Notes Chapter 8 Science Curiosity Class 8 CBSE Board

Nature of Matter: Elements, Compounds, and Mixtures

What is Matter?

• Everything around us like air, water, food, books, clothes, etc., is matter.
• Matter is made up of tiny particles.
• Many things around us are not pure substances but mixtures.

8.1 What Are Mixtures?

• A mixture is made by mixing two or more substances where each substance retains its own properties.
• Substances in a mixture are called components.
• Components do not react chemically with each other.

Types of Mixtures:

1. Non-uniform Mixtures (Heterogeneous):

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

2. Uniform Mixtures (Homogeneous):

• Components are mixed so well that we can’t see them separately.
• Example: sugar in water, air, vinegar.

Alloys – Special Mixtures

• Mixture of metals.
• Uniform throughout.

Examples:

• Brass = Copper + Zinc
• Bronze = Copper + Tin
• Stainless Steel = Iron + Chromium + Nickel + Carbon

8.1.1 Is Air a Mixture?

• Yes, air is a uniform mixture of nitrogen, oxygen, carbon dioxide, water vapour, etc.
• Activity: Lime water turns milky when carbon dioxide is passed through it → proves CO₂ is present in air.

8.1.2 Types of Mixtures (based on states of matter):

8.2 What Are Pure Substances?

• A pure substance has only one kind of particle.
• It cannot be separated by physical methods.
• In science, pure means no mixing with other substances.
• Example: distilled water, oxygen gas, iron.

8.3 Types of Pure Substances

1. Elements

• Simplest form of matter.
• Cannot be broken into simpler substances.
• Made of atoms (same kind).
• Examples: Hydrogen, Oxygen, Iron, Gold.

Types of Elements:

1. Metals: Iron, Gold, Aluminium

2. Non-metals: Oxygen, Sulfur, Carbon

3. Metalloids (semi-metals): Boron, Silicon (studied in higher classes)

2. Compounds

• Formed when two or more elements combine chemically in a fixed ratio.
• Properties of a compound are different from the elements.
• Cannot be separated by physical methods.

Examples:

• Water = Hydrogen + Oxygen (2:1)
• Salt (NaCl) = Sodium + Chlorine
• Sugar = Carbon + Hydrogen + Oxygen

Activities to Understand Compounds:

1. Electrolysis of Water:

• Water breaks into Hydrogen (burns with pop) and Oxygen (glows flame).

2. Heating Sugar:

• Turns brown → Black → Leaves carbon.
• Proves sugar is a compound (carbon, hydrogen, oxygen).

3. Iron + Sulfur:

Before heating: Mixture (can be separated).

After heating: Compound called iron sulfide (cannot be separated, non-magnetic, smells bad when acid is added).

8.4 Uses of Elements, Compounds, and Mixtures

• Air – mixture, essential for life.
• Water – compound, needed by all living beings.
• Metals – used in buildings, vehicles, etc.
• Alloys – stronger than pure metals, used in tools and structures.
• Compounds – used in medicines, fertilizers, cleaning materials.

8.5 What Are Minerals?

Naturally found solid substances on Earth.

Can be:

• Elements: Gold, Silver
• Compounds: Quartz (SiO₂), Mica

Used in making:

• Cement, talcum powder, ceramics, etc.

Dhokra Art (Our Scientific Heritage)

Traditional Indian art using metal mixtures (like bronze).

Made using lost-wax method.

Shows use of mixtures in art.

Key Points to Remember

1. Mixture: Two or more substances mixed without chemical change.

2. Pure substance: Only one type of particle; either element or compound.

3. Element: Simplest pure substance made of same type of atoms.

4. Compound: Made of two or more elements chemically combined.

5. Alloy: Uniform mixture of metals (e.g., brass, bronze).

6. Mineral: Natural solid; mostly compounds but can be elements too.