Which metal sulphate, composed of potassium, aluminium, and sulphate ions in the ratio 1 : 1 : 2, plays a role as a flame retardant, a mordant and an astringent?

Potash alum

The correct answer is option 2. Potash alum.

Potash alum, also known as potassium aluminum sulfate, has the chemical formula \(KAl(SO_4)_2·12H_2O\). In this compound, the ratio of potassium ions \((K^+)\) to aluminum ions \((Al^{3+})\) to sulfate ions \((SO_4^{2-})\) is indeed \(1:1:2\).

Potash alum is commonly used for various purposes:

1. Flame retardant: Potash alum is used as a flame retardant, particularly in textiles and paper products, to reduce the flammability of these materials.

2. Mordant: In the dyeing and printing of textiles, potash alum acts as a mordant, which helps to fix dyes onto the fabric by forming insoluble complexes with the dyes, thereby enhancing colorfastness.
 
3. Astringent:  Potash alum has astringent properties, meaning it causes contraction or tightening of tissues. It is used in cosmetic products such as aftershaves and antiperspirants for its astringent effects on the skin.

Gypsum (option 1) is calcium sulfate dihydrate \((CaSO_4·2H_2O)\), not potash alum. Epsom salts (option 3) refer to magnesium sulfate heptahydrate \((MgSO_4·7H_2O)\), and celestite (option 4) is a mineral form of strontium sulfate \((SrSO_4)\), neither of which matches the description provided for potash alum.