NCERT Solutions Curiosity Chapter 7 Chapter opener — Probe and ponder

Because a stone and a grain of sand are solids, and water is a liquid — and the difference lies entirely in how strongly their constituent particles are held together.

Inside every stone and every grain of sand the particles are closely packed and the interparticle attractions are very strong. They hold each particle in a fixed position, so a grain keeps its own definite shape and can sit still on the grains below it without collapsing. Grain rests on grain, and the heap stands.

In water the interparticle attractions are much weaker. The particles are free to move past one another, so no layer of water can hold up the layer above it. The moment you try to heap water, the particles at the bottom simply slide sideways, and the water spreads out until its surface is level.

Because a liquid has a definite volume but no fixed shape. Your folded hands act as the container; open them and there is no container left.

The particles of water are free to move, but only within a limited space. When you cup your hands, the water cannot escape, so its particles settle along the hollow of your palms and the water copies that shape exactly — the same thing that happens when 200 mL of water is poured from Container A into Container B in Activity 7.4.

Open your hands and the walls are gone. The interparticle attraction in water is far too weak to hold the particles in a bowl shape on their own, so they slide past one another and the water flows away. Its volume has not changed at all — only its shape has.

Because air is matter — it too is made of constituent particles, and every one of those particles has its own mass.

We cannot see air for two reasons: its particles are extremely small, and in a gas the interparticle spaces are maximum and the attraction is negligible, so the particles are scattered far apart and never form a surface for light to bounce off. Not seeing something is not the same as there being nothing there — exactly the lesson of Activity 7.2, where dissolved sugar cannot be seen but can certainly be tasted.

When you blow up a balloon you force a very large number of air particles into it. Their masses add up, the Earth pulls on all of them, and the inflated balloon therefore weighs more than the same balloon empty.

The particles are largely the same ones; the mixture they make up is not.

Particles of matter are not created or destroyed by ordinary changes — grinding, dissolving, melting and boiling only rearrange them. So the constituent particles in the air around you have been in use for a very long time: because gas particles move freely in all directions and never stop, air is stirred and re-stirred continuously, and a particle breathed out in one place can be taken up by a plant far away and returned to the air again.

What has changed is how much of each substance the air contains. Burning fuel in vehicles, factories and homes keeps adding new substances, along with the dust we call Suspended Particulate Matter (see A step further, page 109). So the air of today is made of the same kinds of particles as before, but not in the same proportions.

A good question for this chapter is one that cannot be answered without saying something about the particles — how far apart they are, how strongly they attract each other, or how they are moving. Write your questions in your notebook and bring them to class.

Sample questions:

• If a solid has no space between its particles, why does a stone crack when it is hammered rather than squash flat?
• A syringe full of air can be squeezed; one full of water cannot. What exactly is being squeezed out?
• Why does a wet floor dry up even on a cold day, when water boils only at 100 °C?
• The smell of food reaches the last room of the house. Does the food itself travel there?
• Ice floats on water. What does that tell us about the spacing of particles in ice?