Name: matter: elements, compounds, and mixtures
Uploaded: 2020-09-03T13:38:25.000Z
Duration: 3 min 35 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.1

Classifying Matter by Composition

La matière est composée d'atomes plus petites unités de base de la matière et des molécules qui sont formées en combinant deux atomes ou plus ensemble. Sur la base de sa composition, la matière peut être classée en deux groupes substances pures et mélanges. Par définition, une matière est dite pure s'elle est constituée d'un seul composant, par une composition uniforme partout.Prenons par exemple l'hydrogène. L'hydrogène est composé de seulement des atomes d'hydrogène. De même, l'eau est composée uniquement de molécules d'eau.Ces compositions ne varient pas d'un échantillon à l'autre. Les substances pures peuvent être classées en deux types éléments et composants, selon qu'ils peuvent ou non être décomposés. Les éléments sont des substances pures qui ne peuvent pas être décomposées en substances plus simples par des moyens chimiques.Par exemple, l'hydrogène est un élément. Les composants, au contraire, sont des substances qui peuvent être séparés en leurs constituants uniquement par des moyens chimiques. Par exemple, l'eau, aussi une substance pure, peut être décomposée chimiquement en deux substances plus simples l'hydrogène et l'oxygène ce qui signifie que c'est un composant.Sur Terre, les composants sont plus courants que les éléments purs car la plupart des éléments se combinent avec d'autres éléments. Par exemple, la craie utilisée pour écrire sur le tableau noir est composé de calcium, de carbone et d'oxygène;le sucre est composé de carbone, d'hydrogène, et d'oxygène. Mélanges, contrairement aux substances pures, sont constitués d'un ou plusieurs composants.Leur proportion peut également varier d'un échantillon à l'autre. Par exemple, la limonade est un mélange de trois composants jus de citron, sucre et eau et leurs proportions peuvent être variés pour rendre la boisson plus ou moins sucrée. Les mélanges eux-mêmes peuvent être classés en deux types:homogène et hétérogène, selon l'uniformité du mélange des substances qu'ils contiennent.Un mélange homogène présente une composition uniforme et apparaît visuellement le même partout. On parle souvent de solution. Par exemple, une cuillerée de sel dissous dans l'eau donne un mélange homogène comme chaque goutte de ce mélange a la même composition.De même, la limonade est également un mélange homogène. Dans un mélange hétérogène, les composants ne sont pas répartis uniformément, et donc la composition varie d'un point à l'autre. Par exemple, du sable mélangé à de la limaille de fer ici, différentes parties de ce mélange auront probablement des quantités de limaille de fer et de sable.Peu importe, tout mélange, qu'il soit homogène ou hétérogène, peut être séparé en ses composants purs par des moyens physiques sans changer les identités des composants. Par exemple, le sel peut être récupéré de la solution aqueuse en chauffant le mélange à sec. De même, la limaille de fer peut être séparée du sable à l'aide d'un aimant.

classification de la matière par composition

<h4>Matière : substances pures et mélanges </h4>
 
 Selon sa composition, la matière peut être classée en deux grandes catégories : les substances pures et les mélanges. 
 
 Une substance pure est une forme de matière qui a une composition constante partout avec des propriétés uniformes. Par exemple, tout échantillon de saccharose a la même composition et les mêmes propriétés physiques, comme le point de fusion, la couleur et le goût sucré, quelle que soit la source à partir de laquelle il a été isolé.  
 
 Un mélange est composé d'au moins deux sortes de matière qui peuvent être présentes en quantités variables et qui peuvent être séparées par des transformations physiques, telles que l'évaporation. Les composants d'un mélange conservent leurs identités distinctes. 
 
 <h4>Substances pures : éléments et composés </h4>
 
 Les substances pures peuvent être divisées en deux classes : les éléments et les composés. Les substances pures qui ne peuvent pas être décomposées en substances plus simples par des transformations chimiques sont appelées des éléments. Le fer, l'argent, l'or, l'aluminium, le soufre, l'oxygène et le cuivre sont des éléments. 
 
 Les substances pures qui peuvent être décomposées par des transformations chimiques sont appelées des composés. Cette décomposition peut produire des éléments ou d'autres composés, ou les deux. L'oxyde de mercure(II), un solide cristallin orange, peut être décomposé par la chaleur en éléments mercure et oxygène. Lorsqu'il est chauffé en l'absence d'air, le composé saccharose est décomposé selon l'élément carbone et le composé eau. Le chlorure d'argent(I) est un solide blanc qui peut être décomposé selon ses éléments, l'argent et le chlore, par absorption de la lumière.  
 
 Les propriétés des éléments combinés sont différentes de celles à l'état de corps simple ou non combiné. Par exemple, le sucre cristallin blanc (saccharose) est un composé résultant de la combinaison chimique de l'élément carbone, qui est un solide noir dans l'une de ses formes non combinées, et des deux éléments hydrogène et oxygène, qui sont des gaz incolores lorsqu'ils ne sont pas combinés. Le corps simple sodium , un élément qui est un solide métallique mou, brillant, et le corps simple chlore, un élément qui est un gaz jaune-vert, se combinent pour former le chlorure de sodium (sel de table), un composé qui est un solide cristallin blanc. 
 
