This is hydrogen.

In this photograph, you can see a weather balloon. It's being filled with gas. Once it is filled, it will be released.

The balloon's job is to carry instruments high into the atmosphere. The instruments make measurements of the weather. Then they send the information back to Earth.

Not every gas is useful for filling weather balloons. The gas must be much less dense than air; otherwise the balloon won't lift the heavy instruments off the ground.

The gas normally used is hydrogen. It’s ideal for this. It’s much less dense than air, and quickly floats upwards when released. In fact, hydrogen has the smallest density of all substances.

A test tube of hydrogen seems to be empty. Hydrogen gas doesn't have a color. (It doesn't have a smell either!) But if you put a burning splint to the mouth of the test tube, you will know that there is something in it. Hydrogen burns with a pop when mixed with air. This test allows you to pick out hydrogen from oxygen, nitrogen, and carbon dioxide.

If you put a jet of burning hydrogen onto a cold surface, a colorless liquid collects. The

Liquid boils at 100C. In other words, the liquid is water.

When hydrogen burns, water is made. Water is a compound, made by joining together the elements hydrogen and oxygen. Its chemical name is hydrogen oxide.

You can use this apparatus to pass an electric current through water. When the current is

Switched on, bubbles of gas appear on the two rods. (If a little acid is added, the gases are made more quickly. The acid helps the current to flow.)

Hydrogen is made at one rod, and oxygen at the other. The electrical energy splits the water into hydrogen and oxygen.