The particles are always moving

Activity 7.8 lets you see particles moving. Drop a few grains of potassium permanganate into a glass of water. Use a spoon or spatula, never your fingers.

At first, pink streaks spread out from the grains. Wait, and the whole glass turns an even pink. Why?

Because water particles are always moving. They pull the potassium permanganate particles out of the grain, then keep bumping and knocking them until they are spread right through the water.

That also explains why some things don't dissolve. In substances such as sand, the particles are held together so strongly that the water particles can't pull them out at all. That's why sand is insoluble in water.

Think like a scientist. Drop a grain into three glasses: hot water, room-temperature water and ice-cold water. The colour spreads fastest in hot water, more slowly at room temperature, and slowest in ice-cold water. Hot particles move faster: heating makes particles move more.

You can't see gas particles, so Activity 7.9 uses your nose. Light an incense stick in one corner of a room and wait. At first you smell it only nearby; soon you smell it all over the room. The constantly moving air particles keep striking the fragrance particles and spread them everywhere. That's also how you smell perfume from across a room.

Particles are at work in everyday jobs too. When soap washes an oily stain, soap particles surround the oil: one end of each soap particle holds on to the oil, and the other end mixes with water. When you rinse, the water carries the soap away, and the oil goes with it.