Acids and alkalis can be found in a chemistry laboratory, but also in our houses! Acids are substances with a tangy taste... that's right, like lemons. Fruit like lemons and oranges contain citric acid, which gives them that sharp taste. They also contain ascorbic acid, which is Vitamin C.
As you can see, acids are useful, good for you and tasty! Well, maybe not in the case of vinegar. However, not all acids can be eaten; some are very harmful and can corrode your skin, whereas weaker ones are irritants, which means they will give you a rash if they come into contact with your skin. Here is the hazard symbol for strong corrosive acids.
At school you are likely to use the following dilute acids: hydrochloric acid, sulfuric acid and nitric acid. All acids contain hydrogen.
Alkalis can also be found in the laboratory or in our households. The most common laboratory alkali is sodium hydroxide. Alkalis, generally, feel a bit like soap. Actually, they were used as cleaning products by ancient Arabs. They were made with ash and animal fat. Today, most cleaning products are alkalis, for example, toothpaste, soap and oven cleaners.
In a way, alkalis are the opposite of acids. There can be weak alkalis and strong alkalis, which are caustic, like caustic soda. Caustic substances can burn our skin.
There is a way to identify whether a substance is an acid or an alkali. It is called the litmus test. A special paper called litmus paper is dipped into the substance. There is red litmus paper and blue litmus paper. No matter what the initial colour is, acids will turn litmus paper red and alkalis will turn litmus paper blue. A neutral substance, one which is neither an acid nor an alkali, will turn litmus paper purple.
There is another test, which can also tell us how acidic or alkaline a substance is - this is the pH test. You can see the pH scale below.
There is a special solution called universal indicator solution. When a few drops of universal indicator are added to a substance, it turns a colour. This colour matches one of the colours on the pH scale and that's how you determine how acidic or alkaline a substance is.
Acids and alkalis react together in a reaction called neutralisation. The products of neutralisation are always water and a salt. Beware: even though common salt, the one we add in our food, can be one of those salts, there are many others and their type depends on the specific acid or alkali that reacts. Salts and water are neutral substances. This is the general neutralisation reaction:
acid + alkali → salt + water