Higher and foundation tier


The word acid sounds dangerous, indeed many acids are dangerous but many acids occur naturally and some are safe enough to eat e.g. citric acid in citrus fruits (oranges, lemons, grapefruits - in fact it is the acid that is responsible for the sour taste of many fruits and other foodstuffs). Other common acids include:

Common everyday acids Name of acid
vinegar ethanoic acid
stomach acid hydrochloric acid
battery acid sulfuric acid
fizzy drinks carbonic acid
common acids


corrosive symbol Hydrochloric, nitric and sulfuric acid are the three common mineral acids used in the school laboratory. These are strong acids and when fairly concentrated they are also corrosive. This means that they will attack and destroy living cells and tissue, the corrosive symbol shown opposite is found on many bottles of acids and alkalis. However most of the acids you use in school are dilute solutions, this means that they have lots of water in them. These dilute acids are irritants and may cause slight inflammation or reddening of the skin


It is possible to decide if a substance is acidic or not by simply adding an indicator to it. Indicators are simply mixtures of dyes which turn different colours in acidic or alkaline conditions. One of the most useful and widely used indicator is universal indicator. We use indicators along with the pH scale to decide how acidic or alkaline a substance is. If a solution has a pH of 7 it is neutral, it will turn universal indicator green. If a solution has a pH of less than 7 it is acidic. If the pH is 1, 2 or 3 we say that the acid is a strong acid, it will turn universal indicator red. Weak acids have a pH above 3 but less than 7, the colours these acids turn universal indicator are shown in the the image below. If the solution has a pH above 7 we say it is an alkali or an alkaline solution. Strong alkalis have a pH of 13 or 14, weak alkalis have a pH of 8 or 9. The colour range for universal indicator is shown below:

colour range for universal indicator

Acids and pH

Acids are solutions which have an excess of hydrogen ions (H+) present. The common properties we associate with acids are all due to the presence of hydrogen ions (H+). The pH of a solution, whether it is an acidic, neutral or an alkaline solution depends on the concentration of the hydrogen ions, H+(aq). The lower the pH the higher the concentration of hydrogen ions, H+(aq). To change the pH of an acid or alkali by 1 the concentration of hydrogen ions must change by a factor of x10.

pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
concentration of hydrogen ions, H+ x10 x10 x10 x10 x10 x10 x10 x10

This means for example:

We simply change the concentration of the hydrogen ion by diluting with water. 1ml of acid added to 9ml ofwater will give a dilution factor of x10.

Common properties of acids

Acids are aqueous solutions (aqueous simply means in water). Acids are formed when a substance dissolves in water to release an excess of hydrogen ions, H+(aq), into the water. All acids contain the element hydrogen in their chemical formula. The table below shows the name and chemical formula of common acids e.g hydrochloric acid contains the elements hydrogen and chlorine, sulfuric acid contains the elements hydrogen, sulfur and oxygen.

Acid Molecular formula
hydrochloric HCl
sulfuric H2SO4
nitric HNO3
ethanoic CH3COOH
carbonic H2CO3

citrus fruits contain citric acid Some acids have a sour taste. Citric acid is often used in sweets to give them a sour taste. Lemons, grapefruit and other citrus fruits are also sour due to the presence of citric acid. Citric acid is often used as a preservative for foods, it helps prevent the browning of meats and vegetables. Citrus fruits also contain another acid called ascorbic acid. This acid is better known as vitamin C.

Fizzy drinks such as Coca Cola are also acidic, they contain a weak acid called carbonic acid, vinegar or ethanoic acid is another common weak acid, we are all familiar with the sharp, sour taste of vinegar on our chips!

Acids in the lab

The acids you are most likely to use in the science lab are hydrochloric acid and sulfuric acid. These are often used as dilute solution, these solutions are likely to be classed as irritants. As the concentration of the acid increases, generally around 2 moldm-3 then the hazard changes from irritant to corrosive.

sulfuric and hydrochloric acids in bottles

Making acids

Acids are normally made by burning a non-metal and then dissolving the non-metal oxide produced in water, e.g. Sulfur is a non-metal, it is a yellow solid that melts easily when heated to form a thick tacky brown liquid which burns with a small blue flame. The smelly and toxic gas sulfur dioxide is released as the sulfur burns. A word and symbolic equation for this combustion reaction is shown below:

Sulfur(s) + oxygen(g) → sulfur dioxide(g)
S(s) + 02(g) → SO2(g)
If the sulfur dioxide which forms dissolves in water then an acid form:
sulfur dioxide(g) + water(l)sulfurous acid(aq)
SO2(g) + H20(l) H2SO3(aq)
This can be easily done in the science lab as shown:

sulfur burning to form sulfur dioxide gas

If the burning sulfur is quickly transferred to a gas jar containing a little water, the toxic sulfur dioxide gas quickly fills the jar. If the jar is shaken gently the sulfur dioxide gas will dissolve in the water forming sulfurous acid. If a little universal indicator is added it quickly turns red showing the presence of the sulfurous acid formed. A similar experiment can be done with other non-metals and the non-metal oxides which are produced are all acidic. That is they dissolve in water to form acids e.g.

nitrogen dioxide(g) + water(l) nitric acid(aq)
carbon dioxide(g) + water(l) carbonic acid(aq)
and in general
non-metal oxide(g) + water(l) acid(aq)

Key Points

Practice questions

Check your understanding - Questions on Acids