Science · Class 8 · Chapter 8

Nature of Matter: Elements, Compounds, and Mixtures

• Mixture — Substances combined without a chemical reaction.
• Alloy — A uniform mixture of two or more metals.
• Pure substance — Only one type of particle; can't be separated physically.
• Element — Identical atoms; cannot be broken down further.
• Atom / Molecule — An element's particle / atoms combined together.
• Compound — Elements chemically combined in a fixed ratio.
• Mineral — A natural solid with fixed composition.

Chapter 8 · Concept 1 of 10

What Are Mixtures?

• When substances are mixed and each keeps its own properties, the result is a mixture — its parts are called components.
• Non-uniform mixture — Components are visible, even with a magnifying glass — like a sprout salad.
• Uniform mixture — Components can't be told apart even under a microscope — like sugar dissolved in water. Alloys (like stainless steel, brass, bronze) are uniform metal mixtures.

Chapter 8 · Concept 2 of 10

Is Air a Mixture?

• Air is a uniform mixture — mostly nitrogen (~78%) and oxygen, plus argon, carbon dioxide, and water vapour.
• Proving CO2 is there — Lime water turns milky when exposed to air, because carbon dioxide reacts with it to form insoluble calcium carbonate.

Chapter 8 · Concept 3 of 10

Types of Mixtures

• Combination | Example
• Gas – Gas — Air
• Gas – Liquid — Soda water
• Solid – Gas — Dust in air
• Liquid – Liquid — Vinegar, oil + water
• Solid – Liquid — Seawater, sand + water
• Solid – Solid — Alloys, baking powder

Chapter 8 · Concept 4 of 10

What Are Pure Substances?

• Everyday "pure" — Means nothing cheap or harmful has been added.
• Scientific "pure" — Contains no other substance at all — only one type of particle, and cannot be separated further by any physical process.

Chapter 8 · Concept 5 of 10

Elements

• Passing electricity through acidified water splits it into hydrogen and oxygen gas — proof water isn't a single pure substance.
• Element — A pure substance made of identical particles (atoms) that cannot be broken down into anything simpler — like gold, carbon, or oxygen.

Chapter 8 · Concept 6 of 10

Metals, Non-metals, and Metalloids

• Metals — Gold, silver, iron, aluminium.
• Non-metals — Carbon, sulfur, hydrogen, oxygen.
• Metalloids — Silicon, boron — properties in between.

Chapter 8 · Concept 7 of 10

Compounds

• Water's hydrogen and oxygen are so tightly bound that no physical method can separate them — that's what makes water a compound.
• Compound — Forms when elements combine chemically in a fixed ratio — with completely different properties from the elements themselves.

Chapter 8 · Concept 8 of 10

Iron + Sulfur: Mixture vs Compound

• | Iron + sulfur (not heated) | Iron sulfide (heated)
• Appearance — Grey specks + yellow powder, visible — Uniform black mass
• Magnet test — Iron is attracted — Not attracted at all
• With dilute HCl — Hydrogen gas (a "pop") — Hydrogen sulfide (rotten-egg smell)

Chapter 8 · Concept 9 of 10

Classifying Matter

• Elements — Pure substances, one kind of atom. Cannot be broken down further.
• Compounds — Pure substances, two or more elements combined chemically.
• Mixtures — Two or more substances combined, no chemical reaction, separable.

Chapter 8 · Concept 10 of 10

Minerals

• Usually compounds, occasionally elements — Most rocks are mixtures of minerals. Native minerals like gold and silver are pure elements found in nature; most others — quartz, calcite, mica — are compounds.

Quick Recap

Check Your Understanding

1. 1Assertion: Air is a mixture. Reason: A mixture forms when substances are mixed without any chemical change. Are both true, and does the reason explain the assertion?
2. 2Water (a compound) has very different properties from the oxygen and hydrogen it's made from. Justify this.
3. 3Which set is correctly matched? (i) Elements — water, nitrogen, iron, air (ii) Uniform mixtures — minerals, seawater, bronze, air (iii) Pure substances — carbon dioxide, iron, oxygen, sugar (iv) Non-uniform mixtures — air, sand, brass, muddy water.