A cliff of tiny shells
What Is Chalk Made Of?
A stick of chalk and a white cliff are the same thing: billions of shells from creatures too small to see.
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Step 1 of 5
It fell as a slow white snow.
In a warm shallow sea, uncountable tiny floating creatures each grew a shell. When they died the shells drifted down, year after year, building a soft white layer metres thick.
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Step 2 of 5
The shells are made of the same stuff as your bones.
Each creature pulled calcium out of the seawater to build its shell. That calcium is what makes chalk white, soft and slightly gritty — and it is the same mineral in eggshell and in bone.
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Step 3 of 5
Soft is the point.
Chalk is one of the softest rocks there is. Drag it over a rough board and a thin line of it simply rubs off — which is exactly why it writes, and why the writing wipes away.
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Step 4 of 5
It fizzes in vinegar.
Drop chalk in vinegar and it bubbles. The acid is breaking the calcium apart and letting a gas escape — the same reaction that makes acid rain eat away at limestone buildings.
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Step 5 of 5
Most classroom chalk is not chalk.
Real chalk crumbles and makes a lot of dust. Most sticks sold today are a harder, cleaner mineral pressed into shape — related, but never anywhere near an ancient sea.
The white cliffs of Dover are hundreds of metres of this. Standing at the bottom, you are looking up at about eighty million years of falling shells.
The short version
Chalk is soft white rock built from billions of tiny sea shells, made of the same calcium as your bones.
Try it yourself
Put a small piece of chalk in vinegar and watch it fizz. That is the acid taking the shells apart.
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