Paul Douglas of Tokoiti asks :-

What forms can hydrogen take?

Keith Gordon, a chemist at Otago University, responded.

Hydrogen is the most common element in the universe (more than 75 per cent by weight) and the simplest (having one electron buzzing about one proton). In nature we find that hydrogen atoms pair up to make hydrogen gas, H2, or with oxygen to give water (H2O) or with carbon and some other elements to give protein, fats, carbohydrates and petrol.

Hydrogen has become very topical in recent years as the idea of it as a replacement for petrol has been suggested. In theory this is a wonderful idea as burning hydrogen just gives water - no CO2 or other pollutants. So how much energy is there in hydrogen? If you had a glass (500 mL) full of petrol it can take your car about 5 km and will have a mass of about 350 grams. With pressurised cylinder gas of the same volume the hydrogen would have a mass of 8 grams and propel the car about 400 m. Hydrogen has about 4 times the energy in it per gram than petrol.

To get a higher fuel density you can liquefy the hydrogen (which needs a temperature of about -250 Celsius), 500 mL of liquid hydrogen has a mass of 36 grams and the car gets 2 km on it. At -260 Celsius it solidifies. It can also be stored as a gas at almost liquid densities between the atoms in precious metals. Palladium is by far the best metal for this. 500 mL of palladium can absorb 34 grams of hydrogen and assuming it could be released this would move the car about 2 km. Of course the total mass of the hydrogen and the 500 mL of palladium is then 6 kg. So if the weight of the fuel matters hydrogen has the edge but takes up much greater volumes.

There are certainly a few problems with making hydrogen a fuel but Ford have a car that uses a hydrogen internal combustion engine and Boeing a plane that uses hydrogen in a fuel cell - so it is being taken seriously by industry.