Nature of Matter: Elements, Compounds, and Mixtures
Chapter Notes | Science Class 8
1. Introduction
What is everything around you really made of?
From the air you breathe to your food, clothes and school books, almost everything is matter made of tiny particles.
Most everyday materials are not single pure substances. They are formed by combining different substances in different ways. Depending on how they are combined, the substances may retain their properties or acquire new properties.
In this chapter, you will study elements, compounds and mixtures. You will learn how they differ, how mixtures can be separated, how compounds are formed and decomposed, and how this knowledge is used in daily life and industry.
2. What Are Mixtures?
A mixture is formed when two or more substances are combined in such a way that each substance retains its own properties and the substances do not chemically react with one another. The individual substances present in a mixture are called its components.
Types of Mixtures
Mixtures are classified as follows:
Heterogeneous or non-uniform mixtures: The different components are not evenly distributed and can generally be distinguished. Examples include sprout salad, sand and water, oil and water, and carbon particles in air.
Homogeneous or uniform mixtures: The components are evenly distributed and cannot be distinguished separately. Examples include sugar dissolved in water, seawater, air, vinegar (acetic acid in water), and alloys such as stainless steel.
Scientific meaning: In science, the components used to describe a mixture are pure substances. Everyday preparations such as poha or sprout salad may themselves contain ingredients that are mixtures.
Variable composition: The proportion of the components in a mixture can vary, so different samples of the same kind of mixture may have different compositions.
Scientific Heritage: Alloys
Alloy or Mishraloha: An alloy is a mixture of two or more metals, or of a metal and another element, prepared to obtain improved properties.
Examples include bronze, which contains copper and tin; brass, which contains copper and zinc; and stainless steel, which contains mainly iron along with nickel, chromium and carbon.
Alloys are usually homogeneous mixtures. They are important in construction, tools and appliances because they combine useful properties of their components.
3. Is Air a Mixture?
Air is a homogeneous mixture of gases. It contains mainly nitrogen and oxygen, along with argon, carbon dioxide, water vapour and small amounts of other gases.
Components of Air — Air is a mixture because:
The gases in air are not chemically bonded to one another. Each gas retains its own properties.
Air is generally uniform in composition at a particular place and time, so it is a homogeneous mixture.
The gases in air cannot be separated by sight. They can be separated by physical methods, such as fractional distillation of liquid air.
Testing for Carbon Dioxide in Air
Limewater is a clear solution of calcium hydroxide. It can be prepared by carefully adding a small amount of calcium oxide (quicklime) to water. Calcium oxide reacts vigorously with water to form calcium hydroxide and releases heat. After stirring, the mixture is filtered to obtain clear limewater.
Expose clear limewater to air.
The limewater turns milky.
This happens because carbon dioxide in air reacts with calcium hydroxide to form insoluble calcium carbonate, which makes the solution appear milky.
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Conclusion: The air around us contains carbon dioxide.
Dust, Particulates and Air Pollution
Place a clean black sheet of paper near an open window for a few hours. Dust may settle on it. You can examine the dust with a magnifying glass, if available.
Dust particles and soot present in air are called particulate pollutants.
Major air pollutants include particulate matter, carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide.
Air quality is often described using the Air Quality Index, or AQI.
4. Types of Mixtures by Physical State
Mixtures can be formed by combining substances in different physical states. NCERT gives the following examples:
5. Why Are Mixtures Separated?
In daily life: Mixtures are separated to obtain a useful component or to remove an unwanted component. For example, stones are removed from rice and tea leaves are separated from tea.
In science and industry: Mixtures are separated to obtain pure substances for research, medicines, analysis and further processing.
Some Methods of Separating Mixtures
Handpicking: Large, unwanted particles such as stones can be picked out by hand from grains.
Sieving: Components of different sizes can be separated using a sieve. For example, bran can be separated from flour.
Magnetic separation: Magnetic substances such as iron can be separated from non-magnetic substances using a magnet.
Sedimentation and decantation: An insoluble, heavier solid can settle at the bottom of a liquid. The clear liquid can then be carefully poured off.
Filtration: An insoluble solid can be separated from a liquid by passing the mixture through a filter. For example, tea leaves can be separated from tea.
Evaporation: A dissolved solid can be obtained by evaporating the liquid. Common salt can be obtained from seawater in this way.
Fractional distillation: Miscible liquids or gases with different boiling points can be separated by carefully controlled heating and cooling. The gases of air can be separated by fractional distillation of liquid air.
