Using acids
It was a wet afternoon at the end of term. Mrs Smith, the teacher, had said Year Six could watch a film. The class had settled down to watch the film when after five minutes they were disturbed. "This is a good bit!" cried Andrew. "The bottle of acid gets knocked over and it burns through everything - just like all acids."

"No, it isn't!" shouted Sarah. "Vinegar is an acid but it doesn't burn you."

"It tastes sour though," chipped in Jawed.

"You shouldn't taste acids. They can harm you," added Paul.

"Look!" shouted Mrs Smith. "Are we watching this film or are we talking about acids?"

"Let's talk about acids," said the class and Mrs Smith agreed.

She began by telling the class about how acids got their name. Their name comes from the Latin word acidus, which means sour tasting. As Andrew listened he was unaware that his body had made two acids that morning and was at that moment making a third. When Andrew had breakfast his stomach made an acid called hydrochloric acid. It helped him digest the protein in his breakfast and killed any germs that he ate with his food. When he ran for the bus, his muscles made lactic acid to give him the energy he needed. While he sat in class his body, like all bodies, was making uric acid from some of the substances he had eaten with his food. He would later get rid of this acid, in urine, when he went to the toilet.

"Most of us put acid on our chips!" exclaimed Mrs Smith and held up a bottle of vinegar.
"It gives them a sharp taste," she continued, "but what does it do when we put it on an eggshell?"

Mrs Smith put some eggshell in a beaker and added vinegar to it. The brown liquid swirled around the broken pieces of eggshell and nothing seemed to be happening. Slowly a small bubble appeared on one piece of shell. More bubbles appeared and all grew in size. Some grew so large they left the shell, rose to the surface of the vinegar and popped.

"What is going on?" asked Sarah.

Mrs Smith answered by telling the class that the vinegar and the eggshell were taking part in an irreversible change. Eggshell is made from a substance called calcium carbonate. The acid in the vinegar attacks the calcium carbonate and breaks it up. When this happens, one of the substances that is produced is called carbon dioxide. This is a gas and it makes bubbles on the surface of the shell.

Although they did not know it, calcium carbonate in the neighbourhood was slowly being broken up by a huge amount of acid.

Across the road from the school was a church made of limestone. It had been built hundreds of years ago and had angry looking gargoyles staring down at passers-by. Over the years their faces had become less threatening and this was due to an acid - in the rain. Limestone is made from calcium carbonate, just like eggshell. When the rain falls on the limestone the acids break down the calcium carbonate, carbon dioxide is released and the other substances that are made in the change are washed away.

Back in school Year Six were preparing to pack away.

"What would happen if you put a whole egg in a bottle of vinegar?" asked Jawed.

"I think all the shell would dissolve and you would be left with an egg in a soft skin," answered Sarah.

Could she be right? You could try it and see.

Are all acids safe?
No, they are not. There are two kinds of acids. There are weak acids and strong acids. Many weak acids are made naturally by living things and most will not harm you. Strong acids are not natural. They are made. They can be harmful if they are not handled with great care.

What harmless acids are found in fruit?
They are citric acid, which is found in oranges and lemons, tartaric acid found in grapes, and ascorbic acid, which is also known as vitamin C, found in oranges, lemons, limes and blackcurrants.

Do living things make harmful acids?
Yes, they do. The nettle makes methanoic acid. It stores this acid in its stings. When you break a sting on a nettle leaf the acid enters your skin and gives you pain. Ants make the same acid and use it in their stings.

Where does the acid in acid rain come from?
Some acid rain is natural. It forms because carbon dioxide in the air dissolves in raindrops and produces a weak acid called carbonic acid. This attacks limestone and is responsible for the formation of potholes and caves that are found in limestone country. A great deal of acid rain is made from gases, which are released from power stations where coal and oil are burnt. One of these gases is called sulphur dioxide. It dissolves in raindrops to form sulphuric acid.

What are the names of some strong acids?
Three widely used acids are sulphuric acid, hydrochloric acid and nitric acid.

What are these acids used for?
Sulphuric acid is used in the making of paint, plastics and detergents. It is also used in car batteries. Hydrochloric acid is used to make materials for photography and in the making of dyes for clothing. Nitric acid is used for making fertilisers for farms and explosives for quarries.

If these acids are so strong why are they not kept in metal containers instead of glass bottles?
They are kept in glass bottles because the acids do not take part in any changes with glass. All the acids take part in an irreversible change with metals. They dissolve the metal and make a gas called hydrogen. If you stored them in a metal tin they would escape from it by making a hole and produce an explosive gas at the same time.

Does the acid in vinegar have a name?
Yes, it does. It is called ethanoic acid. It used to be called acetic acid and some older people may still think that this is the correct name.

What happens when you put vinegar on bicarbonate of soda?
The scientific name for bicarbonate of soda is sodium bicarbonate. It contains atoms of sodium, carbon, oxygen and hydrogen. They are linked together in a certain way and form a white powder. When vinegar is added to bicarbonate of soda, the arrangement of the atoms is destroyed in an irreversible change. The atoms rearrange themselves into other substances and one of these substances is carbon dioxide. This forms bubbles of gas and makes the vinegar fizz.

Why does a seltzer tablet only fizz when it is wet?
A seltzer tablet contains sodium bicarbonate and tartaric acid. When they are dry the two substances cannot reach each other so no change takes place between them. When a seltzer tablet is put in water the two substances meet, an irreversible change takes place and carbon dioxide is produced to make the tablet fizz.