Grade 8 Chapter 6: Composition of Matter

Introduction

Everything around us, whether solid, liquid, or gas, is made of matter. But what exactly is matter composed of? This chapter delves into the fundamental classification of matter into elements, compounds, and mixtures. We will explore their distinct properties, learn how to differentiate between various types of mixtures, and understand the different techniques used to separate their components. Additionally, we will examine the concepts of physical and chemical changes and the fundamental laws governing chemical combinations.

Classification of Matter

Matter can be classified based on its chemical composition into three main categories:

1. Elements

An element is a pure substance that cannot be broken down into simpler substances by ordinary chemical means. It consists of only one type of atom.

• Properties:
  • Made up of only one kind of atom.
  • Cannot be decomposed into simpler substances.
  • Each element has unique properties.
• Examples: Hydrogen (H), Oxygen (O), Iron (Fe), Gold (Au), Carbon (C).
• Elements exist as atoms or molecules (e.g., O₂ is a molecule of the element oxygen).

2. Compounds

A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio by mass. The properties of a compound are entirely different from those of its constituent elements.

• Properties:
  • Formed by chemical combination of two or more elements.
  • Constituent elements are present in a fixed proportion by mass.
  • Properties of the compound are different from its constituent elements.
  • Can only be separated into its constituent elements by chemical or electrochemical reactions.
• Examples: Water (H₂O - Hydrogen and Oxygen), Carbon dioxide (CO₂ - Carbon and Oxygen), Sodium chloride (NaCl - Sodium and Chlorine).

3. Mixtures

A mixture is a substance formed when two or more substances (elements or compounds) are physically combined in any proportion, without chemical reaction. The components retain their individual properties.

• Properties:
  • Formed by physical combination of two or more substances.
  • Constituents are present in any proportion.
  • Components retain their original properties.
  • Can be separated by physical methods.
• Examples: Air (mixture of gases), Salt solution (salt and water), Soil, Lemonade.

Types of Mixtures

Mixtures are further classified into two types:

• Homogeneous Mixture: A mixture in which the components are uniformly distributed throughout, and their individual boundaries are not visible. It has a uniform composition.
  • Examples: Salt solution, Sugar solution, Air, Alloys (like brass).
• Heterogeneous Mixture: A mixture in which the components are not uniformly distributed, and their individual boundaries are visible. It has a non-uniform composition.
  • Examples: Sand and water, Oil and water, Mixture of iron filings and sulfur, Soil, Cereal with milk.

Methods of Separating Mixtures

Various physical methods can be used to separate the components of a mixture, depending on their properties:

• Decantation: Used to separate insoluble solids from liquids (e.g., sand from water) or two immiscible liquids (e.g., oil from water) by carefully pouring off the top layer.
• Filtration: Used to separate insoluble solids from liquids using a filter paper or filter medium (e.g., chalk powder from water).
• Evaporation: Used to separate a soluble solid from its liquid solvent by heating the solution, causing the liquid to evaporate and leaving the solid behind (e.g., salt from salt solution).
• Distillation: Used to separate components of a liquid mixture based on differences in their boiling points. The liquid with the lower boiling point evaporates first, is condensed, and collected (e.g., separating alcohol from water).
• Chromatography: Used to separate components of a mixture that dissolve in the same solvent but are absorbed differently by a stationary phase (e.g., separating colors in ink, pigments from plant extracts).
• Centrifugation: Used to separate fine suspended particles from a liquid by spinning the mixture at high speed. Denser particles settle at the bottom (e.g., separating cream from milk, blood cells from plasma).
• Magnetic Separation: Used to separate magnetic substances from non-magnetic substances (e.g., iron filings from sand).
• Sublimation: Used to separate a solid that sublimes (changes directly from solid to gas) from a non-subliming solid (e.g., separating camphor from salt).

Physical and Chemical Changes

Matter can undergo changes, which are classified as physical or chemical.

• Physical Change: A temporary change in which only the physical properties (like shape, size, state) of a substance change, but its chemical composition remains the same. No new substance is formed, and the change is often reversible.
  • Examples: Melting of ice, boiling of water, tearing of paper, dissolving salt in water, breaking of glass.
• Chemical Change: A permanent change in which the chemical composition of a substance changes, resulting in the formation of one or more new substances with entirely different properties. The change is generally irreversible.
  • Examples: Burning of wood, rusting of iron, cooking food, ripening of fruit, digestion of food, burning of a candle.

Laws of Chemical Combination

Chemical reactions follow certain fundamental laws, which govern how elements combine to form compounds.

1. Law of Conservation of Matter (Mass)

Proposed by Antoine Lavoisier. It states that "Matter can neither be created nor destroyed in a chemical reaction." This means that the total mass of the reactants before a chemical reaction is equal to the total mass of the products after the reaction.

Total mass of reactants = Total mass of products

• Example: When carbon burns in oxygen to form carbon dioxide, the mass of carbon and oxygen consumed is equal to the mass of carbon dioxide formed.

2. Law of Constant Proportions (Law of Definite Proportions)

Proposed by Joseph Proust. It states that "In a chemical compound, the elements are always present in definite proportions by mass, irrespective of the source or method of preparation."

• Example: Water (H₂O) always consists of hydrogen and oxygen combined in a fixed mass ratio of 1:8, regardless of where the water comes from (river, tap, or laboratory).

Summary

• Matter is classified into Elements (pure, one type of atom), Compounds (two or more elements chemically combined in fixed ratio), and Mixtures (two or more substances physically combined in any ratio).
• Mixtures can be homogeneous (uniform composition, e.g., salt solution) or heterogeneous (non-uniform composition, e.g., sand and water).
• Components of mixtures can be separated by physical methods like decantation, filtration, evaporation, distillation, chromatography, centrifugation, magnetic separation, and sublimation.
• Physical changes are temporary, do not form new substances, and are often reversible (e.g., melting ice).
• Chemical changes are permanent, form new substances with different properties, and are generally irreversible (e.g., burning wood).
• Laws of Chemical Combination:
  • Law of Conservation of Matter: Mass is conserved in a chemical reaction.
  • Law of Constant Proportions: Elements in a compound are always in fixed mass ratios.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 6: Composition of Matter.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.