Notes Chapter 8 Science Curiosity Class 8 CBSE Board

Notes For All Chapters – Science Curiosity Class 8

Nature of Matter: Elements, Compounds, and Mixtures

Introduction

• Everything around us (air, water, food, clothes, trees, metals, etc.) is matter.
• Matter is made of tiny particles.
• Most things are made of more than one substance, often mixed together.

8.1 Mixtures

• When two or more substances are mixed but each retains its own properties → called a mixture.
• The substances that make a mixture are called components.
• Components of a mixture do not react chemically.

Types of Mixtures

1. Non-uniform mixtures (Heterogeneous)

• Components can be seen separately.
• Example: sprout salad, poha.

2. Uniform mixtures (Homogeneous)

• Components cannot be seen separately.
• Example: sugar in water, lemonade.

Alloys

• Uniform mixtures of metals with other metals or non-metals.
• Examples: stainless steel (iron, nickel, chromium, carbon), brass (copper + zinc), bronze (copper + tin).
• In ancient India, alloys were called Mishraloha, used for health benefits (e.g., bronze for digestion).

8.1.1 Air as a Mixture

• Air is a uniform mixture of gases: nitrogen, oxygen, argon, carbon dioxide, and water vapour.
• Oxygen supports life and combustion.
• Nitrogen makes up 78% of air but does not support combustion.
• Presence of carbon dioxide in air → shown by lime water turning milky.
• Air also contains dust particles (pollutants), visible in sunlight beams.
• Major pollutants: particulate matter (dust, soot), carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide.
• Air Quality Index (AQI) describes air quality.

8.1.2 Types of Mixtures (Based on Components’ States)

• Gas + Gas → Air (uniform).
• Gas + Liquid → Aerated water, oxygen in water.
• Solid + Gas → Smoke (carbon particles in air).
• Liquid + Liquid → Vinegar (uniform), oil and water (non-uniform).
• Solid + Liquid → Sand in water (non-uniform), seawater (uniform).
• Solid + Solid → Baking powder, alloys.

8.2 Pure Substances

• In daily life, “pure” means unadulterated (not mixed with harmful substances).
• In science, a pure substance contains only one kind of particle.
• Cannot be separated by physical methods.
• All particles of a pure substance behave the same.

8.3 Types of Pure Substances

8.3.1 Elements

• Simplest substances, cannot be broken further.
• Made up of identical particles called atoms.
• Examples: gold, silver, sulfur, carbon.

Atoms of some elements combine to form molecules.

• Hydrogen → H₂
• Oxygen → O₂

Classified as:

• Metals (iron, copper, aluminium).
• Non-metals (carbon, sulfur, oxygen).
• Metalloids (boron, silicon) → properties between metals and non-metals.

At present, 118 elements are known.

• Most are solids.
• 11 exist as gases (like oxygen, nitrogen, helium).
• 2 exist as liquids at room temperature: mercury (metal), bromine (non-metal).

8.3.2 Compounds

• Formed when two or more elements combine chemically in fixed ratios.
• Properties of compounds are different from their constituent elements.
• Cannot be separated by physical methods.

Examples:

• Water (H₂O) → made of hydrogen and oxygen (ratio 2:1).
• Common salt (NaCl) → sodium + chlorine (ratio 1:1).
• Sugar → made of carbon, hydrogen, oxygen.
• Iron sulfide (FeS) → formed by heating iron + sulfur; different from mixture of iron and sulfur.

8.4 Uses of Elements, Compounds, and Mixtures

• Air (mixture) → breathing, burning.
• Water (compound) → essential for life.
• Iron, aluminium (elements) → bridges, vehicles, buildings.
• Alloys (mixtures) like stainless steel → strong and durable.
• Fertilisers and medicines (compounds) → support farming and healthcare.
• Scientists develop new materials like graphene aerogel (very light, highly absorbent, used in cleaning oil spills, energy devices, coatings).

8.5 Minerals

• Most rocks are mixtures of minerals.

Minerals may be:

• Pure elements (gold, silver, copper, sulfur, carbon).
• Compounds (quartz, calcite, mica, pyroxene, olivine, talc).

Many useful products are made from minerals:

• Cement from calcite, quartz, alumina, iron oxide.
• Talcum powder from talc.

Our Scientific Heritage – Dhokra Art

• Ancient Indian craft using alloys (brass, bronze).
• Made with wax models and molten metal.
• Reflects creativity and use of mixtures in art.

Snapshots (Summary)

• Mixtures → two or more substances mixed, retain individual properties, no chemical reaction.
• Components of mixtures are called constituents.
• Pure substances → contain only one kind of particle.
• Pure substances are of two types:
  • Elements (cannot be broken further).
  • Compounds (formed by elements in fixed ratio, different properties).
• Minerals are naturally occurring substances, mostly compounds but sometimes pure elements.