Class 7 Science Acid Base and Salts Exam Notes

Covers the following topics:

This Class 7 Science exam notes page on Acid, Base and Salts covers acid-base indicators (litmus, phenolphthalein, methyl orange, olfactory and natural indicators), organic and inorganic acids with their properties, bases and alkalis with their properties, neutralisation reactions, and salts, with a summary of common indicator colours.

Topics Covered

1. Acid
2. Properties of acids
3. Bases
4. Properties of bases
5. Salt

INTRODUCTION

The earliest classification of inorganic compounds, including the large number of organic compounds, was based on their taste. On this basis, the compounds were classified into acids, bases and salts. We use many such compounds in our daily lives, such as curd, pickle, amla, soap and detergents, tamarind, toothpaste, etc. We must learn to classify them based on scientific tests.

Test to Distinguish Between Acids and Bases

There are many substances which show one colour or odour in the acidic medium and a different colour or odour in the basic medium. Such substances are called acid-base indicators.

A. Indicators Showing Different Colours in Acidic and Basic Medium

(a) Litmus solution: Litmus solution, used as an indicator, is a purple coloured dye extracted from the lichen plant. It is the most commonly used indicator in the science laboratory. In a neutral solution, it has purple colour. In an acidic solution, it turns red, whereas in a basic solution it turns blue. There are two types of litmus solution, blue and red litmus solution. Red litmus solution is obtained by acidifying the purple litmus extract, whereas blue litmus solution is obtained by making the purple litmus extract alkaline.
To test whether a given sample is acidic or basic, take a few drops of distilled water in a test tube and two drops of blue litmus solution. Add a few drops of the sample substance to be tested. If the blue litmus solution changes into red colour, the substance is acidic. For instance, lemon juice, vinegar, orange juice, juice of unripe mangoes and tamarind all turn blue litmus solution red. Thus, they are all acidic substances.
We may repeat the above experiment with red litmus solution. Those substances which turn red litmus solution into blue are bases. For instance, cucumber, washing soda solution, baking soda solution and bitter gourd turn red litmus solution blue. Thus, they are bases.

(b) Synthetic indicators such as phenolphthalein and methyl orange: Methyl orange is a compound prepared industrially or in the laboratory. There are many such man-made substances which can act as synthetic indicators.
Phenolphthalein is colourless in neutral solution and in acidic solution, but turns pink in basic solution.
Methyl orange is orange in neutral solution, red in acidic solution and yellow in basic solution.
Ask your teacher to help you classify various substances using these indicators.

B. Indicators Giving Different Odours in Acidic and Basic Medium (Olfactory Indicators)

(a) Clove oil: Take two test tubes and mark them test tube 'A' and test tube 'B'. Add some hydrochloric acid in test tube 'A' and some sodium hydroxide in test tube 'B'. Record the odour of clove oil. Now add a few drops of clove oil in each test tube and shake gently. Smell the sample in test tube 'A' and 'B'. Record the odour of each test tube. You will notice that the odour of the acid and of the base after the addition of clove oil is different. Ask your teacher to help you demonstrate the experiment in the laboratory.

(b) Vanilla essence: Repeat the experiment (a). Take another two test tubes and mark them 'A' and 'B'. Add some acid in 'A' and sodium hydroxide in 'B'. Add some vanilla essence in each test tube (A) and (B). Vanilla retains its smell in acidic medium but loses its smell in basic medium.

C. Natural Indicators

Turmeric (haldi), red cabbage and China rose petals are the natural indicators.
Turmeric is yellow in colour and leaves a yellow stain on clothes. When such a stain is washed with detergents, the stain becomes brownish red. Detergents have the base sodium hydroxide. This shows that turmeric changes in a base to brownish red colour. When the cloth is washed with lemon, it regains its yellow colour. This shows that the brownish red colour of turmeric changes to yellow in acids.
China rose petals act as a natural indicator. The China rose indicator turns magenta (dark pink) in acidic solutions. It turns green in basic solutions and does not show any colour change in neutral solution.

Colour of Natural and Synthetic Indicators

ACID

An acid (from the Latin word acidus, meaning sour) is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water. Chemicals or substances having the property of an acid are said to be acidic. Acids are sour in taste. Acids are of organic and inorganic nature. Acids found in plants and animals are organic in nature. Organic acids are weak, whereas inorganic acids are strong.

