Saltpetre is another name for ______.

potassium nitrate

The correct answer is option 2. potassium nitrate.

Saltpetre, also known as saltpeter or niter, refers to a chemical compound with the chemical formula KNO₃. It is commonly known as potassium nitrate. Let's break down the explanation:

1. Chemical Formula: The chemical formula KNO₃ indicates that saltpetre is composed of potassium ions (K⁺), nitrate ions (NO₃⁻), and one potassium ion for every nitrate ion.

2. Potassium Nitrate (KNO₃): The compound potassium nitrate is often used as a fertilizer, food preservative, and in the manufacturing of gunpowder. It has various applications in agriculture, food industry, and pyrotechnics.

3. Not Sodium Chloride (NaCl): Sodium chloride is common table salt and has the chemical formula NaCl. Saltpetre specifically refers to potassium nitrate, not sodium chloride.

Therefore, when saltpetre is mentioned, it is synonymous with potassium nitrate (KNO₃), and the correct option is 2. potassium nitrate.