In our daily life we come across many substances with different tastes and properties. Some things like lemon juice and curd taste sour, while others like soap solution taste bitter and feel slippery to touch. These differences arise because substances can be classified into acids, bases and salts based on their chemical nature. In this chapter we learn how to identify these substances and how they react with each other.
Taste is one way to identify acids and bases, but tasting substances in a laboratory can be dangerous. Scientists use safer methods. They use natural indicators extracted from plants, such as litmus solution obtained from lichens, or synthetic indicators like methyl orange and phenolphthalein. Indicators change their colour when added to acids or bases and help us identify the nature of a substance safely.
Acids are substances that taste sour. They are present in many food items; for example, citric acid is present in citrus fruits like lemons and oranges, acetic acid in vinegar, lactic acid in curd, and tartaric acid in tamarind. Bases are substances that taste bitter and feel soapy to touch. They are found in things like soap, baking soda and window cleaners, and substances like milk of magnesia contain a base. Solutions that are neither acidic nor basic are called neutral, and pure water is an example.
To test these substances, we use indicators. Litmus is the most common natural indicator and is obtained from lichens. It is available in the form of blue and red litmus paper. Blue litmus turns red in an acid, while red litmus turns blue in a base. Neutral solutions do not change the colour of litmus. Other natural indicators can be prepared from red cabbage, turmeric and coloured petals such as those of hydrangea. Turmeric solution turns red in a base but remains yellow in an acid, and red cabbage juice turns red in acids and green in bases.
When an acid reacts with a base, a chemical reaction called neutralisation takes place. In this reaction, the acid and base cancel each other's effect to form a salt and water. For example, when hydrochloric acid reacts with sodium hydroxide, it forms sodium chloride (common salt) and water. The heat that is sometimes released during this reaction is called heat of neutralisation.
The general equation of neutralisation can be written as follows.
During neutralisation, the acidic and basic nature of both the substances is lost. If the acidic and basic solutions are mixed in the right amounts, the resulting solution becomes neutral. The pH of the neutral solution is about 7. Neutralisation is used in many everyday situations. When we get stung by a bee, the sting injects an acid into our skin; applying baking soda (a mild base) neutralises it. When an ant bites us, formic acid is injected, and rubbing a base like baking soda or calamine solution gives relief. When soil is too acidic, farmers add basic substances like quicklime or slaked lime, and when the soil is too basic, they add organic matter or acids to neutralise it. Indigestion caused by excess acid in the stomach is treated with antacids, which are mild bases like milk of magnesia.
A salt is the product formed when an acid reacts with a base in neutralisation. Salts have a salty taste and are widely used in daily life. The most common salt is sodium chloride, commonly called common salt or table salt, which is used to add taste to food and to preserve it. Common salt is obtained from sea water, from lakes and from rock salt deposits in the Earth.
Salts have many important uses. Common salt is used in cooking and food preservation. Ammonium nitrate is used as a fertiliser in agriculture. Baking soda (sodium hydrogencarbonate) is used in making cakes, bread and biscuits, and also as a mild antacid. Washing soda (sodium carbonate) is used for washing clothes and cleaning purposes. Many salts are also used in medicines, in industries and in making glass, soap and paper. Thus, salts form an important part of our daily lives.
Substances around us can be classified as acids, bases or salts, and this classification has many practical applications. In the human stomach, hydrochloric acid helps in digestion, but excess acid causes acidity and indigestion, which is treated with antacids. In soil, plants grow best in a neutral or slightly acidic medium; farmers test the soil and add lime or organic matter to adjust its acidity.
In the kitchen, baking soda and vinegar are both useful but for different reasons - vinegar (acetic acid) is used in pickles and chutneys, while baking soda (a base) is used to make dough rise. In washing, soap is a basic substance that removes dirt, while many cleaning powders contain bases. In nature, the sting of a bee is acidic, while wasp stings are basic; so the treatment differs - baking soda for bee stings and vinegar for wasp stings. Understanding the nature of acids, bases and salts helps us handle these substances safely and use them properly.
| Substance | Taste | Examples | Effect on Litmus |
|---|---|---|---|
| Acid | Sour | Lemon, vinegar, curd, tamarind | Blue litmus turns red |
| Base | Bitter, soapy | Soap, baking soda, milk of magnesia | Red litmus turns blue |
| Salt | Salty | Common salt | No change |
| Neutral | No taste | Pure water | No change |
| Use | Substance | Reason |
|---|---|---|
| Bee sting relief | Baking soda (base) | Neutralises the acid injected |
| Ant bite relief | Calamine/baking soda | Neutralises formic acid |
| Acidity relief | Antacid (milk of magnesia) | Neutralises excess stomach acid |
| Soil treatment | Quicklime/slaked lime | Neutralises acidic soil |
Acids, bases and salts are fundamental groups of substances that surround us in everyday life. Acids taste sour and are found in many foods, while bases taste bitter and soapy and are used in cleaning and medicine. Indicators like litmus help us identify them safely without tasting. When acids and bases react, neutralisation produces salt and water, and this reaction is used to treat bee stings, acidity and acidic soil. Salts such as common salt, baking soda and washing soda have countless domestic and industrial uses. This chapter teaches us to classify substances and apply the principles of chemistry to solve real-life problems.