Breaking chalk down as far as it will go

On a riverbank or a beach you can pick pebbles and stones out of the sand. Where do they come from?

High in the mountains, rocks slowly break down through erosion. Rivers carry the broken pieces along, grinding them smaller and smaller into pebbles, stones and sand, and carry huge amounts of them down to the plains. Big rocks end up as fine grains of sand and clay.

Is a grain of sand the smallest piece a rock can be broken into? Activity 7.1 tries the question with something easier to break: chalk.

Snap a chalk stick in two. Keep breaking it until your fingers can't break it any more. Then grind the bits in a mortar and pestle and look at the powder through a magnifying glass. Every tiny grain is still chalk.

Has grinding turned the chalk into something else? No. From Grade 7 (Changes Around Us): grinding is a physical change. No new substance is made; only the size of each bit gets smaller.

Now imagine grinding on and on, far past what any mortar could do. In the end you'd reach pieces of chalk so small they could not be broken any further.

Those tiniest units are the building blocks chalk is made of. Even the smallest speck of chalk you can pick up contains an enormous number of them. They are called the constituent particles of chalk. A constituent particle is the basic unit that a bigger piece of a substance is made of, the way water, sugar, lemon and masala are the ingredients of a glass of shikanji. A grain of sand is the same: not the smallest unit of the rock, but itself made of huge numbers of constituent particles.