Gas explosions and the Pike River coal mine disaster.

29 miners were killed recently either as a direct result of a methane gas explosion or its aftermath. Why do some gas air mixtures combust explosively.

In the science laboratory you should be familiar with the “pop test” for hydrogen. The” pop” is caused as the hydrogen in a test tube explodes when a lit match is placed under it. Have you noticed that sometimes there is no explosion or only a gentle noise. The full explosion is only observed when the hydrogen air mixture is just right.
What about the science?

For combustion to take place you need a fuel and air. In the case of gasses there are a number of possibilities.

  • There is insufficient fuel in the mixture for combustion to occur
  • There is insufficient oxygen present to allow combustion to take place
  • The fuel/air mixture allows the fuel present to combust explosively.

The mixture that has just enough fuel to burn is known as the lower explosion limit. The mixture that has just enough oxygen present to combust is known as the upper explosion limit. Gas air mixtures between these limits will explode if there is a source of ignition.
The lower explosion limit for methane one of the gasses found in mines is 5% and the upper limit is 15%. This is why ventillation in a mine is so importan. Once the lower explosion limit is reached it only takes a hot surface to cause ignition of the gas and an explosion.

In the experiment demonstrated the fuel used to cause an explosion is hydrogen.


  1. The can has two holes in it. Why do you think the hydrogen burns for a while before exploding?
  2. Why do you think there might have been a sequence of explosions in the Pike River mine?
  3. It has been reported that the safest time to have entered the mine would have been during the two hours immediately after the original explosion. Does this make sense?
  4. How would you make the mine safe for people to enter?
  5. Is coal mining on the west coast too dangerous?
  6. Should we be conserving the environment and limiting green house gas emissions?

Year 11 chemistry:balancing chemical equations, reactions of metals and acids

Year 11 chemistry:balancing chemical equations, reactions of metals and acids

Chemistry has its own language with words and sentences.

Chemistry has its own language with words and sentences.

To a chemist the words are chemical formulae. When chemical formulae are joined together in a sentence we call this a chemical equation.

Hydrogen and oxygen mixtures explode when lit. In a test tube this explosion is heard as a “pop”. We call the test for hydrogen “the pop test”

The space shuttle is launched into space with three main engines. Each engine produces 1.8 Meganewtons of thrust. The engines are powered by liquid hydrogen and oxygen fuel stored in the large central rust coloured tank.

The space shuttle is blasted into space on the back of a mega “pop test” The two tanks on either side of the shuttle are solid fuel boosters.

Hydrogen molecules and oxygen molecules react with each other to form water molecules. A chemist rewrites this sentence as a word equation

hydrogen + oxygen —–> water

Chemists use chemical formulae instead of words. Finally chemists need to count the numbers of atoms involved. We need to have the same numbers of atoms after the reaction that we had at the start.

When we do this we end up with a fully balanced chemical equation. As far as NCEA level 1 is concerned here are the steps.

  • word equation…..achievement
  • with chemical formulae……merit
  • a fully balanced chemical equation…..excellence

Check out the video to see how we get a fully balanced equation for the “pop test”

try balancing the equations that describe metals reacting with acids.

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