In the preparation of alum, which method is commonly used to remove excess reactants from the alum product?

• A. Heating the alum crystals to high temperatures
• B. Adding concentrated sulfuric acid to dissolve impurities
• C. Washing the alum crystals with cold water
• D. Filtering the solution without washing the crystals

Answer: (C)

Verified step by step guidance

1. Understand that in the preparation of alum, the product often contains excess reactants or impurities that need to be removed to purify the crystals.
2. Recognize that heating alum crystals to high temperatures is not commonly used because alum can decompose or lose water of crystallization upon heating, which is undesirable.
3. Adding concentrated sulfuric acid to dissolve impurities is not typical because it can alter the composition of the alum or introduce new impurities.
4. Know that washing the alum crystals with cold water is the common method used to remove excess reactants and soluble impurities without dissolving the alum itself, as alum is less soluble in cold water.
5. Filtering the solution without washing the crystals would leave impurities on the crystal surface, so washing is necessary to obtain pure alum crystals.