Is Matter Around Us Pure? — Class 9 (CBSE)

A scientist's "pure" is much stricter than a grocer's "pure." Pure milk on a packet means "no added water." Pure to a chemist means "made of only one kind of particle." This chapter teaches you to think like a chemist about every glass of water, every breath of air, and every spoonful of food.

1. The story — purity matters

In 1947 the Indian railway network was already vast, but mineral water was unknown. Travellers carried earthen pots ("surai") of water. By the 1980s, packaged "pure" water became a Rs. 50,000-crore industry in India. The word "pure" sells. But what does it really mean?

To a chemist, pure matter consists of only ONE kind of particle. Distilled water is pure (only H₂O molecules). The water in your bottle is NOT pure — it contains dissolved minerals, oxygen, sometimes chlorine. It's a mixture.

This chapter does three things:

• Classifies all matter into pure substances vs mixtures (and further).
• Teaches you to recognise solutions, suspensions and colloids by particle size and the Tyndall effect.
• Gives you the standard methods to physically SEPARATE the components of a mixture.

2. The classification tree — memorise this picture

                    Matter
                      |
        +-------------+-------------+
        |                           |
   Pure substance              Mixture
        |                           |
   +----+----+              +-------+-------+
   |         |              |               |
 Element  Compound      Homogeneous   Heterogeneous
   |         |              |               |
   H, O    H₂O,         salt soln,    sand+water,
   Au, Cu   CO₂,         air,         oil+water,
            NaCl         alloys       chalk+water

Two-sentence summary:

• Pure substances (elements & compounds) have a fixed composition and fixed properties.
• Mixtures (homogeneous & heterogeneous) have variable composition and the properties of their constituents.

3. Pure substances — element vs compound

Element

• Made of ONLY ONE type of atom. Cannot be broken down by ordinary chemical means.
• Currently 118 known elements (94 natural, 24 synthetic).
• Categories: Metals (iron, copper, gold), Non-metals (oxygen, carbon, sulphur), Metalloids (silicon, germanium — properties between metal and non-metal).

Compound

• Two or more elements in a fixed ratio, chemically combined.
• Properties are completely different from those of the constituent elements.
• Example: water (H₂O) is liquid; hydrogen and oxygen alone are gases. Salt (NaCl) is edible; sodium is a violently reactive metal and chlorine is a poison gas. Chemical combination changes everything.
• Constituents can be separated only by chemical methods (electrolysis, decomposition), not physical ones.

Element vs Compound — the 4-test memorisation table

4. Mixtures — homogeneous vs heterogeneous

Homogeneous mixture (= solution)

• Uniform composition throughout. Cannot see boundaries between components.
• One phase visible everywhere.
• Examples: salt water, sugar water, air, brass (Cu + Zn), bronze, stainless steel.

Heterogeneous mixture

• Non-uniform; you can see (sometimes only with a microscope) different regions.
• Two or more visible phases.
• Examples: sand + water, oil + water, chalk + water, granite, soil.

The line between homogeneous and heterogeneous isn't always crisp — it depends on the scale you look at. Colloids look homogeneous to the eye but are heterogeneous under a microscope (they're an in-between case, more in §6).

5. Solutions — homogeneous mixtures explored

A solution is a homogeneous mixture of two or more substances. The component present in larger amount is the solvent; the smaller one is the solute.

Properties of a true solution

1. Homogeneous — uniform.
2. Particle size very small ($<10^{-9}\text{m}=1\text{nm}$).
3. Particles do NOT settle when left undisturbed.
4. Particles pass through filter paper — too small to be filtered.
5. Light passes straight through — no scattering (no Tyndall effect).
6. Stable — components don't separate over time.

Saturated and unsaturated solutions

• Unsaturated: more solute can still dissolve at that temperature.
• Saturated: cannot dissolve any more solute at that temperature.
• Supersaturated: more solute dissolved than usual (achieved by heating then cooling carefully); unstable.

Concentration formulas — memorise both

Mass percentage (most common in CBSE): $\text{Mass \%}=\frac{\text{mass of solute}}{\text{mass of solution}}\times 100$

Volume percentage (for liquid solutes): $\text{Volume \%}=\frac{\text{volume of solute}}{\text{volume of solution}}\times 100$

Note: mass of solution = mass of solute + mass of solvent. Don't confuse with "mass of solvent" — common 1-mark trap.

6. Suspensions and colloids — the in-between cases

Suspension

• A heterogeneous mixture where solute particles are big enough to be visible (or large enough to settle out).
• Particle size $>10^{-6}\text{m}=1\mu \text{m}$.
• Particles settle down when left.
• Particles are retained by filter paper.
• Example: chalk + water, muddy water, paints (before stirring).

Colloid

• A heterogeneous mixture that LOOKS homogeneous to the naked eye.
• Particle size between $10^{-9}\text{m}$ and $10^{-6}\text{m}$.
• Particles do NOT settle.
• Particles pass through filter paper but are stopped by special ultrafilters.
• Show the Tyndall effect: scatter light, making a visible beam.

Tyndall effect — the colloid signature

When a beam of light passes through:

• A solution → light passes through invisibly.
• A colloid → light is scattered by colloidal particles → the beam is visible.

Examples you've seen:

• Sunbeam through a dusty room.
• Sunlight through forest mist or fog.
• Headlights cutting through mist on a foggy night.

Types of colloids (memorise this table — 2-mark question)

The three particle-size regimes — memorise

7. Separation techniques — which method for which mixture?

CBSE tests this with great consistency. Match each method to the mixture type.

Filtration

For: insoluble solid + liquid (suspension). Examples: sand from water, tea leaves from tea.

Evaporation

For: soluble solid from liquid (solution). Examples: salt from sea water.

Centrifugation

For: small particles that won't filter (colloids, blood). Spin fast → heavier particles move outward → light supernatant on top. Example: separating cream from milk, separating plasma from blood cells.

Sublimation

For: separating a subliming solid from a non-subliming solid. Example: ammonium chloride mixed with sand.

Distillation

For: separating two miscible liquids with large boiling-point difference (> 25 °C). The component with the lower boiling point evaporates first, condenses in the condenser, collected as distillate. Example: water and acetone, water and alcohol.

Fractional distillation

For: two miscible liquids with small boiling-point difference (< 25 °C). Uses a fractionating column packed with glass beads. Example: petrol and diesel from crude oil, separating gases of liquefied air (O₂, N₂, Ar all have similar BPs).

Chromatography

For: separating dyes, pigments — typically dissolved solid solutes. Different components travel different distances on the chromatography paper. Example: separating the dyes in black ink, plant pigments.

Separating funnel

For: two immiscible liquids (won't mix — form layers). Example: oil and water, kerosene and water.

Crystallisation

For: getting pure solid crystals from a solution (better than evaporation for impurities-laden solutions). Example: pure copper sulphate crystals from impure sample.

8. Closing thought

Almost nothing in your life is pure — and that's a good thing. Pure water has no minerals; you'd get deficiencies. Pure oxygen is dangerous; humans live in 21% oxygen with 78% nitrogen. Bronze, an alloy of copper and tin, is stronger than either pure metal. Steel, an iron-carbon mixture, built the modern world.

Mixtures are everywhere because they're useful. What chemistry gives you is the language to describe them precisely (solution vs colloid vs suspension), the tools to measure them (Tyndall effect, mass %), and the methods to separate them (eight techniques you just learned). Walk into any chemical industry, soap factory, oil refinery or pharma plant and you'll see these eight separation techniques scaled up to industrial size.