Nature of Matter: Elements, Compounds, and Mixtures
NCERT-Class-8-Science (Curiosity)-Chapter-8
Notes
Matter and Non-Matter :
Matter is anything that has mass and occupies space. Everything around us—air, water, clothes, trees—is made of matter.
- Matter is made of tiny particles like atoms and molecules.
- Atoms are the smallest units of elements and keep their chemical properties.
- Molecules are formed when two or more atoms join together. Example: two oxygen atoms form an O₂ molecule.
Not matter: Light, sound, heat, electricity, thoughts, and emotions are not matter because they have no mass and do not occupy space.
Primary Classification of Matter:
Matter is first grouped based on what it is made of and how its particles are bonded. It is divided into pure substances and mixtures.
Pure Substances vs Mixtures
| Feature | Pure Substances | Mixtures |
| Composition | Fixed and definite | Variable; any proportion |
| Formation | Made by chemical reactions | Formed by physically mixing substances |
| Properties | New and unique properties | Each component keeps its own properties |
| Separation | Can be separated only chemically | Can be separated by physical methods |
| Energy Change | Often involves energy changes | No energy change in formation |
| Homogeneity | Always uniform (homogeneous) | Can be homogeneous or heterogeneous |
| Examples | Elements (gold), compounds (water, salt) | Air, seawater, soil, alloys |
What Are Mixtures?
A mixture is an impure substance made by physically combining two or more substances without any chemical reaction.
- Each component keeps its own properties.
- The composition can vary—substances can be mixed in any proportion.
- The components can be separated by physical methods like filtration, evaporation, or magnetism.
(A) Types of Mixtures
Mixtures are classified based on how evenly their components are mixed.
(i) Homogeneous (Uniform) Mixtures
- Same composition throughout.
- Components are mixed evenly and cannot be seen separately.
Examples: Air, saltwater, sugar solution, vinegar, alloys.
Alloys: Uniform mixtures of metals, like stainless steel, brass, and bronze.
(ii) Heterogeneous (Non-uniform) Mixtures
- Composition is not the same everywhere.
- Components can be seen separately.
Examples: Sand and water, muddy water, soil, poha, sprout salad.
(B) Classification by Physical State of Components :
Mixtures can be formed from components in any physical state.
| Mixture Type | Components | Example(s) |
| Gas in Gas | Gas + Gas | Air |
| Gas in Liquid | Gas + Liquid | Aerated water (soda), Oxygen dissolved in water |
| Solid in Gas | Solid + Gas | Carbon particles in air |
| Liquid in Liquid | Liquid + Liquid | Acetic acid in water (vinegar), Oil and water |
| Solid in Liquid | Solid + Liquid | Sand and water, Seawater |
| Solid in Solid | Solid + Solid | Baking powder, Alloys |
What Are Pure Substances? :
In everyday language, “pure” means something that has no mixing, like pure ghee or pure milk.
But in science, a pure substance has only one type of particle (one kind of atom or molecule) and cannot be separated by physical methods.
So, scientifically, milk is not pure—it is a mixture.
- Pure Substance : A pure substance is a kind of matter that has the same composition throughout. It has only one type of particle and cannot be separated by physical methods.
What Are the Types of Pure Substances?
(A) Elements :
Elements are the simplest pure substances.
- Definition: An element is made of only one kind of atom. It cannot be broken into simpler substances by physical or chemical methods.
- Number of elements: There are 118 elements.
- State at room temperature: Most elements are solids.
- 11 are gases (like oxygen, helium, nitrogen).
- 2 are liquids (mercury and bromine).
Classification of Elements :
Elements are classified based on their physical and chemical properties into three main categories.
| Category | Properties | Examples |
| Metals | Are lustrous (shiny), malleable, ductile, and good conductors of heat and electricity. | Iron (Fe), Copper (Cu), Gold (Au), Silver (Ag), Magnesium (Mg) |
| Non-metals | Are non-lustrous (dull), brittle, and poor conductors of heat and electricity. | Carbon (C), Oxygen (O), Sulfur (S), Hydrogen (H) |
| Metalloids | Have properties that are intermediate between those of metals and non-metals. Also known as semimetals. | Boron (B), Silicon (Si), Arsenic (As) |
(B) Compounds: Chemical Combinations :
Compounds are pure substances made when two or more different elements join chemically.
- Definition: A compound is formed when elements combine in a fixed ratio and are held together by chemical bonds.
- Different Properties: A compound has properties that are totally different from the elements it is made of.
- Example: Water (H₂O) puts out fire, but it is made from hydrogen (a gas that burns) and oxygen (a gas that helps burning).
- Separation: The elements in a compound can be separated only by chemical reactions, not by physical methods.