 <h4>Mélanges : homogènes et hétérogènes  </h4>
 
 Un mélange homogène, également appelé solution, présente une composition uniforme et une apparence visuelle qui est la même partout. Un exemple de solution est une boisson de sport, composée d'eau, de sucre, de colorants, d'arômes, et d'électrolytes mélangés uniformément. Chaque goutte d'une boisson de sport a le même goût parce que chaque goutte contient les mêmes quantités d'eau, de sucre et des autres composants. Il est important de noter que la composition d'une boisson de sport peut varier — elle peut être fabriquée avec un peu plus ou un peu moins de sucre, d'arômes ou des autres composants, et être toujours une boisson de sport. D'autres exemples de mélanges homogènes comprennent l'air, le sirop d'érable, l'essence et une solution de sel dans l'eau.  
 
 Un mélange dont la composition varie d'un point à un autre est appelé un mélange hétérogène. La vinaigrette italienne est un exemple de mélange hétérogène. Sa composition peut varier parce qu'elle peut être préparée à partir de diverses quantités d'huile, de vinaigre et d'herbes. Elle n'est pas la même d'un point à l'autre du mélange — une goutte peut être principalement du vinaigre, alors qu'une autre goutte peut être principalement de l'huile ou des herbes, car l'huile et le vinaigre sont séparés et les herbes se déposent. D'autres exemples de mélanges hétérogènes sont les biscuits aux pépites de chocolat (on peut voir les morceaux distincts de chocolat, de noix, de pâte à biscuits) et le granit (on peut voir le quartz, le mica, le feldspath et plus).  
 
 Tout mélange, qu'il soit homogène ou hétérogène, peut être séparé en constituants purs par des moyens physiques sans changer l'identité des constituants. Bien qu'il y ait un peu plus de 100 éléments, des dizaines de millions de composés chimiques résultent de combinaisons différentes de ces éléments. Chaque composé a une composition spécifique et possède des propriétés chimiques et physiques définies qui le distinguent de tous les autres composés. Et, bien sûr, il existe d'innombrables façons de combiner des éléments et des composés pour former différents mélanges. 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/1-2-phases-and-classification-of-matter">Openstax, Chimie 2e, Section 1.2 : Phases et classification de la matière.  </a>

Classification de la matière par composition

Matière : substances pures et mélanges
Selon sa composition, la matière peut être classée en deux grandes catégories : les substances pures et ...

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Introduction: Matter and Measurement

Introduction : La matière et la mesure

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

matter: elements, compounds, and mixtures

Matter can be grouped by composition into pure substances and mixtures. A pure substance has a constant composition throughout and uniform properties. For example, any sample of sucrose has the same composition and the same physical properties, such as melting point, color, and sweetness, no matter where it is isolated from.

A mixture contains two or more kinds of matter in varying amounts. Its parts can be separated by physical changes, such as evaporation. The components keep their own identities, so the mixture is not a new substance in the same way a compound is.

Pure substances are divided into elements and compounds. Elements cannot be broken down into simpler substances by chemical changes. Iron, silver, gold, aluminum, sulfur, oxygen, and copper are examples of elements.

Compounds are pure substances that can be broken down by chemical changes. This breakdown may produce elements, other compounds, or both. Mercury(II) oxide can be heated to form mercury and oxygen, sucrose can break down into carbon and water when heated without air, and silver(I) chloride can be broken down by light into silver and chlorine.

The properties of combined elements are different from the properties of the elements in their free state. White crystalline sugar comes from carbon, hydrogen, and oxygen, but those elements look very different when uncombined. Free sodium is a soft, shiny metal, and free chlorine is a yellow-green gas, yet together they form sodium chloride, a white crystalline solid.

Mixtures can be homogeneous or heterogeneous. A homogeneous mixture, also called a solution, has a uniform composition and looks the same throughout. Sports drink is one example. It contains water, sugar, coloring, flavoring, and electrolytes mixed evenly, so each drop tastes the same. Its exact composition can still vary and remain a sports drink. Air, maple syrup, gasoline, and salt water are other homogeneous mixtures.

A heterogeneous mixture has a composition that varies from point to point. Italian dressing is one example, since the amounts of oil, vinegar, and herbs can differ and the parts may separate or settle. Chocolate chip cookies and granite are also heterogeneous mixtures because their different parts can be seen.

Any mixture, whether homogeneous or heterogeneous, can be separated into pure components by physical means without changing what the components are. There are just over 100 elements, but they combine in many ways to form tens of millions of compounds. Each compound has a specific composition and its own chemical and physical properties. Elements, compounds, and mixtures can combine in countless ways to make matter.

Matter: Elements, Compounds, and Mixtures

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classifying matter by composition

Matter can be grouped by composition into pure substances and mixtures. A pure substance has a constant composition throughout and uniform properties. For example, any sample of sucrose has the same composition and the same physical properties, such as melting point, color, and sweetness, no matter where it is isolated from. A mixture contains two or more kinds of matter in varying amounts. Its parts can be separated by physical changes, such as evaporation. The components keep their own...

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