When we fry an egg, it is impossible to 'unfry' it.

A lot of reactions work in the same way, once it is done, it is irreversible.

A good example is the combustion of magnesium to form magnesium oxide.

Reversible reactions are reactions where formed products can revert, or go back, to their

original reactants. Since the reaction can proceed in either direction,

we use this reaction arrow to represent reversibility. An example of a reversible reaction is the

thermal decomposition of ammonium chloride to form ammonia and hydrogen chloride.

Place ammonium chloride in a boiling tube and heat using a Bunsen burner.

Ensure that this is conducted in a fume cupboard as ammonia and hydrogen chloride are both

corrosive gases. If we hold red litmus paper at the mouth of

the boiling tube, it turns blue, which confirms the presence of ammonia.

Blue litmus paper turns red, which confirms the presence of hydrogen chloride.

Remove the boiling tube from the heat and let cool.

Have a look near the mouth of the boiling tube.

You will see that there are white crystals, these are ammonium chloride crystals.

So what has happened here? Ammonium chloride has thermally decomposed

to ammonia and hydrogen chloride. The formed ammonia and hydrogen chloride have

reacted to produce ammonium chloride. Hence, ammonium chloride ammonia + hydrogen

chloride. Therefore, this is a reversible reaction.

Why is the thermal decomposition of ammonium chloride sometimes incorrectly called a 'sublimation

reaction'? Please pause the lesson to think about this

and resume when you are done. Here is a hint, think about phase changes

and what happens to a substance during a phase change.

When a substance goes from the solid phase to a gas phase, it is said that it has sublimed.

This phase change is not a 'reaction', as the substance itself has not changed.

In our example, solid ammonium chloride did not form gaseous ammonium chloride, but rather,

it decomposed under heat to form ammonia and hydrogen chloride.

Therefore, it is not a 'sublimation reaction'.