Examples:
- Water (H₂O): Made from hydrogen and oxygen in a 2:1 ratio.
- Sodium Chloride (NaCl): Common salt, made from sodium (a reactive metal) and chlorine (a poisonous gas).
- Sugar (C₁₂H₂₂O₁₁): Made of carbon, hydrogen, and oxygen. On heating, it breaks into carbon (charcoal) and water.
- Carbon Dioxide (CO₂): Made from carbon and oxygen.
Experimental Evidence and Case Studies :
The distinctions between elements, compounds, and mixtures are validated through key experiments.
(A) Case Study: Air as a Uniform Mixture
Air is a good example of a uniform (homogeneous) mixture.
Composition: Air mainly has nitrogen (78%), oxygen (21%), and small amounts of argon, carbon dioxide, and water vapour.
Each gas keeps its own properties—for example, oxygen supports breathing and burning, while nitrogen does not.
Proof that air is a mixture:
- Carbon dioxide test: When lime water is exposed to air, it turns milky because carbon dioxide in the air reacts with it to form calcium carbonate.
- Dust particles: If you keep a clean black paper near a window, you will see dust settling on it. This shows that air also contains tiny solid particles.
(B) Case Study: Water as a Compound :
Electrolysis of water shows that water is a compound.
The Experiment: When electricity is passed through water with a little dilute sulfuric acid, water breaks down and gas bubbles appear at the two electrodes.
The Gases Formed:
- Hydrogen gas is collected at one electrode. It burns with a “pop” sound.
- Oxygen gas is collected at the other electrode. It makes a flame burn brighter.
Conclusion: Because water can be chemically broken into two elements (hydrogen and oxygen) that have different properties, water is a compound.
(C) Differentiating a Mixture from a Compound: The Iron and Sulfur Experiment : This experiment provides a direct comparison between a mixture and a compound formed from the same starting materials.
| Test | Sample A (Mixture of Iron & Sulfur) | Sample B (Compound - Iron Sulfide) |
| Formation | Iron filings and sulfur powder are physically mixed. | The mixture (Sample A) is heated strongly, causing a chemical reaction. |
| Appearance | A non-uniform, heterogeneous mixture. Yellow sulfur particles and black iron filings are visible. | A uniform, homogeneous black mass. |
| Magnet Test | Iron filings are attracted to a magnet, allowing for physical separation of the components. | The substance is not attracted to a magnet. Iron has lost its magnetic property. |
| Reaction with HCl | Iron reacts with dilute hydrochloric acid to produce colorless, odorless hydrogen gas (H₂), which burns with a "pop" sound. Sulfur does not react. | The entire substance reacts with dilute hydrochloric acid to produce hydrogen sulfide gas (H₂S), which has the smell of rotten eggs. |
This case study proves that in
- Sample A (mixture), iron and sulfur retain their individual properties,
- Sample B (compound), a new substance with entirely different properties has been formed.
How Do We Use Elements, Compounds, and Mixtures? :
Elements, compounds, and mixtures are the basic forms of matter. They are used everywhere in daily life and in different industries.
(i) Elements :
Elements are the simplest substances. They cannot be broken into anything simpler.
Uses of Elements :
- Construction & Manufacturing: Metals like iron, copper, aluminum are used to build houses, make wires, and create packaging.
- Breathing & Burning: Oxygen is needed for breathing and burning.
- Technology & Electronics: Silicon is used in computers and mobiles. Copper, aluminum, cobalt, lithium, gold, and silver are used in phones and other devices.
- Jewelry & Money: Gold and silver are used to make ornaments and in electronics.
- Environmental Cleanup: Graphene aerogel (made from carbon) is very light and absorbs oil, helping in cleaning oil spills.
- Found in Nature: Elements like gold, silver, copper, sulfur, carbon are found as pure minerals in the earth.
(ii) Compounds :
Compounds are substances formed when two or more elements combine chemically. They have new properties.
Uses of Compounds :
- Life & Nutrition:
- Water (H₂O): Needed for drinking, cooking, industries.
- Salt (NaCl): Used in food and as a preservative.
- Sugar: Gives energy.
- Carbon dioxide (CO₂): Used in breathing processes and photosynthesis.
- Medicine & Industry:
- Many medicines are chemical compounds.
- Fertilizers, vaccines, and drugs are made by creating new compounds.
- Sulfur dioxide (SO₂): Used as a preservative and to make sulfuric acid.
- Carbon dioxide (CO₂): Used in fire extinguishers.
- Building Materials: Compounds from minerals form cement (like calcite, alumina, iron oxide).
(iii) Mixtures :
Mixtures are made by physically combining substances. The parts keep their own properties and can be separated easily. Most things around us are mixtures.
