Question

In thermite welding, to ignite the mixture we use:
(A) Magnesium ribbon
(B) Potassium ribbon
(C) Calcium ribbon
(D) Manganese ribbon

Answer 1

A mixture of metal powder and metal oxide is known as thermite. When ignited, the thermite undergoes an exothermic reaction. This reaction occurring is a redox reaction as both oxidation and reduction reactions occur.

Complete step by step solution:
A huge amount of heat is required for carrying out the thermite welding. In the process, aluminium powder and a metal oxide are heated at very high temperatures and the reaction that occurs is a highly exothermic one. An extra material is required to maintain the high temperature and keep the mixture ignited. For this we use a magnesium ribbon.
The correct answer is A.

Additional information
A magnesium ribbon is a very thin strip made up of pure magnesium metal with a thickness of less than $0.1$ millimeters. Since the reaction proceeds at a very high temperature, it is easier to heat the ribbon upto the required temperature using a simple splinter. The higher surface area of the magnesium ribbon provides increased exposure to more nitrogen molecules and thus ensuring complete burning of magnesium.

Note:
We know that magnesium is an alkaline earth metal having the atomic number 2. It is a highly reactive metal and the only group 2 metal that can burn in air and react with nitrogen to form a nitride. The burning reaction of magnesium ribbon is a highly exothermic one and takes place at high temperature of around 800 degree Celsius. Magnesium nitride formed is a greenish yellow coloured powder.