Chapter 8: A Journey through States of Water (Chemistry)

(iv) The conversion of water vapour into its liquid state. ((i) and (ii) describe evaporation.)

(iv)

(i) (b) water colours (and also (c) acrylic colours): the colours dry only when the water in them evaporates. Crayons and pencil colours are dry, so no evaporation is involved.

(ii) (b) ink pen: the liquid ink dries by evaporation. (Ball-point ink is a thick paste that dries very quickly; a pencil leaves dry graphite.)

(i) water colours (also acrylic colours) (ii) ink pen

Natural grass is a living plant. It takes up water from the soil and gives out water vapour from its leaves (transpiration), and water also evaporates from the moist soil below it. Evaporation takes heat from the surroundings, so the air around natural grass becomes cooler. Plastic grass has no water to evaporate; it also absorbs the Sun's heat and becomes hot, so the space around it feels warmer.

Water evaporating from natural grass (and moist soil) takes away heat and cools the air; plastic grass has no water to evaporate and gets hot in the Sun.

Petrol, kerosene, spirit, nail-polish remover (acetone), perfume, alcohol (sanitiser) and milk/curd water. (Petrol, spirit, perfume and sanitiser evaporate much faster than water.)

Petrol, kerosene, spirit, perfume, sanitiser (alcohol) and nail-polish remover.

Evaporation needs heat, but it also depends on air movement. When water evaporates from wet clothes, the air just around them becomes full of moisture (humid), which slows down further evaporation. A fan blows this moist air away and brings in drier air, so water keeps evaporating quickly. The heat needed for evaporation is taken from the clothes and the surrounding air; that is why we feel cool under a fan. So a fan dries clothes faster even though it does not heat them.

A fan carries away the moist air around the clothes and replaces it with drier air, so evaporation goes on faster; the heat for evaporation comes from the surroundings, which is why it feels cool.

In 3–4 days in the open, most of the water in the sludge evaporates (helped by the Sun and wind), and the extra water also drains back into the drain.

• Lower transport cost: the dried sludge is much lighter and smaller in volume, so fewer trips are needed and it is easier to load.
• Safety: dry sludge does not splash or drip, is less slippery and less smelly, and drying in sunlight kills many germs, so it is safer and more hygienic for the workers to handle.

The water evaporates, so the sludge becomes lighter (cheaper to transport) and drier (less messy, smelly and germ-laden, so safer to handle).

Activities involving evaporation: drying washed clothes, drying wet utensils and floors after mopping, drying hair after a bath, keeping water cool in an earthen pot (matka), drying papad, pickles or grains in the Sun, sweating and cooling of the body, and boiling water or making tea (steam).

How it helps: knowing that evaporation is faster in sunlight, wind and with a larger spread surface, and slower in humid weather, we can dry clothes quickly (spread them out, use sunlight and a fan), keep water cool in earthen pots, preserve food by drying, and understand why we feel cool when we sweat.

Drying clothes, mopped floors and hair, cooling water in a matka, drying food, sweating. Knowing what speeds up evaporation helps us dry things faster, keep water cool and preserve food.

As ice and snow: glaciers and snow on high mountains (like the Himalayas), polar ice caps and icebergs, frozen lakes and rivers in very cold places, hailstones, and frost on leaves on cold mornings.

As ice and snow: glaciers, snow-covered mountains, polar ice caps, icebergs, frozen lakes, hail and frost.

Everyone has a right to clean water, but the amount of usable fresh water on the Earth is very small. If each of us wastes or pollutes water, there will not be enough for others, for animals and plants, or for future generations. So before demanding water as our right, we must take the responsibility to use it wisely: close taps while brushing, fix leaking taps, reuse water (e.g. water used for washing vegetables can water plants), harvest rainwater, and never throw waste into rivers and ponds. If all of us act responsibly, everyone's right to water can be protected.

Fresh water is limited and shared by all, so we must save it, avoid wasting and polluting it, and harvest rainwater; only then can everyone, now and in future, enjoy the right to water.

Sprinkle some water on the seat, or wipe it with a wet cloth. The water evaporates, taking heat from the seat, and cools it down quickly. (Fanning it or parking in the shade, or covering the seat with a light-coloured cloth, also helps.)

Sprinkle water or wipe with a wet cloth: as the water evaporates it takes away heat and cools the seat.