# Acids Bases and Salts Class 10 Notes

Contents

## Chapter 2Acids, Bases And SaltsClass 10 Notes

### Indicators :

Indicators are Substances which indicate the acidic or basic nature of the solution by the colur change.

#### Natural Indicators :

The indicators obtained from natural sources are called Natural Indicatores.
Examples of some common natural indicators used widely to show acidic or basic character of substances.

##### (1) Litmus

Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to the division Thallophyta, and is commonly used as an indicator.
An acid turns blue litmus paper red.
An base turns red litmus paper blue.

##### (2) Turmeric

Tumeric is yellow in colur. Tumeric solution or paper turns reddish brown with basae. Turmeric does not change colur with acid.

##### (3) Red cabbage

The juice of red cabbage is originally purple in colur. Juice of red cabbage turns reddish with acid and turns greenish with base.

### Olfactory Indicator :

The substances which changes their smell when mixed with acid or base are called Olfactory Indicator. For example; Onion, Vanilla etc.
Onion loses its smell when added with base. It does not change its smell with acid.
Vanilla smell vanishes with base, but its smell does not vanish with an acid.

### Synthetic Indicator :

The Indicators which are synthesised in the laboratory are known as Synthetic Indicators.
For example ; Phenophthalein, methyl orange, etc.

### Acids :

acids are sour in taste and change the colour of blue litmus to red.
Acid have a sour taste.
Turns blue litmus red.
Release H+ ions in aqueous solution.
Acids solution conducts electricity.

#### Common in Acids :

Acids give hydrogen gas when they react with meal. This show all acids contains hydrogen. When acid is dissolved in water, it dissociates hydrogen. The dissociation of hydrogen ion in aqueous solution is the common property in all acids. Because of the dissociation of hydrogen ion in aqueous solution, an acid show acidic behaviour.

##### Examples :

Hydrochloric acid (HCl) gives hyrogen ion (H+) and chloride ion (Cl-) when it is dissolved in water.

Acetic acid give ion (CH3COO-) and hydrogen ion (H+).

#### (i) Reaction of acids with metal :

Acids give hydrogen gas along with respective salt when they react with meta.
Acid + Metal → Salt + Hydrogen gas

Examples :
Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zink metal.

Sodium Sulphate and Hydrogen gas is formed when Sulphuric acid react with Sodium metal.

#### Test For Hydrogen gas :

The gas evolved after reaction of acid with metal can be tested bringing a lighted candle near it.
The gas burns with a pop sound, than it confirms the evolution of hydrogen gas.

#### (ii) Reaction of acids with metal carbonate :

Acids gives carbon dioxide and respective salts along with water when they react with metal carbonates.
Metal carbonate + Acid → Salt + Carbon dioxide + water

Examples :
Hydrochloric acid gives carbon dioxide gas, sodium chloride along with water when reacts with sodium carbonate.

Sulphuric acid gives calcium sulphate, carbon dioxide gas, calcium sulphate and water when it react with calcium carbonate.

#### (iii) Reaction of acid with metal hydrogen carbonates :

Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water

##### Example :

When sodium hydrogen-carbonate react with hydrochloric acid it gives sodium chloride and water and carbon dioxide.

Suphuric acid gives sodium sulphate, Carbon dioxide gas and water when it reacts with sodium bicarbonate.

#### Test for Evolution of Carbon Dioxide gas :

When Carbon dioxide gas passed through lime water it turns lime water milky.

#### (iv) Reaction of Metallic Oxides with Acids

Metal Oxides are basic in nature. When an acid and metallic oxides reacts both neutralised each other.
Metal oxide + Acid → Salt + Water

### Acids

#### Strong Acids :

An acid which is completely ionised in water and produces (H+) is called strong Acid.

### Acids and Bases React with each other :

Base + Acid → Salt + Water

#### Neutralisation Reaction :

An acid neutralizes a base when they react with each other and respective salt and water are formed.
The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.

### Bases :

bases are bitter and change the colour of the red litmus to blue.

#### Common in all bases :

A base dissociates hydroxide ion in water, which is responsible for the basic behaviour of a compound.

##### Example :

When sodium hydroxide is dissolved in water, it dissociates in hydroxide ion and sodium ion.

Similarly, when potassium hydroxide is dissolved in water. it dissociates in hydroxide ion and potassium ion.

### Chemical Properties :

#### (i) Reaction of Bases with metals :

When alkali (base) reacts with metal, it produces salt and hydrogen gas.
Base + Metal → Salt + Hydrogen gas

##### Examples :

Sodium hydroxide give hydrogen gas and sodium zincate when react with zinc metal.

Sodium aluminate and hydrogen gas are formed when hydroxide reacts with aluminium metal.

#### (ii) Reaction of Base with non-metallic oxides :

Non-metallic oxides are acidic in nature. when carbon dioxide reacts with non-metallic oxide, both neutralize each other resulting in respective salts and water.

Base + Non-metal oxide → Salt + Water
(Non-metallic oxides are acidic in nature)

#### Acid or a Base in a Water Solution

##### Acid in water solution :

hydrogen ions in HCl are produced in the presence of water. The separation of H+ ion from HCl molecules cannot occur in the absence of water.

Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+ (aq) or hydronium ion (H3O+).

##### Base in water solution :

Bases generate hydroxide (OH-) ions in water. Bases which are soluble in water are called alkalis.

All bases do not dissolve in water. An alkali is a base that dissolves in water. They are soapy to touch, bitter and corrosive. Never taste or touch them as they may cause harm.
we have identified that all acids generate H+(aq) and all bases generate OH- (aq), we can view the neutralisation reaction as follows –

what is involved when water is mixed with an acid or a base.

• The process of dissolving an acid or a base in water is a highly exothermic one.
• Care must be taken while mixing concentrated nitric acid or sulphuric acid with water.
• The acid must always be added slowly to water with constant stirring.
• If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating

#### Dilution :

Mixing an acid or base with water results in decrease in the
concentration of ions (H3O+/OH-) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.

### Strength of Acid and Base :

#### Strong Acids :

Acids in which complete dissociation of hydrogen ion take place is called Strong Acids.
In mineral acid, such as hydrochloric acid, nitriac acid, etc. hydrogen ion dissociates completely and hence they are considered as strong acids.

#### Strong Bases :

Bases in which complete dissociation of hydroxide ion takes place are called Strong Bases.

A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power

On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value.

#### Universal Indicator :

Using a litmus paper, phenophthalein, methyl orange, etc. Only the acidic or basic character of a solution can be determined, but the use these indicator does not give the idea about the strength of acid or base. So, to get the strength as well as acidic and basic nature of a given solution universal indicator is used.

### Importance of pH in Everyday Life

#### plants and animals pH sensitive :

• Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change.
• When pH of rain water is less than 5.6, it is called acid rain.
• When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such riversbecomes difficult

#### pH of the soil in your backyard :

Plants require a specific pH range for their healthy growth. To find out the pH required for the healthy growth of a plant.

#### pH in our digestive system :

It is very interesting to note that our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach.
During indigestion the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, people use bases called antacids.
Antacids neutralise the excess acid. Magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose.

#### pH change as the cause of tooth decay

The bacteria present in our mouth converts the sugar into acids. When the pH of acid formed in the mouth fails below 5.5, tooth-decaying starts. The excess acid has to be removed by cleaning the teeth with good quality toothpaste because these kinds of toothpaste are alkaline in nature.

#### Self defence by animals and plants through chemical warfare :

Have you ever been stung by a honey-bee? Bee-sting leaves an acid which causes pain and irritation. Use of a mild base like baking soda on the stung area gives relief. Stinging hair of nettle leaves inject methanoic acid causing burning pain.

### Salts :

Salts are the ionic compound which are produced after the neutralization reaction between acid and base.
Salts are electrically neutral.
There are number of salts but sodium chloride is the most common among them.
Sodium chloride is also known as table salt or common salt. Sodium chloride is used enhance the taste of food.

#### Common salt — A raw material for chemicals

The common salt thus obtained is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more. Let us see how one substance is used for making all these different substances.

#### Sodium hydroxide

When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide

Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode.

#### Bleaching powder

Chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This chlorine gas is used for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2]. Bleaching powder is represented as CaOCl2, though the actual composition is quite complex.

#### Uses of Bleaching powder

(i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;
(ii) as an oxidising agent in many chemical industries; and
(iii) to make drinking water free from germs.

#### Baking soda

The baking soda is commonly used in the kitchen for making tasty crispy pakoras, etc. Sometimes it is added for faster cooking. The chemical name of the compound is sodium hydrogencarbonate (NaHCO3). It is produced using sodium chloride as one of the raw materials.

It is a mild non-corrosive basic salt. The following reaction takes place when it is heated during cooking –

Sodium hydrogencarbonate has got various uses in the household.

#### Uses of Baking soda

(i) For making baking powder, which is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place –

Carbon dioxide produced during the reaction can cause bread or cake to rise making them soft and spongy.
(ii) Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.
(iii) It is also used in soda-acid fire extinguishers.

#### Washing soda

Another chemical that can be obtained from sodium chloride is Na2CO3.10H2O (washing soda). You have seen above that sodium carbonate can be obtained by heating baking soda; recrystallisation of sodium carbonate gives washing soda. It is also a basic salt.

#### Uses of washing soda

(i) Sodium carbonate (washing soda) is used in glass, soap and paper industries.
(ii) It is used in the manufacture of sodium compounds such as borax.
(iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.
(iv) It is used for removing permanent hardness of water.

#### Crystals of Salts really Dry

Copper sulphate crystals which seem to be dry contain water of crystallisation. When we heat the crystals, this water is removed and the salt turns white. If you moisten the crystals again with water, you will find that blue colour of the crystals reappears.

##### The water of Crystallization :

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is CuSO4.5H2O. Now you would be able to answer the question whether the molecule of Na2CO3.10H2O is wet.

One other salt, which possesses water of crystallisation is gypsum. It has two water molecules as water of cyrstallisation. It has the chemical formula CaSO4.2H2O. Let us look into the use of this salt.

#### Plaster of Paris

On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO4.1/2H2O). This is called Plaster of Paris, the substance which doctors use as plaster for supporting fractured bones in the right position.
Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.

Acids Bases and Salts Class 10