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NCERT Solutions for Class 9th Science Chapter 5 Chapter opener — Think It Over

Book page 72 Updated on2026-09-08

Q1.
Why do suspended particles settle in muddy water over time but not in milk?
Answer

Because the two mixtures hold particles of very different size. Muddy water is a suspension; milk is a colloid.

Mud particles: diameter > 1000 nm
Milk (fat and protein) particles: diameter 1 – 1000 nm
Why it happens: Two effects fight each other inside the liquid. Gravity pulls a particle down, and the ceaseless random bombardment by water molecules knocks it about and keeps it up. The downward pull grows with the volume of the particle (as the cube of its radius), while the jostling depends on its surface. So a big mud particle is dragged down far faster than the molecules can keep it afloat — it sediments in minutes. A milk particle is a thousand times smaller; the molecular bombardment is enough to keep it permanently dispersed. The protein molecules in milk also coat each fat globule and stop the globules from joining into bigger, heavier ones.
Check it yourself: Leave a glass of muddy water and a glass of milk side by side overnight. The mud makes a clear layer at the bottom; the milk stays uniform. That single observation is enough to call one a suspension and the other a colloid.
Q2.
How is evaporation different from boiling?
Answer

Evaporation is a surface change that goes on at every temperature; boiling is a bulk change that happens only at one fixed temperature.

Point of comparisonEvaporationBoiling
Where it happensOnly at the free surface of the liquidThroughout the liquid — bubbles form inside it
TemperatureAt all temperatures below the boiling pointOnly at the boiling point (100 °C for water at normal pressure)
SpeedSlow and quietFast and vigorous
Energy sourceDraws heat from the liquid itself and the surroundings, so the liquid coolsHeat must be supplied continuously from outside
Everyday exampleWet clothes drying, sweat cooling youWater bubbling in a kettle
Why it happens: In a liquid, molecules move with a spread of speeds. At any temperature a few surface molecules happen to be fast enough to break free — that is evaporation, and because only the fastest leave, the liquid left behind cools down. Boiling begins when the temperature is high enough that the vapour pressure inside the liquid equals the atmospheric pressure pushing down on it. Only then can a bubble of vapour form inside the liquid without being crushed, and the whole bulk turns to vapour.
Q3.
Why do you see bright rays of sunlight when it passes through small gaps between the leaves of a dense tree?
Answer

Because the air under the tree carries dust, smoke and tiny water droplets, and these particles scatter the sunlight sideways into your eye. This is the Tyndall effect.

Why it happens: Light travels in a straight line, so a beam moving away from you cannot be seen at all — you only see light that actually enters your eye. When the beam runs through clean air nothing turns it aside. But when it meets suspended particles of about the size of the wavelength of light, each particle scatters a little of the beam in all directions, and some of that scattered light reaches you. You then see the path of the beam itself, standing out as a bright ray against the dark shade of the tree.
Did you know? The same effect makes the beams from stadium floodlights visible (Fig. 5.24), shows up the path of a cinema projector in a dusty hall, and is exactly what you see when you shine a laser through milk in Activity 5.1.
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