Acids are chemicals which will turn litmus paper red. Litmus is a coloured chemical which can change from red to blue and back again. Which colour it is depends upon the concentration of Hydrogen ions.If the concentration of Hydrogen ions is higher than it is in pure water then the litmus will turn red. If it is lower than in pure water the litmus will turn blue.
Acids all produce Hydrogen ions.
Have a look at my pH page for more information about pH (this is a measure of the concentration of Hydrogen ions). Hydrogen Chloride is a gas; it consists of molecules each containing one atom of Hydrogen and one atom of Chlorine. When this gas is dissolved in water the molecules of Hydrogen Chloride dissociate: this means that the Hydrogen and Chlorine atom get separated. When they do this it is rather like getting divorced, i.e. it is not fair, one partner gets a bit more than the other. In this case the Chlorine atom gets an extra electron and becomes a Chloride ion. The Hydrogen atom loses its electron and becomes a Hydrogen ion (actually this is just a single proton). Have a look at my ions page to find out more about ions.
Hydrogen ions have a positive charge and Chloride ions have a negative charge. It is when Hydrogen ions combine with litmus that the litmus turns red. So when litmus loses its Hydrogen ions it turns blue. This is what happens when you test an alkali with litmus. An alkali is the opposite of an acid; alkalis produce virtually no hydrogen ions.
Here are some of my favourite acids (as you will see they all produce Hydrogen ions when they dissociate); I have put them in rank order according to how much I like them:
Hydrochloric Acid: this is a solution of Hydrogen Chloride dissolved in water. It has a pH of about 2. I like it because it helps me to digest protein in my stomach. I can remember my father drinking bench strength HCl to help him digest his food after being ill. His stomach had stopped making the acid. Do NOT try this yourself, it does not taste nice, and it will dissolve your teeth. (I was only one year old when he did this, so I can’t really remember this but he told me about it. When you vomit (are sick) the contents of your stomach are thrown up and out through your mouth: it tasted nice when you ate the food, but with the acid in it, it tastes horrible!
HCl = H+ + Cl–
Sulphuric Acid: this is a solution of Sulphur Trioxide in water. Do NOT try to dissolve crystals of Sulphur Trioxide in water as it is very dangerous. This is my second favourite acid because I put it in my motorcycle battery. Do NOT mess around with battery acid, if you get it on your skin it will burn nice little holes in you; it will also do this to your clothes if it gets on them. Be careful when you charge your motorcycle battery: when you take the little caps of the battery cells it can spit acid whilst you charge it. If you forget to take the caps off your battery might explode. You have been warned. Pouring water into concentrated Sulphuric acid is not a very good idea; if you really must dilute concentrated Sulphuric Acid to make battery acid, do it the other way around. Pour a very little acid at a time into the water and make sure you let it cool down each time you add the acid before the next batch. You have been warned, do NOT mess with this acid.
H2SO4 = 2H+ + SO42-
Ethanoic Acid: biologists tend to call this Acetic Acid. It is my third favourite acid because I pour it all over my food to make it taste sharper. Mind you, I only use very dilute acid. I also keep food in this acid so that it does not go rotten. I expect you do this to, but you probably call it vinegar which means sour wine. The easiest way to make this acid is to take some fruit and let it ferment. What happens is that yeast turns the sugar into Ethanol (alcohol); if you now put some Acetobacter in this wine it will turn all the Ethanol into Ethanoic Acid (vinegar). Acetobacter is a bacterium which makes wine go sour by producing Acetic Acid.
CH3COOH = H+ + CH3COO–
Carbonic Acid: this is also fun, because you can drink it. Why drink tap water when you could drink this acid; it will cost a bit more! The equation below does not show you everything that this acid can do. It is made by dissolving Carbon Dioxide in water under pressure. When you pull the ring on the top of your fizzy drink you remove the pressure and allow the Carbon Dioxide to escape. You should be able to work out the equation for this.
H2CO3 = 2H+ + CO3 2-
Nitric Acid: this is really nasty especially if it is concentrated. A drop of Nitric Acid on your skin will produce a super burn. I once tried putting some concentrated Nitric Acid onto an old tree stump in my dad’s garden. The tree stump got very hot and started to burn.
HNO3 = H + + NO3–
If you have read this page carefully and had a look up my ions page, you should be able to write down the formulae of some other acids. You should also be able to work out what acids are found in “acid rain”. Some countries like to burn fossil fuels to make oxides of Nitrogen and Sulphur. When these oxides in the smoke from factories dissolves in the water in clouds it makes acid rain. So if one country wants to annoy another country they could make lots of smoke which will cause acid rain and kill the other country’s trees!
Other Acids: Hydrochloric acid, sulphuric acid, nitric acid, carbonic acid and phosphoric acid are all inorganic acids. Ethanoic acid is an organic acid. There are some other interesting, useful and annoying acids. One of the most annoying acids is Lactic Acid. Lactic means milk. This acid is produced when milk goes sour; so I find it annoying. You may also find it a nuisance when your muscles make it and you get cramp. Citric acid is found in fruits, especially citrus fruits: this is what makes them taste sour. Ascorbic acid is another one found in fruits. Ascorbic acid is also found in vegetables: it annoyed me when I was a child because my parents always wanted me to eat up my vegetables because they contained ascorbic acid, only they called it Vitamin C!!!