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Scientists use something called the pH scale to measure the strength of an acidic or alkaline liquid. Although there may be many types of ions in a solution, the pH focuses on the concentration of hydrogen ions (H+) and hydroxide ions (OH-). The scale goes from values very close to 0 through 14.

Distilled water is 7. Acids can have a pH of between 0 and a number very close to below 7, while alkalis have a pH of between a number very close to above 7 and 14.

Most of the liquids we find every day have a pH near 7. They are either a little below or above that mark. If we go into a chemistry lab, we could find solutions with a pH of 1 and others with a pH of 14.

There are also very strong acids with pH values of below 1, such as battery acid. Alkalis with pH values of near 14 include drain cleaners and sodium hydroxide (NaOH). These chemicals are corrosive and very dangerous.

Also you can easily tell if a substance is an acid or not by its effect on litmus paper. Blue litmus paper is used for testing acids. If wet blue litmus paper turns red, the substance is acidic. Wet red litmus paper is used to test alkalis, which will turn the paper blue.

Properties of acids and alkalis that distinguish them from other substances


•have a sour taste and feel like water.
•turn litmus solution red, as well as turn blue litmus paper red.
•have pH numbers less than 7.
•react with metals, forming hydrogen and a salt.
•react with carbonates, forming a salt, water and carbon dioxide.
•react with alkalis, forming a salt and water.


•feel soapy to touch and taste bitter.
•turn litmus solution blue, as well as turn red litmus paper blue.
•have pH numbers greater than 7.
•react with ammonium compounds to give off ammonia gas (except for ammonia).
•react with acids, forming a salt and water.
•do not react with metals and carbonates.

The process in which an acid reacts with an alkali to form a neutral solution containing a salt and water is called neutralisation.

As an acid is added to an alkali, the pH of the solution decrease. The end point of neutralisation is when pH value of the solution is 7. When this happens, it means that the alkali has been neutralised.

The balance of acids and alkalis is crucial for our good health and for our environment. If one dominates too much over the other, all sorts of things can go wrong and life on earth would be in peril.

Acid rain, fish deaths from acid rain, dirty swimming pools, Legionnaire bacteria outbreaks and indigestion are examples of acids and alkalis being out of balance.


The following are the examples of the application of neutralisation in our daily life:

1.Shampoos are usually slightly alkaline. They neutralise the acidic oil that the skin secretes to keep our hair healthy and strong.

2.Hair conditioner, which is slightly acidic, neutralises the residue of shampoo on the hair and makes the hair smooth and shiny.

3.Bee stings are acidic. They can be treated with bicarbonate powder, which is a weak alkali.

4.Toothpastes are alkaline. They neutralise the acid produced by bacteria that act on the food in our mouth.

5.Plants grow well in soil that is neutral. Farmers add slaked lime to reduce the acidity in the soil.

•The plant hydrangeas only produces pink or white flowers in an alkaline soil, and blue flowers in an acidic soil.

•In order to digest food and kill the kinds of bacteria and viruses that come with the food, the inside of our stomach is acidic. As we drink more alkaline water; more hydrochloric acid is secreted to maintain the acidic pH value in our stomach.

Acids and alkalis are two important kinds of solutions.

Acid comes from the latin word acidus, which means "sharp" or "sour". Some of the acids can be found in:

•Lemons/lemon juice, which contains citric acid,
•Oranges, which contain ascorbic acids,
•Ant and bee stings, which contain methanoic acid,
•Tea, which contains tannic acid,
•Soft drinks, which contain carbonic acid,
•Vinegar, which contains ethanoic acid, and
•Our body, which contains small compounds called amino acids.
There are different strengths of acids. The number of H+ ions formed in the solution determines the strength of an acid.

Examples of strong acids: hydrochloric acid, sulphuric acid and nitric acid.

Examples of weak acids: ethanoic acid, citric acid and carbonic acid.

Alkali comes from the Arabic word al-qaliy, which means "the ashes". Alkalis are present in many cleaning substances used in our homes today, especially in kitchen cleaners like oven spray, floor cleaners and creams for sinks.

Kitchen cleaners are alkaline because they contain ammonia or sodium hydroxide, which attack grease. Other common alkalis include sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonia.

There are different strengths of alkalis. The number of OH- ions in the solution determines the strength of an alkali.

Some examples of strong alkalis are calcium hydroxide, sodium hydroxide and potassium hydroxide, while the most recognisable and common weak alkali is ammonia.


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