There are many different types of chemical but two important ones are acids and bases. But what are they and why are they important?
You will no doubt have used universal indicator paper many times in your school laboratory and should know that acids have a pH of less than 7.
But what is it that makes an acid acidic? Let’s take a look at some formulas:
- HCl – hydrochloric acid
- HNO3 – nitric acid
- HF – hydrofluoric acid (the one that corrodes glass and is stored in plastic containers)
- H2SO4 – sulphuric acid
- H3PO4 – phosphoric acid
They are all ionic compounds that dissolve in water and when ionic compounds dissolve in water, the ions come apart. In the case of acids, the cation is always the H+ ion and it is that which makes them acids.
Strong and Weak Acids
When a strong acid dissolves in water, pretty much all of the ions in the structure separate from each other so there are lots of hydrogen ions floating around . If an acid is weak, only a few of the ions separate.
The technical term for a chemical coming apart is dissociation so we say that a strong acid is fully dissociated in water and a weak acid is weakly dissociated in water..
When you measure the pH of a liquid, the universal indicator detects the amount of dissociation of the acid. Fully dissociated acids turn it red (pH1 and 2) whilst weakly dissociated acids turn it orange (pH3 to 6). Weak acids are less corrosive than strong acids.
A base is the chemical opposite of an acid, in fact they react together in a neutralisation reaction to produce salts. Let’s look at the chemical formulae of some bases:
- NaOH – sodium hydroxide
- Na2CO3 – sodium carbonate
- MgO – magnesium oxide
- CaO – calcium oxide
- CaCO3 – calcium carbonate
- KOH – potassium hydroxide
- CuO – copper oxide
From the formulae, you can see that they are usually either the oxides, hydroxides and occasionally carbonates of metals. The situation is more complex than this but for starters, that rule usually works.
Some of the bases that are metal hydroxides dissolve in water. They dissociate to produce the metal ion and the hydroxide ion. A substance that produces hydroxide ions like this is called an alkali. ALL alkalis are bases but NOT ALL bases are alkalis, there are therefore more bases than alkalis. Like acids, strong alkalis are fully dissociated in water and vice versa. Strong alkalis have pH values of 12 to 14 (blue) whilst weaker alkalis give pH values of 8 to 11 (shades of blue-green).
Why are Acids and Bases Important?
They are used in many industries to manufacture a wide variety of everyday items, building materials and medicines, for example aspirin (made from acetylsalicylic acid).
Uses of acids
- Sulphuric, hydrochloric and phosphoric acids are used to clean rust from metals; phosphoric acid leaves a protective coating on the metal surface that can be painted over.
- Nitric acid and phosphoric acids are used in the manufacture of fertilisers, phosphoric acid is used to manufacture detergents for washing clothes and in dishwashers.
- Weak acids are found in foods, for example, acetic acid is the acid found in vinegar. The sour taste in foods is because of these weak acids.
- Acids are important in your body too. Amino acids are weak acids that join together to form DNA, without these acids, life as we know it would not exist. Your stomach contains a very dilute solution of hydrochloric acid.
- Sodium hydroxide is a strong base (and a strong alkali as it dissolves in water) that is used in manufacture of soap, paper, bleach and some types of synthetic fibre used to make your clothes. You will probably have used it in science lessons.
- Calcium hydroxide has been used in the manufacture of cement since Roman times.
- Magnesium hydroxide is a weak base that is used as an ‘antacid’ to neutralise excess hydrochloric acid in the stomach to cure indigestion.
- Sodium carbonate is used as washing soda and for softening hard water. It is a weak base.
- Sodium hydrogen carbonate is a weak base used as baking soda in cooking food, for making baking powders, as an antacid to cure indigestion and in soda acid fire extinguisher.
Without acids and bases, many things that seem very ordinary and that you may take for granted would not exist and your life would be very different indeed. What acids, bases and items made from them can you find in your home?