6. What Are Pure Substances?
Everyday Meaning of "Pure"
On food packets, the word "pure" usually means that the product is unadulterated and does not contain cheaper or harmful substances added to it.
Adulteration is the illegal mixing of inferior or harmful substances with a product. It reduces the quality and safety of the product.
Scientific Definition of a Pure Substance
A pure substance contains only one kind of matter or one type of particle throughout the sample.
Examples: Chemically pure sugar, distilled water, chemically pure baking soda, pure iron and pure gold.
A pure substance cannot be separated into other substances by physical processes such as filtering, sieving or simple evaporation.
Milk, soil and packed fruit juice are mixtures, not pure substances.
Classification of Common Substances:
Milk: A mixture containing water, fats, proteins and minerals.
Packaged fruit juice: A mixture containing water, sugars, flavours, vitamins and, in some cases, added preservatives.
Baking soda: A pure substance if it is chemically pure sodium bicarbonate.
Sugar: A pure substance if it is pure sucrose.
Soil: A mixture of sand, clay, organic matter, water and air.
In science, "pure" means that only one substance is present and that the same kind of particle is found throughout the sample.
7. Types of Pure Substances
Pure substances are divided mainly into elements and compounds.
Physical Changes
Physical changes such as melting, freezing and evaporation change the state or form of a substance but not its chemical identity.
Ice, liquid water and steam are all the same substance, water. Their particles have the same chemical composition even though their physical states are different.
Chemical Decomposition: An Example of Water
In the NCERT activity, a few drops of dilute sulfuric acid are added to water so that electricity can pass through it. When electric current is passed through the water, gases are collected at the two terminals.
The gas that burns with a "pop" is hydrogen, while the gas that makes a flame glow brighter is oxygen. This chemical change shows that water is a compound made from hydrogen and oxygen.
2H₂O ──(electricity)──> 2H₂ + O₂
8. Elements
Elements are pure substances made up of only one kind of atom.
Elements cannot be broken down into simpler substances by ordinary chemical methods.
Each element contains atoms of the same kind.
Examples: Hydrogen, oxygen, gold, silver, sulfur and carbon.
Molecules and Forms of Elements
Many elements exist as molecules, in which two or more atoms of the same element are joined together. For example, hydrogen (H₂) and oxygen (O₂) commonly exist as diatomic molecules.
Metals: Metals such as gold and iron are generally shiny, malleable and good conductors of heat and electricity.
Non-metals: Non-metals such as oxygen and sulfur are often poor conductors. They may be gases, liquids or brittle solids.
Metalloids: Metalloids such as silicon and boron have properties between those of metals and non-metals. They are studied in greater detail in higher classes.
Key Facts About Elements
There are 118 known elements.
Most elements are solids at room temperature. Eleven elements are gases, including oxygen, helium and nitrogen.
Two elements are liquids at room temperature: mercury, which is a metal, and bromine, which is a non-metal.
Some elements, such as gallium and caesium, are solids that melt at temperatures close to room temperature. Their melting behaviour is a special case.
9. Compounds
Compounds are pure substances formed when two or more elements chemically combine in a fixed ratio.
Key Properties of Compounds
Compounds are formed when elements combine in fixed ratios. For example, water always contains hydrogen and oxygen in the ratio 2:1 by number of atoms.
A compound has properties different from those of its constituent elements. Hydrogen is a flammable gas and oxygen supports combustion, but water can be used to extinguish many fires.
The components of a compound cannot be separated by simple physical methods. Chemical methods are needed to decompose a compound.
Examples of Compounds
Water: A compound of hydrogen and oxygen, represented by H₂O.
Sodium chloride or common salt: A compound of sodium and chlorine, represented by NaCl. Its sodium and chlorine particles are present in a fixed 1:1 ratio. Sodium is a soft metal and chlorine is a hazardous gas, but sodium chloride has properties different from both. The constituent elements cannot be separated from sodium chloride by physical methods.
Sugar: A compound containing carbon, hydrogen and oxygen. On heating, sugar first turns brown and later chars. Water droplets appear near the open end of the tube, while carbon is left behind. Thus, heating decomposes sugar to give carbon and water, showing that sugar is a compound rather than an element.
Interesting fact: Modern mobile phones use more than 45 elements, including aluminium, copper, silicon, cobalt, lithium, gold and silver, for their different parts.
10. Mixture and Compound: Iron and Sulfur
We can compare the behaviour of iron and sulfur when they are simply mixed and when they react on heating. This classic classroom experiment shows the difference between a mixture and a compound.
