Maharashtra Board Class 9 Science Chapter 5 Acids Bases and Salts PDF Download

5. Acids, Bases And Salts

We have earlier seen that some foodstuffs are sour to taste while some others are bitter and slippery or soapy to touch. When these substances are studied scientifically, it is found that they contain acidic and basic substances respectively. We have also learnt an easy and safe method of using an indicator like litmus to identify acids and bases.

How are acids and alkalies identified with the help of litmus paper?

We shall now learn more about acids and bases. But let us first review what the molecules of compounds are made up of?

How are the following substances classified into three groups with the help of litmus? Lemon, tamarind, baking soda, buttermilk, vinegar, orange, milk, lime, tomato, milk of magnesia, water, alum.

Fill in the columns in the part a of the following table.

Some compounds are seen to have H⁺ as the basic radical in their molecules. These are all acids. Some compounds are seen to have OH^- as the acidic radical in their molecule. All these compounds are bases. Those ionic compounds which have a basic radical other than H⁺ and an acidic radical other than OH^- are called salts.

Teacher's Note

Acids and bases are everywhere in our life. Lemon juice is acidic and soap is basic. When you mix these, they neutralize each other. This is very important in chemistry.

Exam Trick

Remember: If a compound has H⁺ ion, it is an acid. If it has OH^- ion, it is a base. All others are salts. Just remember this simple rule.

Points to Remember

Acids have H⁺ ions as the basic radical.
Bases have OH^- ions as the acidic radical.
Salts have other ions, not H⁺ or OH^-.
We use litmus paper to test acids and bases.
Acids are sour and bases are bitter or soapy to touch.

Now complete part B of the above table. It will be clear that there are three types of ionic compounds and these are acids, bases and salts.

Ionic Compounds: Recapitulation

The molecule of an ionic compound has two constituents, namely, cation (positive ion/basic radical) and anion (negative ion/acidic radical). There is a force of attraction between these ions as they are oppositely charged, and that is called the ionic bond. The force of attraction between one positive charge on a cation and one negative charge on an anion makes one ionic bond.

5.1 Formation Of The Compound NaCl

Electron Configuration

While studying static electricity we have learnt that there is a natural tendency of any body to change from an electrically charged state into an electrically neutral state. Why, then, is an electrically charged ion formed from an electrically balanced, that is, neutral atom? The explanation lies in the electronic configuration of atoms. Let us see once again how the sodium and chlorine atoms form the Na⁺ and Cl^- and as a result, how the NaCl salt is formed.

The outermost shell of the sodium and the chlorine atoms is not a complete octet. However outermost shell in both the Na⁺ and Cl^- ions is a complete octet.

An electronic configuration with a complete octet indicates a stable state. Further, an ionic bond is formed between the oppositely charged Na⁺ and Cl^- ions and therefore an ionic compound NaCl having very high stability is formed.

Teacher's Note

When sodium atom loses one electron, it becomes Na⁺ ion. When chlorine atom gains that electron, it becomes Cl^- ion. These two ions attract each other and form NaCl salt. This is how salt is made in nature.

Exam Trick

Remember: An atom becomes an ion when it loses or gains electrons. Na loses 1 electron to become Na⁺. Cl gains 1 electron to become Cl^-. Then they stick together to form NaCl salt.

Points to Remember

Sodium atom has electronic configuration (2,8,1).
Chlorine atom has electronic configuration (2,8,7).
After forming ions, both have complete octets like noble gases.
Ionic bonds form between oppositely charged ions.
NaCl is very stable because of the strong ionic bond.

Dissociation Of Ionic Compounds

On mixing the substances as shown here, what are the resulting mixtures formed by mixing the following substances called?

Water and salt. Water and sugar. Water and sand. Water and sawdust.

On dissolving in water, an ionic compound forms an aqueous solution. In the solid state, the oppositely charged ions in the ionic compound are sitting side by side. When an ionic compound begins to dissolve in water, the water molecules push themselves in between the ions of the compound and separate them from each other. That is to say, an ionic compound dissociates while forming an aqueous solution.

Teacher's Note

When you mix salt with water, the salt dissolves. The salt crystals break apart into Na⁺ and Cl^- ions. Water molecules separate these ions. In India, many people make salt by evaporating seawater in salt pans.

Exam Trick

Remember: Dissociation means breaking apart into ions. When salt dissolves in water, it dissociates into Na⁺ and Cl^- ions. These ions are surrounded by water molecules, shown as (aq).

Points to Remember

When ionic compounds dissolve in water, they break into ions.
Water molecules separate the positive and negative ions.
The symbol (aq) means the ion is surrounded by water molecules.
Dissociation is a physical change, not a chemical change.
Different ionic compounds dissociate into different types of ions.

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