EN हिं

NCERT Solutions for Class 9th Science Chapter 5 .5 Tyndall Effect — Pause and Ponder

Book page 885 Updated on2026-09-08

Q9.
Clouds are made up of tiny water droplets or ice crystals floating in the air. Based on what you know about solutions, suspensions and colloids, what type of mixture do you think clouds are and why?
Answer

A cloud is a colloid — an aerosol, with tiny liquid water droplets or ice crystals as the dispersed phase and air as the dispersion medium.

TestWhat a cloud doesConclusion
Are the droplets dissolved in the air?No — they stay as separate droplets of liquid waterNot a solution
Do they settle out?No — a cloud floats for hours or daysNot a suspension
Does it scatter light?Yes — that is why clouds look white and cast shadowsShows the Tyndall effect
Do you see separate droplets?No — the cloud looks like one smooth massParticles are of colloidal size → a colloid
Why the droplets do not fall: Cloud droplets are only a few micrometres across, so their weight is minute while the drag of the air on them is relatively large; rising air currents easily hold them up. They fall as rain only when many droplets coalesce into a drop big enough for its weight to beat the updraught. That is also why a cloud is white — the droplets scatter all the colours of sunlight almost equally, and all colours together look white.
Q10.
Why do cities with a lot of smoke and dust in the air often look hazy?
Answer

Because the smoke and dust particles suspended in the city air scatter sunlight in all directions — the Tyndall effect on the scale of a whole city.

Why it produces haze: Light coming from a distant building travels to your eye in a straight line and carries the image of that building. On the way it meets millions of smoke and dust particles, and each of them turns some of that light aside. Two things follow. First, the light from the building is weakened, so the building looks faint. Second, sunlight that was never headed towards you is scattered into your line of sight, adding a uniform grey-white glow over the whole scene. The image loses contrast and everything looks washed out and blurred — that is haze.
Did you know? Particles of colloidal size — from vehicle exhaust, construction dust and the burning of fuel — are the worst offenders. They are small enough to stay suspended in the air for days, and small enough to be breathed deep into the lungs, which is why the same haze that hides the skyline is also a health warning.
Was this helpful? Report an error