NCERT Solutions for Class 9th Science Chapter 5 .1 How Can We Classify Mixtures? — Activity 5.1: Let us experiment — Group activity

Book page 735 Updated on2026-09-19

Q1.

Are the particles visible in each mixture? Record your observations.

Answer

Only in beaker B. The three mixtures are salt in water (A), chalk powder in water (B) and a few drops of milk in water (C).

Beaker	Mixture	Are particles visible?	Appearance
A	Common salt + 50 mL water	No	Clear and transparent — you can read print through it
B	Chalk powder + 50 mL water	Yes	Cloudy; white specks clearly seen swirling in the water
C	A few drops of milk + 50 mL water	No	Uniformly milky-white, but no separate specks are seen

Why it happens: Your eye can only resolve particles bigger than about 1000 nm. Salt has broken up into separate ions smaller than 1 nm, so beaker A looks like pure water. Chalk particles are far bigger than 1000 nm and scatter enough light to be seen individually. Milk particles lie in between (1 – 1000 nm), so no single particle can be picked out, yet together they make the liquid look white.

Q2.

Direct the light from a laser pointer through the beakers containing the mixtures (Fig. 5.3) and observe it from the side of the beaker in a direction perpendicular to the laser beam. Record your observations.

Fig. 5.3, page 73 — redrawn sketch of the set-up: a laser beam sent through each of the three beakers A, B and C and viewed from the side.

Answer

The path of the beam is invisible in A, brightly visible in B and visible in C.

A laser beam through the three mixtures of Activity 5.1 — invisible in the solution, bright in the suspension, visible in the colloid.

Beaker	Path of the laser beam seen from the side	What it tells you
A — salt and water	Not visible	A true solution; particles are far too small to scatter light
B — chalk and water	Bright, sharply visible	A suspension; large particles scatter light strongly
C — milk and water	Visible	A colloid; the mixture looks uniform yet still scatters light

Why it happens: You see a beam only if some light is turned sideways towards your eye. Particles much smaller than the wavelength of light (about 400 – 700 nm) barely disturb the wave, so a solution lets the beam pass unseen. Particles comparable to or larger than the wavelength scatter it in every direction, so the beam path lights up. This is the Tyndall effect, and it is the neatest way to tell a colloid from a solution when both look clear.

Safety first: Never look straight into the laser beam. Observe only from the side, at right angles to the beam.

Q3.

Predict what you would observe in each of the beakers if you leave them undisturbed for a few minutes.

Answer

Beaker A (salt and water): no change at all. The solution stays clear and uniformly salty from top to bottom.
Beaker B (chalk and water): the chalk settles as a white layer at the bottom, leaving clearer water above. This settling is called sedimentation.
Beaker C (milk and water): no change. It stays evenly milky; nothing settles even after hours.

Why it happens: Settling needs the weight of a particle to beat the random molecular jostling that keeps it suspended. Chalk particles (> 1000 nm) are heavy enough, so they lose and fall. Salt exists as separate ions and milk particles are only 1 – 1000 nm across, so in both A and C the jostling wins and nothing settles.

Q4.

Set up a filtration apparatus and filter each mixture separately. Is there any residue left on the filter paper?

Answer

Only beaker B leaves a residue.

Beaker	Residue on the filter paper	Filtrate
A — salt and water	None	Clear salt solution (still salty — the salt passes through)
B — chalk and water	White chalk residue	Clear water
C — milk and water	None	Milky liquid — unchanged

Why it happens: Filter paper is a sieve with pores of roughly 1000 nm. Anything bigger is held back; anything smaller goes through with the liquid. Chalk particles are larger than the pores, so they stay behind. Dissolved salt ions and colloidal milk particles slip straight through, which is why filtration cannot separate a solution or a colloid.

Q5.

Based on your observations, do you think these are the same types of mixtures or are they different?

Answer

They are three different types of mixture. Put the four tests side by side and each beaker gives its own pattern of answers.

Test	A — salt + water	B — chalk + water	C — milk + water
Particles visible?	No	Yes	No
Tyndall effect?	No	Yes	Yes
Settles on standing?	No	Yes	No
Residue on filtering?	No	Yes	No
Type of mixture	Solution (homogeneous)	Suspension (heterogeneous)	Colloid (heterogeneous)

Why it happens: One property — the size of the dispersed particles — decides every one of these results. Below 1 nm you get a solution, between 1 nm and 1000 nm a colloid, above 1000 nm a suspension. The colloid is the interesting middle case: it looks homogeneous like A but scatters light like B.

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