Identify the compound of calcium which is used for plastering of fractured bones. With the help of chemical equation showing how is it prepared and what special precautions should be taken during the preparation of this compound.

Or

‘Sweet tooth may lead to tooth decay’. Explain why? What is the role of toothpaste in preventing cavities?

The compound of calcium which is used for plastering of fractured bones is Plaster of Paris. It is also called as Calcium Sulphate Hemihydrate.

Explanation:

1) Calcium sulphate occurs in nature in the form of anhydrite (CaSO₄) and gypsum (CaSO₄. 2H₂O). Large deposits of gypsum are found in India mainly including the states of Punjab and Rajasthan.

2) The effect of heating gypsum is quite interesting. Initial heating changes its crystal structure. On further heating to 390 K, it forms hemihydrate with the molecular formula CaSO₄. 1/2 H₂O or (CaSO₄)₂. H₂O. This hemihydrate is termed as Plaster of Paris.

3) The reaction is as follows [Reaction temperature is less than equal to 390 K]:

2CaSO₄. 2H₂O → CaSO₄. 1/2 H2O + 3H₂O

4) On further heating above 473 K, the water of crystallization from the molecule vanishes and the anhydrous calcium sulphate is obtained. It is known as dead burnt plaster and does not set when mixed with water.

5) The reaction is as follows [Reaction temperature is greater than equal to 473 K]:

CaSO₄. 1/2 H₂O → 2CaSO₄ + H₂O

6) The main precaution which needs to be taken is that plaster of Paris should not be heated above 473 K as it will lose all the molecules of water of crystallization and it will turn into dead burnt plaster, and it will not set even when it is mixed with water.

Additional Information:

Plaster of Paris is used extensively for making statues, models, moulds for casting etc. There were large deposits of gypsum at Montmartre which near Paris and hence this plaster came to be known as Plaster of Paris.

OR

Toothpaste is normally basic substances which reduces the acidic medium of the mouth and hence prevent tooth decay.

Explanation:

1. The tooth of human consists of three main parts which include enamel, dentine and pulp.

2. Enamel is the hardest occurring substance in the human body, and it is the outermost part of teeth. Dentine is the middle region and pulp contains the blood vessels.

3. Our teeth are made up of a compound of calcium, and hence whenever a food item is attached to the teeth, bacteria’s present in the mouth attack the food item and decrease the pH of the mouth making it acidic.

4. As the mouth becomes acidic, the enamel starts to corrode away and hence tooth decay begins. To avoid the development of acidic pH, it is necessary to introduce a base in the mouth.

5. Toothpaste is normally the bases which help to neutralize the acidic medium of the mouth and restore the pH to 7 and hence preventing the tooth decay.