Uses of Mixtures :
- Air: A uniform mixture of gases needed for breathing and weather.
- Alloys (Metal Mixtures): Stainless steel (Iron + nickel + chromium + carbon), Bronze (Copper + tin), Brass (Copper + zinc). Alloys are stronger and resist rust.
- Food: Milk, juices, salads, and granola are mixtures.
- Agriculture: Soil is a mixture needed for plant growth.
- Everyday Products: Paints, soaps, and cosmetics are mixtures.
- Construction: Materials like wood, steel, and concrete are mixtures.
Conclusion : Understanding elements, compounds, and mixtures helps us understand the world around us and is important for progress in science, engineering, medicine, and technology.
What Are Minerals? :
- Minerals are naturally occurring, solid, inorganic substances.
- They have a fixed chemical composition and a crystalline structure.
- Most rocks are mixtures of different minerals.
Types of Minerals :
- Most minerals are compounds, such as quartz, calcite, and mica.
- Some minerals are pure elements, called native minerals. Examples: gold, silver, copper, carbon, and sulfur.
Uses of Minerals :
Many materials come from minerals.
- Cement is made from minerals like calcite and quartz.
- Talcum powder is made from the mineral talc.
- Gold is a mineral found in nature and also a metal (an element).
Glossary of Key Terms :
| Term | Definition |
| Adulteration | An illegal process of adding substances which are cheaper, or of a poor quality, to a product, usually to increase quantity or reduce manufacturing cost. |
| Air Quality Index (AQI) | A numerical scale used to measure and describe the quality of air, indicating whether it is pure or polluted. |
| Alloy | A uniform mixture of two or more metals, or a metal with a small amount of non-metal, created to have distinct properties. Examples include stainless steel, brass, and bronze. |
| Atom | The smallest unit of an element that retains its chemical properties. It is the building block of all matter. |
| Chemical Formula | A representation of the elements in a compound and their ratio (e.g., H₂O for water). |
| Components | The individual substances that are combined to form a mixture. Each component retains its individual physical and chemical properties within the mixture. |
| Compound | A pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass. Compounds have properties entirely different from their constituent elements and can only be broken down by chemical means. |
| Element | The simplest form of matter; a pure substance made of only one type of atom that cannot be broken down into simpler substances by physical or chemical means. |
| Matter | Anything that has mass and occupies space. It is made of atoms and molecules. |
| Metal | A class of elements that are typically shiny (lustrous), malleable, ductile, and good conductors of heat and electricity. Examples include iron, gold, and copper. |
| Metalloid | Elements that have properties intermediate between those of metals and non-metals, also known as semimetals. Examples include boron and silicon. |
| Mineral | A naturally occurring, solid, inorganic substance with a definite chemical composition and crystalline structure. Most are compounds, but some are pure elements. |
| Mixture | A substance formed by combining two or more substances (elements or compounds) physically, not chemically. The components retain their individual properties, have a variable composition, and can be separated by physical methods. |
| Molecule | The smallest unit of an element or compound that can exist independently. It is formed when two or more atoms combine chemically. |
| Non-metal | A class of elements that are generally non-lustrous, dull, brittle, and poor conductors of heat and electricity. Examples include carbon, oxygen, and sulfur. |
| Non-uniform Mixture | A mixture where the composition varies in different parts and the individual components are often easily visible. Examples include sand and water, or a sprout salad. |
| Pure Substance | A substance made up of only one type of particle (atom or molecule) with a uniform composition and definite properties throughout. It cannot be separated into other substances by physical means. |
| Symbol | A one or two letter abbreviation used to represent an element (e.g., H for Hydrogen, Au for Gold). |
| Uniform Mixture | A mixture where the composition is consistent throughout, and the components are not easily distinguishable. Examples include air, saltwater, and alloys. |
Practice Questions : (Find answers in Exam Master)
- Differentiate between a uniform mixture and a non-uniform mixture, providing an example for each.
- Explain why air is classified as a uniform mixture and list its main components.
- How does the scientific definition of a "pure substance" differ from its common, everyday meaning?
- Define an element and explain how its atoms can form a molecule.
- Why is water (H₂O) considered a compound, and what does the process of passing electricity through it demonstrate?
- Describe what an alloy is and provide two examples mentioned in the texts.
- In the experiment comparing a mixture of iron and sulfur (Sample A) to the heated substance (Sample B), why did a magnet attract iron from Sample A but not from Sample B?
- What are the defining characteristics of a compound that distinguish it from the elements it is made of?
- What are metalloids, and which two examples are provided in the source material?
- What are minerals, and can they be either elements or compounds?
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