Middle School

**Materials:**

- Chalk
- Calcium hydroxide
- Filter paper
- Phenolphthalein
- Heat source
- Two Pyrex beakers

**Directions:**

1. Crush some classroom chalk (a mixture of limestone and gypsum) finely.
2. Mix the crushed chalk into water in a beaker.
3. Pour the mixture through filter paper. The water appears to be "pure."
4. Boil the water or let it evaporate away. Observe what is left. Note that water also dissolves and transports minerals.
5. In a beaker, mix calcium hydroxide and water.
6. Filter the mixture until clear.
7. Fill another container with water.
8. Obtain some phenolphthalein solution and add a few drops to a glass of "pure" water.
9. Add a few drops to the water mixed with calcium hydroxide.
10. Give the results and explain them.

Answer :

Upon dissolving and then boiling or evaporating water containing dissolved calcium carbonate from chalk, a calcium carbonate residue remains. Calcium hydroxide in water with phenolphthalein turns pink because it's a basic solution, while pure water remains colorless with phenolphthalein.

When classroom chalk (which is mainly made up of calcium carbonate) is crushed and mixed with water, and then the mixture is filtered, the water may appear 'pure' but still contains dissolved calcium ions. Upon boiling or evaporation, a residue of calcium carbonate would be left behind. This is because the solubility of calcium carbonate in water is very low, and so it precipitates out when the water is removed.

In the second experiment, mixing calcium hydroxide with water results in a solution that turns pink or fuchsia when phenolphthalein, a pH indicator, is added. This is due to the solution being basic as calcium hydroxide is an alkali. Therefore, the addition of phenolphthalein to pure water, which is neutral, would not result in any significant color change.
phemolphthalein changes colour.(i forgot from which to which) This is because calcium hydrroxide is basic.

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