Many chemical reactions take place only very slowly, if at all, without a catalyst. A catalyst is a substance that controls the rate of a chemical reaction but is not itself in any way changed or used up. The control of a chemical reaction by a catalyst is called catalysis.
The reaction between zinc and dilute sulphuric acid takes place only very slowly without a catalyst, but a few drops of copper sulphate solution will catalyse the reaction and cause it to take place very much more rapidly.
Similarly hydrogen peroxide normally decomposes very slowly but in the presence of a catalyst its rate of decomposition is greatly increased.
This is an example of catalytic decomposition. The reaction can be catalysed by manganese dioxide, but also by platinum gauze or copper gauze and some other substances. If we use concentrated hydrogen peroxide, in the presence of a catalyst the reaction is so violent and so much heat is evolved that we get not liquid water but superheated steam. Hydrogen peroxide has been used as a rocket fuel, for example in the rocket-propelled fighter (Komet) developed by Germany during the last stages of the Second World War.
It seems that catalysis takes place on the surface of the catalyst, so the larger the surface area of the catalyst the more effective it is. Catalysts are often used in the form of a powder or a gauze (woven with a very open weave, like a gauze bandage).
Most modern cars are fitted with a catalytic converter (cat). This is usually a piece of platinum gauze, with a very large surface area, in the exhaust system, which catalyses the conversion of some of the more dangerous or unpleasant exhaust emissions into relatively less harmful substances.
The exhaust emissions from cars using leaded petrol contain lead compounds which coat the surface of the platinum gauze and reduce and eventually destroy its effectiveness as a catalyst. You must therefore never use leaded petrol in a car fitted with a cat. Most cars built within the past twenty years are fitted with cats.
Catalysts are very widely used in industry. The Haber Process for making ammonia uses iron as a catalyst. Oil refineries use a tall tower called a cat cracker, in which some of the more complex substances in the petroleum are broken down (cracked) by catalytic action into simpler substances.
Catalysts are also used in the manufacture of many important chemicals including sulphuric and nitric acids and artificial fertilizers.
Most of the chemical reactions taking place in our bodies, and inside all other living things, are controlled by catalysts. We call these reactions biochemical reactions, and the biological catalysts that control them are called enzymes.
Most non-biological chemical reactions that require a catalyst can be catalysed by several different substances, and many substances, particularly platinum gauze, can catalyse many different reactions. But the action of an enzyme is very specific: each enzyme can catalyse only one biochemical reaction, and each biochemical reaction can be catalysed by only one enzyme.
Enzymes
are used in digestion and in almost all other of the
chemical reactions taking place in our bodies. If an enzyme is
absent the biochemical reaction which it catalyses cannot take place and
we shall be ill and may even die. There is more about digestion in another page on this web site - to read it please click here 
Enzymes are proteins and are very complex compounds of carbon, hydrogen, oxygen, nitrogen and many other elements. Trace elements (elements which we need in minute quantities, such as selenium, zinc, iodine and magnesium) are needed to make many enzymes, and if some of these trace elements are missing from our diet we cannot make all the enzymes which we need.
Similarly many hereditary diseases such as cystic fibrosis are caused by a single faulty gene which prevents us from making one particular enzyme.
When an organism dies the enzymes that were active at the time of death will continue to be active, and the biochemical reactions catalysed by them will therefore continue to take place. This is why fruit and vegetables start to lose their colour and freshness once they are picked, and is the cause of conditions such as rigor mortis in Man and many other animals.
An enzyme present in saliva catalyses the conversion of starches, which are not soluble in water, into sugars, which are. Biological washing powders contain enzymes which work in a similar way on many types of biological stain such as grass, blood and food, converting insoluble substances into soluble substances.
Enzymes are also used in making butter, cheese and yoghurt, and today are used in many industrial processes.