Sciencemadness Discussion Board

Chalk composition: CaCO3 vs CaSO4

fusso - 22-9-2019 at 07:34
From TKOR's vid their bought chalk don't dissolve in HCl. They also provided a DIY recipe using CaSO4. What's the difference between CaCO3 and CaSO4 chalk?

RedDwarf - 22-9-2019 at 08:31

If you're talking geologically or chemically then chalk is calcium carbonate, but the word chalk is often used to describe other (compressed) powders such as blackboard/sidewalk chalk, billiard chalk or climbing chalk (normally but not always magnesium carbonate) which can be of different compositions. Context, as ever in language, is everything!

fusso - 22-9-2019 at 09:00

I'm talking about writing/blackboard chalk.

PirateDocBrown - 22-9-2019 at 15:32

Blackboard chalk is almost always gypsum, CaSO4.2H2O.

zed - 2-10-2019 at 13:32

As I recall, gymnasts may utilize CaCO3 chalk.

I'll check!

Well, I checked, and I stand corrected. Seems Gymnast's chalk may be MgCO3.

Plenty of other sources of CaCO3. At least, if you are in the USA.

[Edited on 2-10-2019 by zed]

hodges - 2-10-2019 at 14:50

Years ago, I remember reacting chalk board chalk with an acid and getting CO2. So the chalk I had was definitely a carbonate. I don't know what chalk board chalk is made of these days. I didn't even realize it was still made given most boards being white boards these days. I guess a few vendors still have chalk boards with ads on them on sidewalks outside their shops.

Sigmatropic - 2-10-2019 at 14:55

Lime, as in lime stone is also calcium carbonate. Giving quicklime on calcination, in turn giving slaked lime on hydration. Also calcium chloride is available as a desiccant. At least one of these is bound to be in your local hardware store.