Option 1: Liquid Sodium is employed as a coolant in nuclear reactors. This is the correct answer. Sodium, being a very efficient heat transfer fluid, is often used as a coolant in certain types of nuclear reactors due to its high thermal conductivity.

Option 2: Calcium carbonate is an ingredient of toothpaste. This statement is generally correct and therefore not the best option. Calcium carbonate is often used as an abrasive in toothpaste to help remove plaque.

Option 3: Bordeaux mixture consists of sodium sulphate and lime. This statement is incorrect. Bordeaux mixture is a combination of copper sulphate and slaked lime, not sodium sulphate and lime.

Option 4: Zinc amalgams are used as a dental filling. This statement is incorrect. Dental fillings are typically composed of an amalgam of various metals, but zinc is not one of them. Amalgam fillings are usually composed of tin, silver, and mercury, not zinc.