CLASS 10 Science CHAPTER 2 Acids, Bases And Salts (NCERT Notes)

CLASS 10 Science 
CHAPTER 2 
Acids, Bases And Salts (NCERT Notes)

Acids are sour in taste and change the colour of blue litmus to red. 

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

Litmus is a natural indicator, turmeric is another such indicator.

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. When the litmus solution is neither acidic nor basic, its colour is purple. There are many other natural materials like red cabbage leaves, turmeric, coloured petals of some flowers such as Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in a solution. These are called acid-base indicators or sometimes simply indicators.

UNDERSTANDING THE CHEMICAL PROPERTIES OF ACIDS AND BASES

Acids and Bases in the Laboratory

Acids change the colour of blue litmus to red. 

Bases change the colour of the red litmus to blue.

There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators. For Example vanilla, onion and clove, can be used as olfactory indicators.

How do Acids and Bases React with Metals? 

When metal combines with acid it forms a compound called a salt. And leave hydrogen gas. The reaction of a metal with an acid :

Acid + Metal → Salt + Hydrogen gas

How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

When sodium carbonate react hydrochloric acid it will form sodium chloride ,water and give out carbon dioxide.

When sodium hydrogencarbonate react hydrochloric acid it will form sodium chloride ,water and give out carbon dioxide.

On passing the carbon dioxide gas evolved through lime water,

On passing excess carbon dioxide the following reaction takes place:

Limestone, chalk and marble are different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water. The reaction can be  –

Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water

How do Acids and Bases React with each other?

The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. In general, a neutralisation reaction can be written as –

Base + Acid → Salt + Water

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Reaction of Metallic Oxides with Acids

The general reaction between a metal oxide and an acid can be written as –

Metal oxide + Acid → Salt + Water 

Metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides.

Reaction of a Non-metallic Oxide with Base

Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water. The reaction between a base and an acid, we can conclude that non- metallic oxides are acidic in nature.

What Happens to an Acid or a Base in a Water Solution

When a acid is dissolved in water

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.

HCl + H2O → H3O+ + Cl–

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+).

H+ + H2O → H3O+

Acids give H3O+ or H+(aq) ion in water.

When a base is dissolved in water

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.

All acids generate H+(aq) and all bases generate OH– (aq), we can view the neutralisation reaction as

follows – 

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.

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.

HOW STRONG ARE ACID OR BASE SOLUTIONS?

A scale used measuring hydrogen ion concentration in a solution, called pH scale . The p in pH stands for ‘potenz’ in German, meaning power. The pH scale measure pH 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. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration in the solution, that is, increase in the strength of alkali. 

Variation of pH with the change in concentration of H+(aq) and OH– (aq) ions.

The strength of acids and bases depends on the number of H+ ions and OH– ions produced. If we take hydrochloric acid and acetic acid of the same concentration, say one molar, then these produce different amounts of hydrogen ions. Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be weak acids.

Importance of pH in Everyday Life

Are 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 rivers becomes difficult. 

What is the 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, you can collect the soil from various places and check the pH of the soil.

pH in our digestive system

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. One such remedy must have been suggested by you at the

beginning of this Chapter. These 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

Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel, made up of calcium phosphate is the hardest substance in the body. It does not dissolve in water, but is corroded when the pH in the mouth is below 5.5. Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.

Self defence by animals and plants through chemical warfare

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.

Nature provides neutralisation options

Nettle is a herbaceous plant which grows in the wild. Its leaves have stinging hair, which cause painful stings when touched accidentally. This is due to the methanoic acid secreted by them. A traditional remedy is rubbing the area with the leaf of the dock plant, which often grows beside the nettle in the wild. 

Some naturally occurring acids

MORE ABOUT SALTS

 

pH of Salts

Salts of a strong acid and a strong base are neutral with pH value of 7. Salts of a strong acid and weak base are acidic with pH value less than 7 and those of a strong base and weak acid are basic in nature, with pH value more than 7.

Chemicals from Common Salt

Salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that you use in food.

Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal.

Common salt — A raw material for chemicals

The common salt is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more. 

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.

  2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)

Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode. The three products produced in this process are all useful. 

Hydrogen gas is used in: Fuel ,Margarine and Ammonia for fertilizer.

Chlorine gas is used in:  Water treatment plant ,Swimming pools, PVC ,Disinfectants, CFCs and Pesticides. 

Sodium hydroxide is used in: De- greasing metals ,Soap and Detergents ,Paper making and Artificial fiber.

Hydrochloric acid is used in: Cleaning steel, Ammonium chloride, Medicine and Cosmetics.

BLEACH is used in: Household bleaches and Bleaching fabric.

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.

  Ca(OH)2 + Cl2 → CaOCl2 + H2O 

Bleaching powder is used –

(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) for disinfecting drinking water to make it free of germs.

Baking soda

The soda commonly used in the kitchen for making tasty crispy pakoras is baking soda. 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 can be used to neutralise an acid. It is a mild non-corrosive base. The following reaction takes place when it is heated during cooking –

Uses of sodium hydrogencarbonate (NaHCO3)

(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 causes 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

Washing soda obtained from sodium chloride is Na2CO3.10H2O (washing soda). 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.

Are the Crystals of Salts really Dry?

Crystals are solid materials where atoms and molecules are arranged in a highly ordered ( periodic) microscopic structure, called crysal lattice.

Crystallization is process by which a chemical is converted from a liquid solution into a solid crystalline state. For Example 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.  Chemical formula for hydrated copper sulphate is Cu SO4. 5H2O. 

The molecule of Na2CO3.10H2O is wet. What does 10H2O signify? Does it make Na2CO3 wet? Salt, which possesses water of crystallisation is gypsum. It has two water molecules as water of cyrstallisation. It has the formula CaSO4.2H2O. 

Plaster of Paris

On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (

CaSO4 .1/2 H2O). 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.

Note that only half a water molecule is shown to be attached as water of crystallisation. How can you get half a water molecule? It is written in this form because two formula units of CaSO4 share one molecule of water. Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth.

( From NCERT Book )