In stirred salt water, how does salt concentration compare at different places?

It is the same everywhere. Dissolved salt spreads evenly through the water.

For 10 g salt in 90 g water, what mass belongs in the percentage denominator?

100 g. Include salt and water, so 10 ÷ 100 × 100 gives 10%.

What can separate from saturated potassium nitrate solution on cooling while water remains?

Potassium nitrate crystals. Cooler water dissolves less of this salt, so excess solute becomes solid.

What does alum make fine mud particles do before they settle?

Join into larger clumps. Alum helps particles clump together rather than acting as a filter.

In the torch test, what observation shows milk is not a true solution?

The beam’s path becomes visible. Its particles are larger than true solution particles and scatter light.

Which layer drains first from a separating funnel holding settled mustard oil and water?

The lower water layer. Water lies below the less dense oil, so it reaches the outlet first.

Where does the water collected in the receiver come from?

Cooled vapour from the flask. Water leaves the salt solution as vapour, then condenses in the cooled condenser.

Where must the ink spot sit so rising water can carry its colours up paper?

Above the water surface. A submerged spot washes into water instead of travelling up the paper.

Teach Exploring Mixtures and their Separation in eight steps.

For each idea: show it, ask before you tell, then say one line.

Gather salt, sand, water, milk, two glasses, a spoon, torch, cup, absorbent kitchen paper, water-soluble black pen, paper and pencil.

Never taste mixtures.

Common misconceptions & worked visual for this chapter · 15 practice questions with answers

Why it matters

Clear salt water still contains dissolved salt.
Milk scatters light because its particles are larger than those in a true solution.
Mustard oil rests above water because it is less dense.

What to teach

How to teach it

Mixtures and solutions

Salt and sand beside separate glasses of water before mixing.

1. Mixtures, solutions, solute and solvent

Solutions stay even throughout; heterogeneous mixtures have uneven composition.

Show: Set a pinch of salt and some sand beside separate glasses of water. Stir them in after predicting.
Ask: Which mixture will stay evenly mixed after stirring?
Say: Salt, the solute, dissolves in water, the solvent, making a homogeneous solution. Sand-water is heterogeneous.
Used in: Salt dissolved in pickling brine helps preserve vegetables.

If your child says…“If the water looks clear, it must be pure.”

Ask: What did we add that we can no longer see?
What’s true: Clear salt water contains both water and dissolved salt.

$\text{Solution mass}=25+100=125\,\text{g}$

$\text{Mass percentage}=\frac{25}{125}\times100\%$

2. Concentration and percentage by mass and volume

Solute amount compared with solution amount.

Show: Write these lines for the sugar solution.
Ask: What percentage of this solution is sugar?
Say: It is 20% sugar by mass.
Used in: Vinegar labels state acid concentration so people can compare strengths.

If your child says…“I divide by the water alone.”

Ask: Does sugar add to solution mass?
What’s true: Include both sugar and water.

Rising solubility curve beside warm solution and cooled solution containing crystals.

3. Solubility, saturation and crystallization

Temperature changes how much solute can dissolve.

Show: Read the diagram’s curve, then follow the cooling arrow.
Ask: Can crystals form here without losing water?
Say: Cooling lowers this salt’s dissolving limit. Extra solute forms crystals. The remaining liquid stays saturated.
Used in: Crystallization can purify solids by separating crystals from their solutions.

If your child says…“Crystals appear only after all the water dries up.”

Ask: Does the cooler beaker still contain water?
What’s true: Crystals can form while water remains.

Particles in mixtures

Three routes show particles settling, separating by spinning, and clumping after alum addition.

4. Suspensions, centrifugation and coagulation

Undissolved particles spread through liquid and can settle.

Show: Look at the diagram's three rows. Follow standing at the top, spinning in the middle, and alum addition at the bottom.
Ask: How could particles leave this cloudy water faster than by standing alone?
Say: Undissolved particles in a suspension can settle. Spinning separates denser particles faster. Alum helps fine particles join into larger clumps that settle.
Used in: Settling lets cooks pour clearer stock away from bits collected at the bottom.

If your child says…“Alum filters the mud out.”

Ask: Where is a filter in the alum row?
What’s true: Alum helps particles clump, then settling separates them from the water above.

Diluted milk and salt water beside a switched-off torch.

5. Colloids and the Tyndall effect

Particles remain dispersed and scatter light without settling on standing.

Show: Stand diluted milk beside clear salt water in glasses, with an unlit torch aimed sideways. After predicting, switch it on and shine through each glass. View from the side.
Ask: Which liquid will reveal the torch beam’s path?
Say: Milk’s particles stay spread through its liquid. These particles scatter light, revealing the beam’s path: the Tyndall effect.
Used in: Colloidal particles in paint help it spread into an even coating.

