NCERT SCIENCE CLASS 10-ACIDS, BASES, AND SALTS NOTES AND STUDY MATERIAL-PART -2

NCERT SCIENCE CLASS 10-ACIDS, BASES, AND SALTS- CHAPTER 2 -NOTES AND STUDY MATERIAL-PART -1

In part 1 notes of acids, bases, and salts, we have discussed indicators and chemical properties of acids and bases. In this session, we can discuss the strength of acids and bases.

WHAT DO ACIDS AND BASES HAVE IN COMMON?

To understand this, an activity can be discussed.

ACTIVITY- Take a beaker in which a rubber cork is placed and over it two nails are fixed which are connected to a six-volt battery with the help of conducting wires. In the circuit, a bulb is also linked.

When this beaker is filled with dilute HCl and when the current is allowed to pass through the circuit, it is found that the bulb glows. But does it show that dilute HCl is an electrolyte? It allows the current to pass through it.

In the same experiment if it is performed by taking dilute H2SO4 in place of dilute HCl it is found that the bulb glows. How does the current flow? The current flow through the circuit with the help of ions present within the solution of acid in water

The same experiment if it is performed with the base like sodium hydroxide solution, in that case also the bulb glows. Sodium hydroxide solution also contains ions due to which current is passed through the circuit.

Repeat the experiment with a glucose solution and an alcohol solution. It is observed in those cases the bulb does not glow showing thereby that glucose solution and alcohol solution are not good conductors of electricity because they are not present in the form of ions. So what is common?

Acids and bases are present in the form of ions in the solution. Hence they can conduct electricity.

WHAT HAPPENS TO THE ACID OR A BASE IN A WATER SOLUTION?

ACTIVITY- Activity can be performed in the lab by taking a test tube in which sodium chloride salt is taken and to it is added dilute sulphuric acid. The reaction will occur and HCl gas is evolved and salt is formed.

If this gas is allowed to pass over blue litmus paper which is dry, it does not change in color. If wet blue litmus paper is now brought near the mouth of the test tube, the HCl gas which is liberated now will turn blue litmus to red in color. What does it show?

When HCl is dissolved in water, it forms ions, and then only it can turn blue litmus to red in color. HCl when it comes in contact with water it forms ions H3O+ and chloride ions.

H3O+ ions are called hydronium ions. Hydrogen ions cannot exist alone. It always combines with water and forms hydronium ions.

H⁺ + H2O → H3O⁺

In the same manner, when sodium hydroxide or any other base is dissolved in water it also forms ions. NaOH when dissolved in water it forms sodium ions and hydroxide ions.

NaOH → Na ⁺ + OH ⁻

Another base potassium hydroxide dissolved in water also dissociates into ions potassium ions and hydroxide. When acid and base react salt and water are formed.

A general acid HX and a general base MOH reacts and it forms salt MX and water.

HX + MOH → MX + H2O

Acids give hydrogen ions and bases give hydroxide ions. So neutralization reaction is actually the combination of hydrogen ions and hydroxide ions to form water.

H ⁺ + OH ⁻ → H2O

WHAT ARE CONCENTRATED ACIDS AND DILUTE ACIDS?

Concentrated acids have a concentration of ions more per unit volume and when it is mixed with water the concentration of the ions per unit volume will decrease.

ACTIVITY-about 5 ml of distilled water is taken and to it add a few drops of concentrated sulphuric acid and touch the test tube. What do you find? The test tube has become hot. It is an exothermic reaction.

The mixing of distilled water to the concentrated acid is known as dilution and the acid is said to be diluted. So a dilute acid is that in which the concentration of ions per unit volume is less.

Care must be taken that acid should be added to distilled water. If the reverse is practiced it will cause the acid to splash out and it may cause burns. It is due to the production of heat. The glass container also may break and it may cause some accidents. So also if you dissolve sodium hydroxide pellets in distilled water and touch the test tube. It becomes hot. Heat is liberated in this case also.

WHAT ARE STRONG ACIDS AND WEAK ACIDS?

Organic acids are those acids that are obtained from plants and animals like citric acid, tartaric acid.

Mineral acids are those acids that are obtained from the earth like hydrochloric acid, sulphuric acid, and nitric acid. These are very strong acids.

Why do we call them strong acids and why do we call organic acids weak acids?

You know about the corrosive nature of acids and bases. If concentrated sulphuric acid or concentrated hydrochloric acid falls on a piece of wood or cloth it may damage them. It may produce a hole in the cloth.

If it falls on the hand, the hand will burn. So the same case will happen with the bases also. If they are concentrated they will cause damage to the skin, to the cloth, and so on.

So acids and bases, if they are concentrated, should be handled with care because they cause damage as they are corrosive in nature.

Acids form ions when dissolved in water. Strong acids are those which are completely ionized when dissolved in water and weak acids are those which are weakly or partially ionized when dissolved in water.

Sulphuric acid is a strong acid whereas acetic acid is a weak acid.

In the same manner, strong bases are those which are completely ionized in water like sodium hydroxide, potassium hydroxide, and weak bases are those which are partially ionized when dissolved in water like ammonium hydroxide or magnesium hydroxide.

All the bases do not dissolve in water. Those bases which dissolve in water are called alkalis.

In the next part of acids, bases, and salts, do you want to learn how to measure the strength of acids?

YOU MAY ALSO READ: PART 1 ACIDS, BASES, AND SALTS

: PART 3 ACIDS, BASES, AND SALTS

: PART 4 ACIDS, BASES, AND SALTS

: PART 5 ACIDS, BASES, AND SALTS