Class 10 Acids, Bases-Salts

INTRODUCTION

We use many acids and bases in our day-to-day life. Many acids and bases are used as raw materials for making various useful products. About one hundred fifteen different elements are known to us. These elements combine to form a large number of compounds such as Acids, Base, and Salts. In this chapter, we will study all these three types of compounds.

LEARNING OBJECTIVES

In this lesson, you will be able to learn about:

Chemical properties of acids and bases.
The strength of acids and bases;
Common Properties of acids and bases.
The significance of pH in our daily life.
Products made from common salt

CHEMICAL PROPERTIES OF ACIDS AND BASES

Acids: Acid is a substance which tastes sour and turns blue litmus into the red, e.g. hydrochloric acid, sulphuric acid, acetic acid, etc.

Base: Base is a substance which tastes bitter and turns red litmus into the blue. A base feels soapy on touch, e.g. sodium hydroxide, calcium hydroxide, sodium hydrogen carbonate, etc.

INDICATORS

Indicators are substances like ‘dye’ which changes colour when it is put into an acid or a base. Indicators do so by showing a change in colour or a change in smell. Many indicators are used in the laboratory but litmus is the most common acid-base indicator.

Litmus: Litmus is a natural indicator which is prepared from an extract of lichen. Litmus solution is of the mauve colour. It is usually available in the form of litmus papers which are of blue or red colour. Cabbage juice, China rose extract, turmeric powder, etc. are other examples of natural indicators.

Exam Tips: Red is the sign of danger. Acids are more corrosive and hence more dangerous. Acids turn blue litmus paper into the red.

Synthetic Indicator: Indicators which are prepared artificially are called synthetic indicators. Phenolphthalein and methyl orange are examples of synthetic indicators. Phenolphthalein turns colourless in acidic solution and turns pink in basic solution. Methyl orange turns red in acidic solution and turns yellow in basic solution.

Olfactory Indicator: Some indicators show a change in their odour in the presence of acids or bases. Such indicators are called olfactory indicators. They are very useful for visually challenged students because such students cannot use other indicators. Clove, vanilla and onion are examples of olfactory indicators.

Activity:

Let us do the following activity to show the use of acid-base indicators in the laboratory.

Take red and blue litmus solutions, methyl orange and phenolphthalein.
Take samples of hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃), acetic acid (CH₃COOH), sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)₂], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)₂] and ammonium hydroxide (NH₂OH).
Take each sample in a watch glass and put a few drops of a particular indicator in it. Observe any change in colour.
Write your observation in the following the table.

Sample	Red litmus	Blue litmus	Phenolphthalein	Methyl Orange
Hydrochloric acid	No change	Red	Colourless	Red
Sulphuric acid	No change	Red	Colourless	Red
Nitric acid	No change	Red	Colourless	Red
Acetic acid	No change	Red	Colourless	Yellow
Sodium hydroxide	Blue	No change	Pink	Yellow
Calcium hydroxide	Blue	No change	Pink	Yellow
Potassium hydroxide	Blue	No change	Pink	Yellow
Magnesium hydroxide	Blue	No change	Pink	Yellow
Ammonium hydroxide	Blue	No change	Pink	Yellow

Caution: Acids and bases are corrosive in nature and hence you should never attempt to taste them in the laboratory. Checking an acid or a base with a suitable indicator is the safe and reliable way to check them.