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Chemistry in the News

What is ammonium nitrate and what happens when it explodes?

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Today’s news has been dominated by the terrible explosion in Beirut, Lebanon. The blame for the explosion has been directed at 3,000 tons of ammonium nitrate that was being stored in a warehouse in Beirut’s port. What is ammonium nitrate, why can it explode, and what happens when it does? This graphic takes a look.

Ammonium nitrate’s major use is as a fertiliser – this accounts for around 78% of its use (by volume) worldwide. It’s a source of nitrogen, important for the growth of plants.

It’s also used in some explosive mixtures which are used for mining and quarrying. Mixed with fuel oil it’s known as ANFO (ammonium nitrate/fuel oil), which accounts for approximately 80% of the 2.7 billion kilograms of explosives used in North America every year. In these explosive mixtures, it acts as an oxidising agent, helping other materials burn.

In its pure form, ammonium nitrate isn’t usually explosive – in fact, it’s safe to handle. However, if it is contaminated with impurities, the risk of it detonating increases. Heating also poses a danger: ammonium nitrate decomposes at around 230˚C, and can explode when heated to between 260–300˚C if it’s confined.

The force of the ammonium nitrate explosion is provided by the rapid generation of gas as it breaks down. The primary reaction when it breaks down produces nitrogen, water vapour and oxygen.

This reaction isn’t the only one taking place, however. You might have wonder what caused the plume of smoke seen after the explosion in Beirut to have an orange-red colour. This is due to other reactions generating nitrogen dioxide. It’s not the only additional product: ammonia and nitrous oxide are amongst the others.

If you want to learn more about the chemistry behind ammonium nitrate explosions, I’ve provided some useful links below. If you’re looking to help the huge numbers of people affected by the explosion in Beirut, the Lebanese Red Cross is accepting donations, and Impact Lebanon are raising disaster relief funds.

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