Chemistry is going on around us all the time and we meet chemical reactions every time we cook, light a fire or watch a firework display. Today I'm going to talk about acids and bases in everyday life. Fundamental to understanding chemistry is knowing about acids and bases, what they are and what reactions they take part in. Let's start by finding out exactly what an acid and a base is then see where you will come across their reactions every day.
Acids
By definition an acid is a substance that releases hydrogen ions (H+) in water.
Examples of some acids are hydrochloric acid (HCl found in your stomach), nitric acid (HNO3 used in fertilisers) sulphuric acid (H2SO4 used in batteries), phosphoric acid (H3PO4 used in colas). Acids that are present in natural substances include ethanoic acid (or acetic acid CH3COOH present in vinegar), citric acid found in citrus fruits and lactic acid found in milk. All of these acids have a hydrogen atom in their formula which can be released when the acid is in contact with water. So HCl reacts in this way when it comes in contact with water.
Bases
One definition of a base that is often used is that it is a substance that releases hydroxide ions (OH-) in water. Another, and better, definition proposed by Johannes Brønsted and the Thomas Lowry independently in 1923, defined a base as a substance that accepts hydrogen ions. This definition is preferable as it includes ammonia which contains no hydroxide ions but does react with acids in the presence of water to form form a neutral solution.
Examples of bases include sodium hydroxide (caustic soda NaOH) found in some oven and drain cleaners, lime Ca(OH)2 used to make cement, magnesium hydroxide Mg(OH)2 used as an antacid, baking soda (NaHCO3) a raising agent used in cooking and ammonia (NH3) found in some cleaning products.
Note that we said above that an acid releases hydrogen ions (H+) in water. This is important as acids and bases only behave as acids and bases when they are dissolved in water. As an example lets look at HCl. HCl is, in fact, a small molecule that is a gas and does not behave like an acid. Only when it is dissolved in water does it become hydrochloric acid and exhibit the properties of an acid.
The pH Scale
The pH scale is used to show how acidic or basic a substance is. pH 7 is neutral, acids have pH's below 7 and bases above 7. The diagram shows the pH of some common substances.
The pH is found by using the following formula:
pH = -log[H+] note - we use square brackets to denote concentration in mol/l so [H+] reads as the concentration of hydrogen ions.
So an acid with a [H+] of 0.1 mol/L has a pH of 1
Making Your Own pH Indicator
It's easy to make an indicator from red cabbage. An indicator is a substance that changes colour according to the pH of the solution. Red cabbage juice contains the pigment flavin which is red in acidic solutions, purple/blue when neutral and green or yellow when basic. Chop up some red cabbage cover with boiling water for about ten minutes to extract the juice or use a blender with some hot water. Filter the resulting solution with a coffee filter to obtain a clear purple coloured solution. You can then use this to test some of the chemicals we've discussed here. The image above (by Yves.brenner) shows a range of household chemicals. From left to right water (neutral), a strong acid (acidic), vinegar (slightly acidic), ammonia (slightly basic) and sodium hydroxide (basic).
Neutralisation
When the hydrogen (from an acid) and the hydroxide ion (from a base) come together a molecule of water is formed which is neutral. A simple way to remember this is that 'an acid and a base gives a salt and water'.
For example:
If we simplify this to the action of the H+ ions (from the acid) with the OH- ions (from the base) we get the neutralisation equation
Acids and Bases in Everyday Life
Now we know what acids and bases are let's see how we might come across their reactions in everyday life.
Baking Soda
Baking soda (sodium hydrogen carbonate NaHCO3) is slightly basic and is used as a raising agent in cakes, scones and other baked goods. Some recipes for scones include tartaric acid to make the scones rise more. This is because, when an acid reacts with a carbonate or a hydrogen carbonate, carbon dioxide is produced. The carbon dioxide expands as the temperature increases and the scone rises.
The Fizz in Antacids
Some antacids fizz when you add them to water. Citric acid is added to antacids so that it reacts with sodium hydrogen carbonate (NaHCO3) to produce carbon dioxide (as noted above) thus making them more pleasant to drink!
Phosphoric Acid
Many colas include phosphoric acid to provide flavor. It's often used in preference to citric acid which is more expensive. Phosphoric acid is also used to make products for dealing with rust such as gels and liquids. The brown flaky iron oxide is converted to black ferric phosphate which is more solid than iron oxide and provides protection against further rusting.
Phosphoric acid
Acid Rain
Acid rain is polluting many parts of the world and is a result of burning fossil fuels. Although coal and oil are made up mostly of carbon compounds they also contain small amounts of sulphur from the proteins that were present in the original plants. Sulphur, when burned, produces sulphur dioxide which dissolves in water in the clouds to form sulphurous acid. This then reaches the earth in the rain. Another cause of acid rain is the nitrogen oxides released by motor vehicles, resulting in the formation of nitric acid and nitrous acid in the clouds. Acid rain damages plants and buildings and can enter the water courses, damaging aquatic life.
Conclusion
The reactions of acids and bases in everyday life is a fascinating subject and many professional chemists have started out in their chosen careers after being intrigued by the chemical processes going on around them every day. This is only a short introduction so I hope, after reading this, you will be as interested in chemistry as I am and be eager to find out more about this interesting subject .
