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NCERT Solutions for Class 9th Science Chapter 5 .4.1 Separation of two immiscible liquids — Activity 5.6: Let us separate

Book page 835 Updated on2026-09-08

Q1.
Let it stand undisturbed. What do you observe?
Answer

The mixture, cloudy at first from being poured, soon clears into two sharply separated layers — a yellow layer of mustard oil (5 mL) floating on a colourless layer of water (20 mL), with a flat, clearly visible boundary between them.

Why it happens: Mustard oil and water are immiscible: their molecules attract their own kind far more strongly than they attract the other kind, so they do not spread through each other. Standing still lets gravity sort the two liquids purely by density — the denser liquid sinks and the lighter one floats — and once sorted, there is nothing to mix them again.
Q2.
The yellow-coloured mustard oil forms the upper layer and water forms the lower layer. Can you explain why?
Answer

Because mustard oil is less dense than water, so an equal volume of it weighs less and it floats.

Density of water = 1.00 g cm⁻³
Density of mustard oil ≈ 0.91 g cm⁻³
Since 0.91 g cm⁻³ < 1.00 g cm⁻³, the oil rises to the top
Why it happens: Take 1 cm³ of each liquid. The water cube has a mass of 1.00 g and the oil cube only about 0.91 g. Gravity pulls harder on the heavier water, so the water sinks to the bottom and pushes the lighter oil up. Put another way, the buoyant force on the oil from the surrounding water is greater than the oil’s own weight, so the oil rises until it sits on top. This density difference is precisely what the separating funnel exploits: open the stopcock and the lower liquid runs out first.
Tip: Discard the small middle portion where the two liquids meet. Collecting it would contaminate both, and losing a millilitre matters far less than losing purity.
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