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NCERT Solutions Curiosity Chapter 1 Water, heat and the materials around us — In-text Questions

Book page 4 Updated on2026-09-05

Q1.
Do you ever wonder why and how it rains?
Answer

Yes. Rain is the visible part of the water cycle, which runs all the time between the sea, the sky and the land.

How it happens — step by step:

  1. Evaporation. The Sun heats the water of oceans, rivers, lakes and even wet clothes. Water slowly changes into an invisible gas called water vapour and rises with the warm air.
  2. Cooling as it rises. Higher up, the air is colder.
  3. Condensation. The vapour cools and turns back into tiny droplets of water around specks of dust. Millions of such droplets floating together are what we see as a cloud.
  4. Precipitation. The droplets keep joining one another until they become too heavy for the air to hold — and they fall as rain. If it is very cold high up, they fall as hail or snow.
  5. The rain flows into rivers and back to the sea, and the whole cycle begins again.
Why the rain comes at a fixed season in India: in summer the land heats up much faster than the Arabian Sea and the Bay of Bengal. Hot air over the land rises, and moist air rushes in from the sea to take its place. These are the monsoon winds. When they strike the Western Ghats and the Himalayas they are forced upward, cool down, and pour rain — which is why Kerala gets the first showers around June and Cherrapunji (Meghalaya) is one of the wettest places on Earth.
Check it yourself: Hold a cold steel glass of water in a room on a humid day. Water drops appear on the outside of the glass. Nothing leaked out — the vapour already present in the air condensed on the cold surface. That is exactly how a cloud is born.
Q2.
Have you noticed that water freezes and becomes ice when we cool it and boils and becomes steam when we heat it?
Answer

Yes. Water is the same substance in all three forms; only its state changes.

StateWhat we call itWhat makes it changeEveryday sight
SolidIceCooling to 0 °C — freezingice cubes in the freezer, hailstones
LiquidWaterWarming ice above 0 °C — meltinga glass of drinking water
GasSteam / water vapourHeating to 100 °C — boilingthe whistle of a pressure cooker
Why it happens: water is made of extremely tiny particles that are always moving.
  • When water is cooled, the particles slow down, come close, lock into fixed places and can only shake where they are. The water becomes hard — ice.
  • When water is heated, the particles move faster and faster until they break free of one another and fly apart into the air — steam.
So heating and cooling do not create a new substance; they only change how freely the particles can move. That is why melting ice gives back exactly the same water.
Did you know? The white cloud you see coming out of a cooker or a kettle is not steam. Real steam is invisible. What you see is steam that has already cooled a little and turned back into tiny water droplets — condensation happening right in front of you.
Everyday use in India: a matka (earthen pot) keeps water cool in summer because water seeps through its tiny pores and evaporates, taking heat away from the pot. Sprinkling water on a terrace before a summer evening cools it for the same reason.
Q3.
How do we understand hot and cold?
Answer

By touch we can only compare — to know exactly how hot something is, we must measure it.

What you observe: our sense of touch is unreliable. Try this simple test:

Left hand → bowl of cold water   |   Right hand → bowl of warm water
After a minute, put both hands into the same bowl of ordinary water.

The same water feels warm to the left hand and cool to the right hand at the same moment. So our skin tells us only whether something is hotter or colder than our hand.

Why we need a proper measure: because “hot” and “cold” are relative words, science uses a number called temperature, measured in degrees Celsius (°C), and an instrument called a thermometer. A thermometer gives the same reading no matter whose hand holds it.
  • A clinical thermometer measures body temperature. Normal human body temperature is about 37 °C (98.6 °F). A higher reading means fever.
  • A laboratory thermometer measures the temperature of water, oil or air over a much wider range.
  • At sea level, water freezes at 0 °C and boils at 100 °C.
Where it helps: a doctor decides on medicine from a thermometer reading, not by touching the forehead. A halwai keeps the milk at the right temperature to set curd. The Meteorological Department warns of a heat wave when the maximum crosses about 45 °C, and schools then change their timings.
Q4.
Then again, there are so many different things around us—the paper we write on, the metal key, the plastic ruler and the rubber eraser in our box, the magnet that keeps the box closed, the clothes we wear, the cup we drink milk in and so many other things. What are they made of? Are they made of different materials? How do we separate different materials from one another?
Answer

Yes — each object is made of a material chosen to suit its job, and the same object may even use two or three materials together.

ObjectMaterialProperty that makes it suitable
Paperwood pulplight, soft, holds ink
Keymetal (brass, steel)hard, strong, does not bend or break
Rulerplasticlight, stiff, cheap, does not rust
Eraserrubbersoft and springy, rubs off pencil marks
Magnet on the boxmagnetic materialattracts iron, so the lid snaps shut
Clothescotton, wool, silk, synthetic fibrecotton lets air through in summer; wool traps air and keeps us warm
Cupglass, ceramic, steel or clayholds liquid, does not dissolve in milk

How we separate materials: we look for one property that only one of the materials has, and then use it.

  • Handpicking — for large, few, clearly different pieces, such as stones from rice.
  • Winnowing — a farmer drops the threshed grain in the breeze; the light husk is blown away and the heavy grain falls straight down.
  • Sieving — flour passes through the holes of a chalni, bran stays back.
  • Magnetic separation — a magnet pulls out iron pins from a heap of sand or pencil shavings.
  • Filtration — tea leaves stay in the strainer while the tea passes through.
  • Sedimentation and decantation — muddy water is left standing, the mud settles and the clear water is poured off.
  • Evaporation — sea water is spread in shallow pans; the water evaporates in the Sun and common salt is left behind. This is how salt is made along the coasts of Gujarat and Tamil Nadu.
The rule behind all of them: separation always uses a difference in a property — size for sieving, weight for winnowing, attraction to a magnet for magnetic separation, solubility and boiling behaviour for evaporation. If two materials share every property, they cannot be separated.
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