Before Heating: Sample A, a Mixture
Mix iron filings and sulfur powder thoroughly in a 7:4 ratio by mass to prepare Sample A. It is a mixture because the two elements have been mixed without a chemical reaction and retain their individual properties.
Appearance: Black iron filings and yellow sulfur powder can be seen separately. Therefore, Sample A is a non-uniform mixture.
Magnet test: A magnet attracts only the iron filings and leaves the sulfur behind. This shows that each component retains its own properties.
Acid test: When dilute hydrochloric acid is added to Sample A, the iron reacts and produces hydrogen gas. The gas burns with a sharp "pop" sound. Sulfur does not react in the same way and remains as a yellow solid.
After Heating: Sample B, a Compound
When part of Sample A is heated gently with continuous stirring until a black mass forms, a chemical reaction takes place. The new black substance is iron sulfide. This is Sample B, a compound.
Appearance: The product is a uniform black mass. Separate iron and sulfur are no longer visible.
Magnet test: A magnet does not attract Sample B because iron has chemically combined with sulfur and its original magnetic behaviour has changed.
Acid test: When dilute hydrochloric acid is added to Sample B, hydrogen sulfide gas is released. It has a characteristic rotten-egg smell and is chemically different from the hydrogen gas produced by iron in the mixture.
Reactions:
Fe + 2HCl → FeCl₂ + H₂↑ (Sample A: Mixture)
FeS + 2HCl → FeCl₂ + H₂S↑ (Sample B: Compound)
Thus, a mixture has variable composition and its components retain their properties, whereas a compound has a fixed composition and properties different from those of its constituent elements.
Comparison: Mixture vs Compound
11. Uses of Elements, Compounds and Mixtures
Elements are the basic building blocks, compounds form new useful substances, and mixtures are prepared or selected for practical uses. This understanding is important in health, food, construction and environmental science.
Everyday Presence
Elements, compounds and mixtures surround us: Air is a mixture, water is a compound, and the metals used in construction may be elements or alloys.
Metals for construction: Elements such as iron and aluminium are used in bridges, buildings and vehicles because of their strength and durability.
Role in Science, Technology and Industry
New discoveries: Understanding how elements combine helps chemists and engineers design new materials and medicines.
Medicines and vaccines: Compounds and chemical reactions are used in the development of medicines and vaccines.
Fertilisers: Compounds such as nitrates and phosphates provide nutrients to plants and can help increase crop yield.
Engineering materials: Alloys and composite mixtures can provide improved strength, corrosion resistance and other desired properties.
Building Materials as Mixtures
Many construction materials are mixtures. Wood, steel and concrete are used because of the combined properties of their components.
Metals are extracted from minerals, which may be naturally occurring elements or compounds. The extracted metals may then be processed to obtain pure metals or alloys.
Special Example: Graphene Aerogel
What is it? Graphene aerogel is a very lightweight, highly porous material made from carbon. It is known as one of the lightest materials on Earth.
Properties: It has a highly porous structure and excellent absorbent capacity.
Uses: It can be used for environmental clean-up, including the absorption of oil spills, as well as in energy-saving devices and special coatings for buildings.
12. What Are Minerals?
Minerals are naturally occurring solid substances found on Earth and have a fixed chemical composition. Most rocks are mixtures of different minerals.
Viewing minerals: Some minerals can be seen with the naked eye, while others may require a magnifying glass or microscope.
Building blocks: Minerals may be native elements or compounds made of two or more elements.
Relation to matter: The elements and compounds present in minerals are basic building blocks of matter. Matter is anything that has mass and occupies space.
Types of Minerals
Native minerals: These occur in elemental form in nature. Examples include gold, silver, copper, sulfur and carbon.
Compound minerals: Most minerals are compounds made of more than one element. Examples include quartz, calcite, mica, pyroxene, olivine and talc.
Everyday Uses of Minerals
Cement: Cement is prepared from materials containing minerals such as calcite, quartz, alumina and iron oxide. It is used in construction.
Talcum powder: Talcum powder is made from the mineral talc and is used for personal care.
Minerals provide raw materials for buildings, tools and many other products. They show how elements and compounds found in nature support human needs.
13. What Is Matter?
Matter is anything that has mass and occupies space. It is made from elements and compounds and forms material objects such as rocks, water and air.
Non-matter: Light, heat, electricity, thoughts and emotions are important phenomena, but they are not classified as matter because they do not have mass and occupy space in the same way as material substances.
Why it matters: Understanding matter and its forms helps us explain everyday observations and supports scientific and technological progress.
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