CBSE Class 9 Science Notes: Is Matter Around Us Pure

🔍 Pure Substances

Definition: Substances with a fixed composition and definite properties. They are made of only one type of particle (atoms or molecules).

• Have a fixed melting and boiling point.
• Can be either elements or compounds.

Examples: Gold (Au), Water ($H_2O$), Sodium Chloride (NaCl).

🌷 Mixtures

Definition: Substances formed by physically combining two or more substances that are not chemically bonded. Their composition can vary.

• Do not have fixed melting/boiling points.
• Can be separated by physical methods.

Examples: Air, Salt water, Sand and iron filings.

🛠 Elements

Definition: The simplest form of matter that cannot be broken down into simpler substances by chemical means. They are composed of only one type of atom.

Core Principle: Atoms of the same element have the same number of protons (atomic number).

Examples: Hydrogen (H), Oxygen (O), Iron (Fe), Copper (Cu).

🍊 Compounds

Definition: Substances formed when two or more elements chemically combine in a fixed ratio.

Core Principle: The properties of a compound are different from the properties of the elements that form it. Compounds can only be separated by chemical methods.

Examples: Water ($H_2O$), Carbon Dioxide ($CO_2$), Sodium Chloride (NaCl).

🍦 Mixtures (Revisited)

Definition: As discussed above.

Distinction from Compounds: Components of a mixture retain their individual properties, while in a compound, elements lose their original properties.

🌾 Homogeneous Mixtures

Definition: Mixtures where the composition is uniform throughout. You can’t visually distinguish the different components.

Examples: Salt solution, sugar solution, air.

🍕 Heterogeneous Mixtures

Definition: Mixtures where the composition is not uniform throughout. You can visually distinguish the different components.

Examples: Sand and water, oil and water, trail mix.

🌝 Colloids

Definition: A type of mixture where particles are dispersed throughout another substance but are larger than those in a solution. Exhibits the Tyndall effect (scattering of light).

Examples: Milk, fog, smoke, paint.

🌞 Suspensions

Definition: A heterogeneous mixture where particles are undissolved and can be seen with the naked eye. Particles eventually settle down.

Examples: Muddy water, milk of magnesia, paint.

👉 Physical Changes

Definition: Changes that alter the form or appearance of a substance but do not change its chemical composition. These are generally reversible.

• No new substance is formed.
• Changes are usually reversible.
• Changes involve a change in state, size, or shape.

Examples: Melting of ice, boiling of water, tearing of paper.

💡 Chemical Changes

Definition: Changes that result in the formation of a new substance with different properties. These are generally irreversible.

• A new substance is formed.
• Changes are usually irreversible.
• Often accompanied by energy changes (heat, light, etc.)

Examples: Burning of wood, rusting of iron, cooking of food.