Organic Acids

Inorganic Acids
Inorganic acids are called mineral acids. They are prepared by dissolving mineral oxides in water. Sulphur dioxide dissolves in water to form sulphurous or sulphuric acid. Carbon dioxide dissolves in water to form carbonic acid. Hydrogen chloride dissolves in water to form hydrochloric acid, etc.
(a) Hydrochloric acid
(b) Nitric acid
(c) Phosphoric acid
(d) Sulphuric acid

Properties of Acids

1. Acids are generally sour in taste. For example, the sour taste of lemon juice is due to citric acid.
2. Strong or concentrated acids, or their fumes, often produce a stinging feeling on mucous membranes.
3. Acids change the colour of pH indicators as follows: they turn blue litmus and methyl orange red, and they turn phenolphthalein colourless.
4. Acids react with metals to produce a metal salt and hydrogen.
5. Acids react with metal carbonates to produce water, CO₂ and a salt.
6. Acids react with metal hydroxides and metal oxides to produce water and a salt.
7. Acids conduct electricity, depending on the degree of dissociation in aqueous solution. Car batteries use acids in them.
8. Acids can be gases, liquids or solids. Respective examples (at 20°C and one atm) are hydrogen chloride, sulphuric acid and citric acid. Solutions of acids in water are liquids, such as hydrochloric acid, which is an aqueous solution of hydrogen chloride. At 20°C and one atm, linear carboxylic acids are liquids and solids beginning from decanoic acid (ten carbon atoms). Perfumed carboxylic acids, the simplest being benzoic acid, are solids.
9. Strong acids and some concentrated weak acids are corrosive and can cause severe burns even after short contact.

BASES

Strong bases, like strong acids, attack living tissue and cause serious burns. They react differently with skin than acids do, so while strong acids are corrosive, we say that strong bases are caustic (corrosive). Bases may also be weak bases, such as ammonia, which is used for cleaning. Arrhenius bases are water-soluble. An alkali is a special example of a base, where in an aqueous environment hydroxide ions (also written as OH^-) are donated. Bases which dissolve in water are called alkalis. Bases are alkalis, but not all alkalis are bases.

The notion of a base as a concept in chemistry was first introduced by the French chemist Guillaume Francois Rouelle in 1754. He noted that acids, which in those days were mostly volatile liquids (like acetic acid), turned into solid salts only when combined with specific substances. These substances formed a concrete base for the salt, and hence the name.

Properties of Bases

Some general properties of bases include:
1. A slippery or soapy feel on the fingers, due to the specification of the lipids in human skin.
2. Concentrated or strong bases are caustic (corrosive) to organic matter and react violently with acidic substances.
3. Aqueous solutions (bases dissolved in water) or molten bases dissociate into ions and conduct electricity.
4. Bases turn litmus paper blue and phenolphthalein pink.

In chemistry, a base is most commonly thought of as an aqueous substance that can accept hydrogen ions. A base is also often referred to as an alkali if OH^- (hydroxide) ions are involved. Examples of simple bases are sodium hydroxide and ammonia. Although ammonia does not directly contain an OH^- group in its formula, it produces one in water, i.e., ammonium hydroxide. All compounds containing hydroxide are bases. For example, sodium hydroxide (NaOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)₂) and calcium hydroxide (Ca(OH)₂), etc.

Bases can be thought of as the chemical opposite of acids. A reaction between an acid and a base is called neutralisation. Acids and bases are seen as opposites because the effect of an acid is to increase the hydronium ion (H₃O⁺) concentration in water, whereas bases reduce this concentration. Bases react with acids to produce water and salts (or their solutions). Acids react with bases to form salt and water.

Uses of Neutralisation

1. Hydrochloric acid reacts with sodium hydroxide to form common salt, called sodium chloride, and water. This reaction supports neutralisation.
2. Farmers use slaked lime (calcium hydroxide, Ca(OH)₂) or quick lime (calcium oxide, CaO) to neutralise the acidic effects of soil in their farms.
3. If the soil is basic, organic matter is used to release or neutralise the bases.
4. Bacteria are continuously produced in our mouth, which cause tooth decay by making acids. The basic nature of toothpaste and tooth powder prevents tooth decay by neutralising the acidic effects.
5. A small amount of hydrochloric acid available in our stomach helps digest food and kills germs. Its excess production in the stomach causes indigestion. To neutralise the effect of acid, magnesium hydroxide (milk of magnesia) tablets are used.
6. Baking soda (sodium bicarbonate, NaHCO₃) is used to treat bee sting or ant sting. Their sting releases formic acid on the skin, which is neutralised by bases.

SALT

A salt, in chemistry, is defined as the product formed from the neutralisation reaction of acids and bases. There are several varieties of salts. Salts that produce hydroxide ions when dissolved in water are basic salts, and salts that produce hydronium ions in water are acid salts. Neutral salts are those that are neither acid nor basic salts.

When salts are dissolved in water, they are called electrolytes and are able to conduct electricity, a property that is shared with molten salts. Sodium chloride (NaCl), magnesium chloride (MgCl₂), calcium carbonate (CaCO₃), calcium sulphate (CaSO₄), etc. are some examples of salts. Not all salts are edible. Salts can be poisonous to the body as well. Not all salts are salty. The salt that we add to our food is sodium chloride (NaCl).

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