If your child says…“Milk is a true solution because it looks even.”

Ask: If both look even, why does only one reveal the beam?
What’s true: Milk has larger particles than a true solution, so it scatters light.

Separating substances

Separating funnel drains water first, then closes before oil enters the receiver.

6. Separating immiscible liquids

Separate liquid layers can be drained one after another.

Show: Look at the diagram's mustard oil above water. Follow the lower layer through the tap into the receiver. Find where the interface reaches the closed tap.
Ask: When should the tap close to keep oil out of the collected water?
Say: The denser water drains first. Close the tap as the boundary between the liquids reaches it.
Used in: Skimming the upper fat layer from soup removes excess fat.

If your child says…“Water always floats on top.”

Ask: Which liquid lies below the other in this funnel?
What’s true: Mustard oil is less dense than water, so water forms the bottom layer.

Distillation collects water; sublimation separates iodine from sand onto a cold surface.

7. Distillation and sublimation

Distillation collects liquid from vapour. Sublimation changes solids directly into vapour.

Show: Follow both vapour paths to the diagram’s cooled collection points.
Ask: What will each cooled vapour become?
Say: Water condenses into liquid, leaving salt behind. Iodine becomes vapour directly, then deposits solid crystals.
Used in: Kalpakkam’s project aimed to supply drinking water and power-plant process water.

If your child says…“Distillation collects only what stays in the pot.”

Ask: What carries water into the receiver?
What’s true: Collected water comes from cooled vapour.

One hand holds an ink-spotted paper strip above a cup of water.

8. Paper chromatography

Dissolved components travel at different speeds along paper.

Show: Before the session, test that the water-soluble black pen separates into different colours. Hold an ink-spotted strip of absorbent kitchen paper above a cup of water. After predicting, dip the bottom edge, keeping the spot above water.
Ask: What would happen if the ink spot started underwater?
Say: The ink’s colours would dissolve in the water.
Used in: Paper chromatography separates food dyes, helping identify which colours a food contains.

If your child says…“The ink spot should go underwater.”

Ask: Where would the ink go if we dipped the spot itself?
What’s true: A submerged spot washes into the water instead of travelling up the paper.

A real problem

Freshwater from seawater at Kalpakkam

The project began in 1998.

It aimed to provide drinking water nearby and process water for the power plant.

Its multi-stage flash plant produces distilled water from seawater.

Blending its distilled water with reverse-osmosis water can improve drinking-water quality.

IAEA BARC IAEA

A glass of clear water on pale stone beside the sea.

Concept Lab · Common misconception

The tempting mistake in Exploring Mixtures and their Separation

Open standalone misconception page →

Does clear water have to be pure?

Wrong

A clear liquid must be pure water.

Why it feels right. I use visible particles to recognise mixtures.

Right

Dissolved substances can be invisible.

Worked example

Pure water

Clear sample→Evaporate→No solid residue

Salt solution

Clear sample→Evaporate→Salt remains

The two clear samples look alike at first. Evaporating them reveals the dissolved solid in one.

Compare pure water and a clear salt solution.
Evaporate a sample of each under supervision.
The salt solution leaves a solid residue.

Ask your child eight quick questions.

One per step. The answer and the reason are given.

In stirred salt water, how does salt concentration compare at different places?

It is the same everywhere. Dissolved salt spreads evenly through the water.
For 10 g salt in 90 g water, what mass belongs in the percentage denominator?

100 g. Include salt and water, so 10 ÷ 100 × 100 gives 10%.
What can separate from saturated potassium nitrate solution on cooling while water remains?

Potassium nitrate crystals. Cooler water dissolves less of this salt, so excess solute becomes solid.
What does alum make fine mud particles do before they settle?

Join into larger clumps. Alum helps particles clump together rather than acting as a filter.
In the torch test, what observation shows milk is not a true solution?

The beam’s path becomes visible. Its particles are larger than true solution particles and scatter light.
Which layer drains first from a separating funnel holding settled mustard oil and water?

The lower water layer. Water lies below the less dense oil, so it reaches the outlet first.
Where does the water collected in the receiver come from?

Cooled vapour from the flask. Water leaves the salt solution as vapour, then condenses in the cooled condenser.
Where must the ink spot sit so rising water can carry its colours up paper?

Above the water surface. A submerged spot washes into water instead of travelling up the paper.

15 more questions with answers

Teach Exploring Mixtures and their Separation in eight steps.

Why it matters