Acids and alkalis

Acids

Acids are substances which ionise to produce hydrogen ions (H+) when in an aqueous solution. The equation representing this ionisation is:

Common acids used in the laboratory include:

• Hydrochloric acid (HCl)
• Nitric acid (HNO₃)
• Sulphuric acid (H²SO₄)
• Ethanoic acid (CH₃COOH)

Alkalis

Alkalis are substances which produce hydroxide ions (OH^–) when added to water to form an aqueous solution. The equation representing this ionisation is:

Common alkalis used in the laboratory include:

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)
• Ammonia (NH₃)

Indicators

Indicators are chemicals which change colour when added to acidic or alkaline solutions. The colour of the solution indicates whether it is acidic or alkaline.

There are lots of different indicators that can be used. Three common indicators which you must be able to describe the use of are:

• Litmus – litmus solution is purple when added to neutral solutions, turns red when added to acidic solutions and turns blue when added to alkaline solutions. Litmus can also be used as an indicator in paper form where it exists as either red litmus paper or blue litmus paper. Red litmus paper turns blue in alkaline solutions. Blue litmus paper turns red in acidic solutions.
• Phenolphthalein – phenolphthalein solution is colourless in neutral and acidic solutions. It turns pink when added to alkaline solutions.
• Methyl orange – methyl orange solution is orange in neutral solutions, turns red when added to acidic solutions and turns yellow when added to alkaline solutions.

The pH scale

The strength of an acidic or alkaline solution is measured using the pH scale as shown in the diagram below:

The pH scale runs from 0 to 14 and is used to classify solutions into their strengths as acids and alkalis.

• A solution with a pH of 0-3 is strongly acidic
• A solution with a pH of 4-6 is weakly acidic
• A solution with a pH of 7 is neutral
• A solution with a pH of 8-10 is weakly alkaline
• A solution with a pH of 11-14 is strongly alkaline

The indicators mentioned previously are used to determine whether a solution is acidic or alkaline, but cannot be used to indicate the strength of the solution.

Universal indicator is another indicator which can be used to establish the acidity or alkalinity of a solution. Universal indicator differs from the indicators listed previously in that it is a mixed indicator which contains several different colour changing substances which all change colour at different strengths of acidity or alkalinity.

This mixture of colour changing substances means that universal indicator can be used to measure the approximate pH value of an aqueous solution. If universal indicator is added to a neutral solution it will appear green. When universal indicator is added to an acidic solution, it will show a colour in the range of light green to red depending upon the strength of the acid. A strong acid will show a red colour and a weak acid will appear yellow. When universal indicator is added to an alkaline solution, it shows a colour ranging from green-blue to purple depending upon the strength of the alkali. A blue colour indicates that the alkali is weak and a strong alkali produces a dark purple colour in the solution.

Neutralisation

A neutral solution has a pH of 7. It is neither acidic nor alkaline in nature. Adding an alkali to an acid causes the pH of the solution to increase from a pH below 7 to a pH of 7.

Adding an acid to an alkali causes the pH of the solution to decrease from above 7 to a pH of 7. The addition of an acid or alkali to produce a neutral solution with a pH of 7 is known as a neutralisation reaction. A reaction that removes acidity or alkalinity is a neutralisation reaction. In a neutralisation reaction, an acid and alkali react to produce a salt and water. The general word equation for this is:

Alkalis are ionic compounds. They are made up of hydroxide ions (OH^–) and metal cations.