Question

3. How are the excess starting materials removed from the alum product?

4. What is the purpose of using isopropyl alcohol in this experiment?

5. During the washing step, how can you tell when your product has been sufficiently purified?

Answer 1

3. How are the excess starting materials removed from the alum product?

A. The excess starting materials are removed from the alum product by washing the precipitate with water and filter the product.

During the preparation of alum the starting material used in excess is H2SO4. By repeatedly washing of the product the sulphuric acid is removed. The washing step takes advantage of the different solubility properties of the impurities if any present and the product formed, any of the ionic impurities will be soluble in water whereas the alum product is slightly soluble in water. It can be confirmed that the no more washing is required by testing for any trace of H₂SO₄ present. Take 2-3 drops of distilled water in test tube add drop of thymol blue. To the test tube add the filtrate (i.e. the product washed water). If thymol blue changes to pink color in test tube, acid is still present and more washing is required to remove acid. If thymol blue changes to yellow or pale orange color, the product is adequately washed. Thymol blue changes to pink color due to presence of the ionic impurity hydronium ion (H₃O₊/H₂SO₄)

4. What is the purpose of using isopropyl alcohol in this experiment?

A. Isopropyl alcohol is used in the experiment to dry the product.

Once the alum product is obtained by vacuum filter method, isopropyl alcohol is added over the surface of the solid. The isopropyl alcohol is more volatile than water, it washes away the water and evaporates rapidly to give the dry product.

5. During the washing step, how can you tell when your product has been sufficiently purified?

A. During the washing step, thymol blue test is performed to confirm if the product is purified or not.

Take 2-3 drops of distilled water in test tube add drop of thymol blue. To the test tube add the filtrate (i.e. the product washed water). If thymol blue changes to pink color in test tube, acid is still present and more washing is required to remove acid. And if the thymol blue changes to yellow or pale orange color, it confirms that the product has been adequately washed.

The thymol blue changes to pink due to presence of ionic impurity hydronium ion.