and operates in ...

Deductive Reasoning

Deductive reasoning, or deduction, is the type of logic used in hypothesis-based science. In deductive reasoning, the pattern of thinking moves in the ...

Correlation and Causation

Correlation and CausationStatistical tests can calculate whether there is a relationship, or correlation, between independent and dependent variables. A ...

Ask questions and define problems related to specialized topics of study based on observations or information from text, phenomena, models, or investigations;

112.51.c.1.A.

Scientific Method

The scientific method is used to solve problems and explain phenomena. The development of the scientific method coincided with changes in philosophy ...

Demonstrating the Scientific Method with Tree Branches
ExpandObtain branches containing a few dozen typical leaves from five different local tree species. ...

Observation and Background Research
ExpandNOTE: The first two steps in the scientific process involve studying and observing natural phenomena of ...

Problem-Solving

A problem-solving strategy is a plan of action used to find a solution. It involves trial and error, algorithms, heuristics, and insights. Effective ...

Problem Definition

Problem definition is the initial and the most essential step in research.
The researcher identifies and explains the specific issue or gap to be ...

Apply science practices related to specialized topics of study to plan and conduct investigations or use engineering practices to design solutions to problems;

112.51.c.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 ...

Decontamination for Laboratory Biosafety

Robert M. Rioux and Zhifeng Chen, Pennsylvania State University, University Park, PA
Decontamination is essential for laboratory biosafety, as the ...

Proper Waste Disposal

Robert M. Rioux and Taslima A. Zaman, Pennsylvania State University, Pennsylvania, PA

Users are responsible for the proper disposal of the waste ...

Use appropriate safety equipment and practices during laboratory, classroom, and field investigations as outlined in Texas Education Agency-approved safety standards;

112.51.c.1.C.

Common Lab Glassware and Uses

Source: Laboratory of Dr. Neal Abrams &#8212; SUNY College of Environmental Science and Forestry
Glassware is a regular appearance in the professional ...

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

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Preparation of LaboratoryExpand
	Here we show the laboratory preparation for 10 students ...

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Determining the Density of an EggExpand
	Density is a measure of how compact a substance or ...

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 ...

Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA

	Preparation of the LaboratoryExpand
	Here, we show the laboratory preparation ...

Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

	Nitration of Methyl BenzoateExpand
	From this lab onward, ...

Use tools appropriate to the specialized topic of study;

SI Units: 2019 Redefinition

Measurement is integral to analytical chemistry. Each record comprises a number &#8211; denoting the magnitude&#160; &#8211; and the unit &#8211; a ...

International System of Units

The International System of Units, known as the SI system, is a universally accepted measurement system recognized and used worldwide. The SI system is ...

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, ...

Collect quantitative data using the International System of Units (SI) or qualitative data as evidence as appropriate to the specialized topic of study;

112.51.c.1.E.

Review and Preview

Data are individual items of information obtained from a population or sample. Data may be classified as qualitative (categorical), quantitative ...

Organize quantitative or qualitative data using representations appropriate to the specialized topic of study such as scatter plots, line graphs, bar graphs, charts, data tables, diagrams, scientific drawings, and student-prepared models;

112.51.c.1.F.

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 explain phenomena or design solutions using appropriate tools and models. The student is expected to:

Measuring with Accuracy in the Lab

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