Std 5 EVS 1 · Chapter 24

24. Substances, Objects and Energy

Complete solutions to the Exercise. Maharashtra State Board (Balbharati), Standard 5 EVS 1.

Exercises

1What’s the solution?

(a) We need to make a sherbet quickly for some guests. But we only have sugar candy in the house.

Pound the sugar candy in a mortar until it becomes a powder, and then stir it into the water.

A whole lump of sugar candy takes a long time to dissolve, because only its outer surface touches the water. When it is pounded, the same sugar is broken into a great many tiny particles, and water can reach all of them at once. So the powder dissolves quickly and the sherbet is ready in a short time.

(b) We need to rub salt on a corncob but only salt crystals are available.

Grind the crystals into a fine powder first, and then rub that on the corncob. Big crystals would only roll off and would not spread evenly, but the fine particles of powdered salt stick all over the cob and give it an even taste.

2Use your brain power!

(a) Why do tablets of camphor decrease in size day by day?

Camphor keeps changing into very tiny particles in the gaseous state, which leave the tablet and mix with the air around it. That is why we can smell camphor from a distance. As these particles go on escaping, less and less of the tablet is left, so it grows smaller day by day and finally disappears altogether. Naphthalene balls become smaller in exactly the same way.

(b) How do we save fuel by using public transport?

A bus carries fifty or sixty people in one vehicle. If all of them travelled separately by car or two-wheeler, forty or fifty vehicles would run on the same route and each would burn its own fuel.

Less fuel per person — one vehicle burns fuel instead of many, so the fuel used for each passenger is far less.

Less crowding — fewer vehicles on the road means fewer traffic jams, and vehicles standing in a jam waste fuel.

Less pollution — less fuel burnt means less smoke and fewer poisonous gases in the air.

This matters because the stores of coal and mineral oil on the earth are limited and cannot be replaced once used.

3Answer the following questions.

(a) When and why do clothes smell of naphthalene?

Naphthalene balls are kept among warm clothes that are stored away. Naphthalene is continuously changing into very small particles in the gaseous state. These particles leave the balls and settle on the clothes kept around them, and so after some days the clothes too begin to smell of naphthalene. As the particles keep leaving, the balls themselves grow smaller and slowly disappear.

(b) In which states is water found in nature?

Water is found in nature in all three states — solid (ice), liquid (water) and gaseous (water vapour, as in a cloud). In all three the particles of water themselves remain the same; only their arrangement is different.

State	Arrangement of particles	Example
Solid	The particles are packed closely together in a regular order and cannot move about, so a solid keeps its own shape.	Ice, coal, aluminium
Liquid	The particles are a little further apart and can slide past one another, so a liquid takes the shape of its container.	Water, kerosene, petrol
Gas	The particles are far apart and move about freely, so a gas spreads out and fills all the space available.	Water vapour, nitrogen, oxygen

The particles of the substance are the same in all three states; it is only their arrangement that differs, and that is what gives solids, liquids and gases their different properties.

(d) What is meant by energy?

Energy is required to do work. The capacity of a body to do work is called energy.

Heat energyReleased when coal, petrol, diesel, CNG or LPG is burnt.

Kinetic energyEnergy in the form of motion. Every moving thing has it — the wind that turns a windmill or moves a sailing boat.

ElectricityUsed to run a fan, a mixer-grinder or a water pump.

Light and soundThe electricity in a television is converted into light and sound energy.

Solar energyUsed in a solar cooker or a solar water heater.

Sunlight, wind and water are non-exhaustible sources of energy and should be used as far as possible, because the stores of coal and mineral oil are limited.

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