Acids, Bases and Salts

The substances we meet in daily life can often be sorted by their chemical nature into acids, bases, and salts. An acid is a substance that tastes sour and turns blue litmus paper red. The word "acid" comes from a Latin word meaning sour. Many familiar foods owe their sour taste to acids: lemons and oranges contain citric acid, vinegar contains acetic acid, curd and sour milk contain lactic acid, and tamarind contains tartaric acid. These acids found in food and living things are called organic acids and are generally weak and safe in small amounts.

Some acids are made and used in laboratories and industry. These are called mineral acids and include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and nitric acid (HNO₃). Mineral acids are usually strong and can be very corrosive, meaning they can eat away skin, cloth, and metals. They must always be handled with great care, and strong acids should never be tasted or touched. Interestingly, our own stomach produces dilute hydrochloric acid, which helps in the digestion of food.

Acids share a set of common properties. They taste sour (though we never taste laboratory acids), they turn blue litmus red, and many of them react with metals to release hydrogen gas. They also react with bases in a special reaction called neutralisation, which we will study later. Because strong acids are corrosive, they are stored carefully and diluted by always adding acid slowly to water (never water to acid), as mixing the wrong way can splash dangerously.

Acids are extremely useful. Sulphuric acid is so important to industry that it is sometimes called the "king of chemicals"; it is used in fertilisers, batteries, and many manufacturing processes. Acids are also used in food preservation, in cleaning, and in our own bodies for digestion.

A base is a substance that is the chemical opposite of an acid. Bases generally taste bitter and feel soapy or slippery to the touch, and they turn red litmus paper blue. Many household substances are basic: baking soda, soap and detergents, lime water, toothpaste, and window-cleaning liquids are all bases. The bitterness of these substances and the slippery feel of soap are typical signs of a base.

A base that dissolves in water is called an alkali. All alkalis are bases, but not all bases are alkalis (some bases do not dissolve in water). Common examples of bases and alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide [Ca(OH)₂] (slaked lime, which forms lime water), and magnesium hydroxide [Mg(OH)₂] (the main ingredient in many antacids, "milk of magnesia"). Strong bases such as sodium hydroxide are highly corrosive, just like strong acids, and must be handled carefully.

Bases share common properties that mirror those of acids. They taste bitter (laboratory bases are never tasted), feel soapy, and turn red litmus blue. They also react with acids in a neutralisation reaction to form a salt and water, cancelling out each other's effects. Because strong bases are corrosive, they too require careful handling and proper storage.

Bases are very useful in everyday life and industry. Sodium hydroxide is used in making soap, paper, and textiles; calcium hydroxide (lime) is used in whitewashing walls and in agriculture to reduce soil acidity; magnesium hydroxide and other mild bases are used as antacids to relieve acidity in the stomach. Understanding bases alongside acids helps us see how the two opposites are used together in cooking, cleaning, medicine, and farming.

We cannot, and must not, find out whether a substance is acidic or basic by tasting or touching it, especially in the laboratory. Instead, scientists use special substances called indicators. An indicator is a substance that changes its colour to tell us whether a given substance is an acid or a base. The colour change acts as a clear, safe signal of the chemical nature of the substance.

The most common indicator is litmus, a natural dye extracted from a plant-like organism called lichen. Litmus is available as a solution and as litmus paper in two colours, blue and red. Acids turn blue litmus red, and bases turn red litmus blue. A neutral substance, such as pure water, does not change the colour of either type of litmus paper. Litmus is the simplest indicator for telling acids and bases apart.

There are several other useful indicators. Phenolphthalein is a synthetic indicator that stays colourless in acids and turns pink in bases. Methyl orange turns red in acids and yellow in bases. There are also many natural indicators that we can prepare at home: turmeric stays yellow in acids but turns red (or reddish-brown) in bases — which is why a turmeric stain on cloth turns red when soap is rubbed on it; red cabbage extract changes through a range of colours; and the petals of flowers such as the China rose (hibiscus) can also be used as indicators.

Indicators are extremely valuable in science and daily life. They allow us to test substances safely and quickly, to follow the progress of a neutralisation reaction, to check the acidity of soil for farming, and to monitor the quality of water. By simply watching a colour change, we learn the chemical nature of a substance without any risk of tasting or touching it.

Indicators like litmus can tell us whether a substance is acidic or basic, but they do not tell us how strong the acid or base is. To measure the strength of acidity or basicity, scientists use the pH scale. The pH scale is a set of numbers running from 0 to 14 that tells us how acidic or basic a solution is. The letters "pH" stand for the "power of hydrogen", as the value depends on the amount of hydrogen present in the solution.

On the pH scale, a value of exactly 7 means the solution is neutral — neither acidic nor basic — like pure water. A value less than 7 means the solution is acidic: the lower the number, the stronger the acid, so a substance of pH 1 or 2 (such as stomach acid or lemon juice) is strongly acidic. A value greater than 7 means the solution is basic (alkaline): the higher the number, the stronger the base, so a substance of pH 13 or 14 (such as sodium hydroxide solution) is strongly basic.

The pH of a solution can be found quickly using universal indicator, which is a mixture of several indicators that shows a whole range of colours for different pH values — typically red for strong acids, through orange, yellow, and green for neutral, to blue and violet for strong bases. By matching the colour to a chart, we can read off the approximate pH. More precise measurements are made with a digital instrument called a pH meter.

Knowing pH is important in many areas of life. Our blood must stay very close to pH 7.4, and even small changes can be harmful. Soil pH affects how well crops grow, so farmers test and adjust it. The pH of rainwater, rivers, and aquariums is monitored to protect living things, and pH balance matters in foods, medicines, cosmetics, and toothpastes. The pH scale therefore turns the idea of "acidic" or "basic" into a precise, useful number.

When an acid and a base are mixed together in the right amounts, they cancel out each other's properties. This reaction between an acid and a base is called neutralisation. In a neutralisation reaction, the acid and base react to form a salt and water, and a large amount of heat is usually released. The general word equation is: acid + base → salt + water. For example, hydrochloric acid reacts with sodium hydroxide to give common salt (sodium chloride) and water.

A salt is the compound formed when the acid and base neutralise each other. Salts are usually neutral, though some can be slightly acidic or basic. The most familiar salt is sodium chloride (NaCl), common table salt, formed from hydrochloric acid and sodium hydroxide. Other useful salts include sodium sulphate (Na₂SO₄) and many salts used in fertilisers, medicines, and food. Salts are an important class of chemical compounds with countless everyday uses.

Neutralisation is at work in many everyday situations. When we suffer from acidity in the stomach (too much acid), we take an antacid, which is a mild base that neutralises the excess acid and relieves the pain. A bee or ant sting injects an acid that causes pain; rubbing a mild base such as baking soda on the sting neutralises it. Soil that is too acidic is treated with lime (a base) to neutralise it, and soil that is too basic is treated with compost or other acidic matter. Even toothpaste, which is basic, neutralises the acids formed by bacteria in the mouth and protects the teeth.

A harmful effect related to acids in the environment is acid rain. When fuels are burnt, gases such as sulphur dioxide and nitrogen oxides are released into the air. These gases dissolve in rainwater to form acids, making the rain acidic — this is acid rain. Acid rain damages buildings and monuments (especially marble ones such as the Taj Mahal, in a process sometimes called "marble cancer"), harms plants and forests, and makes the water of rivers and lakes acidic, threatening fish and other aquatic life. Reducing the burning of polluting fuels helps prevent acid rain.