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Does the amount of calcium carbonate in chalk influence its charecteristics ? If so please mention them

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    – Poutnik
    Jun 12 at 11:39

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Amorphous pure calcium carbonate $\ce{CaCO3}$ is a white, common and rather soft mineral, called chalk or lime. It can be cut down in cylinders as thick as a finger. Using it on a blackboard will make a broad and thick line, and your fingers (and trousers) will quickly be full of white marks. It is too soft to be useful in a classroom. On the other hand, the same cylindrical pieces can also be made with plaster, which is hydrated calcium sulfate. But the obtained piece is too hard to be used on a blackboard. It will scratch the board. But if you make the appropriate mixture of plaster and lime, you can produce a piece of chalk useful on a blackboard.

So to produce a piece of chalk useful in a classroom, lime is reduced to powder, mixed with plaster ($\ce{CaSO4·\frac{1}{2} H2O}$). The mixture is then got wet. This will transform the plaster $\ce{CaSO4·\frac{1}{2}H2O}$ into gypsum $\ce{CaSO4·2H2O}$. This operation also transforms the mixture of two powders into a solid block, ready to be used for writing on a blackboard. Neither too soft, nor too hard.

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