CBSE Class 7 Science Notes Chapter 5 Acids, Bases and Salts
In our daily life, we use a large number of edible substances such as lemon, baking soda, tamarind, common salt, sugar, curd and vinegar. Some of these substances taste sour, some taste bitter, some taste sweet and some taste salty.
Acids, bases and salts are the three important groups of chemical substances that are used by us in different ways. Some of the acids, bases and salts occur in nature and they can be made artificially in factories also.
Edible substances and their tastes
| Substance | Taste (Sour/Bitter/Any other) |
| Lemon Juice | Sour |
| Orange Juice | Sour |
| Vinegar | Sour |
| Curd | Sour |
| Tamarind(imli) | Sour |
| Sugar | Sweet |
| Common Salt | Salty |
| Amla | Sour |
| Baking Soda | Bitter |
| Grapes | Sweet/Sour |
Acids and Bases
The word acid has been derived from a Latin word ‘acidus’ which means ‘sour’. Thus, all sour substances essentially contain acids. Substances like lemon juice, orange juice, unripe mango and curd taste sour. They taste sour because they contain substances called acids in them. The chemical nature of such substances is acidic. The acids in these substances are natural acids.
However, there are other substances like baking soda it does not taste sour.
It means that it has no acids in it. It is bitter in taste. And if prepare a solution of baking soda in water and rub it between your fingers, it feels soapy. Substance like these which are bitter in taste and feel soapy on touch are known as bases. The chemical nature of such substances is said to be basic. All the acids mentioned in table occur in nature.
Acids and their Sources
| Name of Acid | Found in |
| Acetic Acid | Vinegar |
| Formic Acid | Ant’s Sting |
| Citric Acid | Citrus fruits such as oranges, lemons, etc.. |
| Lactic Acid | Curd |
| Oxalic Acid | Spinach |
| Ascorbic Acid(Vitamin C) | Amla, Citrus fruits |
| Tartaric Acid | Tamarind, grapes, unripe mangoes, etc.. |
Bases and their Sources
| Name of Base | Found in |
| Calcium Hydroxide | Lime Water |
| Ammonium Hydroxide | Window Cleaner |
| Sodium Hydroxide/Potassium Hydroxide | Soap |
| Magnesium Hydroxide | Milk of Magnesia |
Natural Indicators Around Us
It is not safe to taste every substance to find out if it is acidic or basic. There are some special substances that have different colours in acidic and basic mediums. These substances are known as indicators. The indicators change their colour when added to a solution containing an acidic or a basic substance.
Some naturally occurring indicators are litmus, turmeric, China rose petals (gudhal) and red cabbage juice. These indicators show different colours in acidic and basic media. They are used to test whether a substance is acidic or basic in nature.
Litmus- A Natural Dye
A naturally occurring indicator, i.e. litmus is obtained from certain lichens (small plants) and used as a dilute solution. Litmus has mauve (purple) colour in water. In an acidic solution, it turns red. When it is added to a basic solution, it turns blue. Usually, it is available as a red and blue litmus paper.
Turmeric is Another Natural Indicator
Turmeric is a bright yellow powder obtained from a plant. It is is called ‘Haldi’ in Hindi. Turmeric contains a yellow dye. Turmeric turns red in basic solution. It is used as indicator in the form of turmeric paper.
China Rose as an Indicator
China rose is a natural indicator. It is called ‘Gudhal’ in Hindi. It is a extracted from the red flowers of China rose plant with water.
Acid Rain
The rain containing excess of acids called an acid rain. The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively. It can cause damage to buildings, historical monuments, plants and animals.
This happens as follows:
- Acid rain makes the water of lakes, ponds and rivers too acidic due to which fish and other aquatic animals get killed.
- Acid rain eats up the leaves of the trees gradually. By losing leaves, the trees die. Acid rain also damages crop plants in the fields.
- Acid rain damages the metal structures like steel bridges, etc when it falls on them.
- Acid rain damages the surfaces of buildings and monuments made up of marble.
Neutralisation
Acids and bases are chemically opposite substances. So, when an acid is mixed with a base, they neutralise (or cancel) the effect of each other. When an acid solution and a base solution are mixed in suitable amounts, both the acidic nature of the acid and the basic nature of the base are destroyed. The resulting solution is neither acidic nor basic. So, the reaction between an acid and base is known as neutralisation. In the process of neutralisation, salt and water are produced with the evolution of heat.
Salt produced in the reaction may be acidic, basic or neutral in nature. The evolved heat raises the temperature of the reaction mixture.
Acid + Base → Salt + Water (Heat is evolved)
e.g. Hydrochloric acid (HCl) (Acid) + Sodium hydroxide (NaOH) (Base) → Sodium chloride (NaCl) (Salt) + Water (H2O)
If dilute sulphuric acid is added to lime water (which is a base), then neutralisation reaction takes place and the reaction mixture becomes hot.
We are going to use an indicator which you have not used so far. It is called phenolphthalein.
Note: Phenolphthalein is an indicator used in the neutralisation process. When the solution is basic, phenolphthalein gives a pink colour but if the solution is acidic, it remains colourless.
Neutralisations in Everyday Life
The neutralisation reactions involving acids and bases play a very important role in our everyday life. The treatment of an ant’s sting, remedy for indigestion, soil treatment and the treatment of factory wastes, all involve neutralisation reaction.
Indigestion
Our stomach produces hydrochloric acid. This hydrochloric acid helps in digesting our food. Sometimes, excess of hydrochloric acid is produced in the stomach which causes indigestion. Due to indigestion, sometimes a person feels pain in the stomach and irritation. To relieve indigestion, we take an antacid such as milk of magnesia. Milk of magnesia contains a base called magnesium hydroxide. Magnesium hydroxide neutralises the excess acid present in the stomach and cures indigestion. Another antacid is baking soda which contains a base sodium hydrogen carbonate.
Ant Bite
When an ant bites, it injects an acidic liquid into the skin of the person which causes burning pain. The sting of an ant contains an acid called formic acid. The effect of the acid can be neutralised by rubbing a mild base like baking soda solution (sodium hydrogen carbonate) or calamine solution. Calamine solution contains a base called zinc carbonate. Thus, being a base, baking soda solution or calamine solution neutralises the acidic liquid injected by the ant and cancels its effect.
Soil Treatment
The soil may be acidic or basic naturally. The plants do not grow well, if the soil at a place is too acidic or too basic. Excessive use of chemical fertilisers makes the soil acidic. When the soil is too acidic, it is treated with bases like quicklime (calcium oxide) or slaked lime (calcium hydroxide). These bases neutralise the excess acid present in the soil and reduce its acidic nature. If the soil is basic, organic matter called manure or compost is added to it. The organic matter releases acids which neutralise the excess bases present in the soil and reduce its basic nature.
Factory Wastes
The waste substances discharged by many factories contain acids. If these factory wastes are allowed to flow into the water bodies (like rivers, ponds, lakes, etc), then the acid present in them will kill fish and other organisms which live in the water bodies. The factory wastes are therefore neutralised by adding basic substances before discharging them into water bodies.
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