Class 6 Science Chapter 8: A Journey through States of Water Notes

Are ice and water the same substance?

Yes. Leave an ice cube in a cup and it becomes water; put water in a freezer and it becomes ice again. Nothing is added or taken away, so ice and water are two forms of the same substance. These forms are called states. They still behave differently: water flows and splashes, and ice does neither.

Where does water disappear to?

Puddles shrink and washed utensils dry. A spoonful of water on a steel plate disappears without the underside of the plate ever getting wet, so the water has not seeped through. It has changed into water vapour, the gaseous state of water. This change is called evaporation, and it goes on all the time, even at room temperature. Read more in evaporation and condensation.

Where do the drops on a cold glass come from?

Put ice cubes in a glass of cold water and drops soon appear on the outside. They do not seep out: the water level inside does not fall, and the glass even gets heavier on a digital balance. The drops come from water vapour in the air, which turns back into liquid when it touches the cold glass. This is condensation. Dew on plants forms the same way. The amount of water vapour in the air is called humidity.

What are the three states of water?

Ice (solid) keeps its own shape. Water (liquid) flows and takes the shape of its container, but its volume stays the same. Water vapour (gas) spreads to fill all the space it gets. Wax, oil and ghee can also be solid or liquid. See three states of water.

How can we change the state of water?

Heating turns ice into water (melting) and water into water vapour. Cooling turns water into ice (freezing) in a freezer, and water vapour into water. Candle wax melts near a flame and turns solid again as it cools, and coconut oil often turns solid in winter. Atmospheric Water Generator (AWG) machines use condensation to collect drinking water from humid air.

What makes water evaporate faster?

A larger exposed area (water on a plate dries before water in a bottle cap), sunlight, moving air and dry air all make water evaporate faster. On rainy, humid days clothes dry slowly. See factors affecting evaporation.

Why does evaporation cool things?

Evaporation takes heat from the surface where it happens. Water seeps through a matka and evaporates from its outside, so the water inside stays cool. We feel cool under a fan because it helps our sweat evaporate. A pot-in-pot cooler keeps fruits and vegetables fresh with wet sand and no electricity.

How do clouds give us rain?

Water vapour is lighter than air, so it rises. High above the ground the air is cooler, and the vapour condenses into tiny droplets around dust particles. Floating droplets form clouds. When many droplets join into drops that are too heavy to float, they fall as rain, and sometimes as hail or snow. Water then flows back through rivers and the ground to the oceans. This never-ending journey is the water cycle. Only a small part of Earth’s water is fit to use, so save it and keep rivers and lakes clean.