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Last Updated: Thursday, 10 August 2006, 13:51 GMT 14:51 UK
Q&A: Liquid explosives
Baggage sign at an airport
Passengers have been banned from taking hand baggage on to flights
An alleged plot to blow up planes from the UK mid-flight and cause "mass murder on an unimaginable scale" has been disrupted, Scotland Yard has said.

It is thought the plan was to detonate explosive devices smuggled in hand luggage on to as many as 10 aircraft.

BBC security correspondent Gordon Corera said the plan "revolved around liquids of some kind".

One theory is that the attack may have involved liquid explosive being carried on to a plane in either drink bottles or cans.

Dr Clifford Jones, an explosives expert from the University of Aberdeen, says even a small amount of liquid explosives carried on to an aircraft would result in a catastrophic explosion.

What are liquid explosives?

The best place to start is with the term "high explosive"; these can be either solid or liquid. Of course, the most famous ones are solid, such as Dynamite and TNT.

One liquid explosive is a general use explosive that is used in quarries.

However, I would not be surprised if it is possible to produce solid explosives in liquid form.

How do they work?

Usually when something burns, it is subsonic and there is very little pressure effect.

With high explosives, the rate of burning is extremely rapid and exceeds the speed of sound. As a result of that there is something called "overpressure" - pressure greater than the surrounding atmospheric pressure.

Massive overpressure is not needed to cause damage. An excess of 1% can break windows, and an overpressure of 10% can harm or kill people and cause structural damage to buildings.

An overpressure of just 2% could break the windows of the aeroplane, and 10% would wreck the aircraft and possibly kill the people in it before it reached the ground.

By the time the damage is caused, the chemistry has finished and physics has taken over.

How are they made?

There are such things as liquid explosives that are high explosives and they behave in exactly the same way as solid explosives, such as TNT.

But there are also explosives that are made by mixing a solid and a liquid - one being the oxidant and the other being the fuel. Unlike most high explosives, they do not contain the fuel and oxidant in the same molecule but they do contain them in sufficiently close contact to cause a blast.

Are the components difficult to get hold of?

No, it is very easy. Ordinary household substances could be used.

Are specialist knowledge or equipment needed to make them?

If someone wanted to obtain a solid high explosive in a liquid form, it would not be difficult for a trained chemical technologist.

But if someone was using a backyard laboratory it is more likely they would go for the two component approach.

Not a lot of experience is needed, the principles are quite simple but it would be a hazardous process of trial and error.

I would not want to be messing about these things. It has been known for schoolboys to go home and attempt this and blow their house up.

Could an explosive device be carried on to an aeroplane?

The size of a device necessary could be carried in hand baggage. Explosives in a toilet bag, certainly inside a shoulder bag would be enough to meet the terrorists' needs.

They could be quite hard to detect because I do not think any of the things we have mentioned would respond to x-rays. For example, a liquid hydrocarbon fuel could pass as mineral water.

The question is how do you get something packed into a bag so it does not look suspicious?

Dr Clifford Jones is a reader at the University of Aberdeen's Department of Engineering

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