chemistry

ACID, BASE AND SALT NCERT CLASS 7^TH

Substances can be divided into three types – Acid, Base and Salt.

ACID: Taste of acid is sour. There are many substances that contain acid and so taste sour. For example – lemon, curd, pickles, orange juice, vinegar, etc.

Substances that taste sour are called acidic. The chemical nature of such substance is known as ACIDIC.

The word acid comes from Latin ‘ACERE’ which means sour.

BASE: Taste of base is bitter. Substances that contain base taste bitter. For example; soap or soap solution, baking soda, washing soda, etc.

The chemical nature of substances that contain base is known as BASIC.

INDICATOR: It is not always possible to know the acidic or basic nature of substances by tasting them. Tasting a substance in laboratory is not also advisable because it may be harmful. Thus, to test the chemical nature (acidic or basic nature) of substance a special kind of substance is used. This special kind of substance which is used to test the acidic or basic nature of anything is known as INDICATOR.

Indicator is a substance that shows the acidic or basic nature of a substance by change in its colour.

Types of Indicator: Indicators can be divided into two types.

Natural Indicator: Indicators that are obtained from naturally occurring substance are called NATURAL INDICATORS. For example; litmus, turmeric, china rose, etc.

Synthetic Indicator: Indicators that are made in laboratory are called SYNTHETIC INDICATORS. For example; phenolphthalein, methyl orange, etc.

NATURAL INDICATORS:

Litmus: Litmus is extracted from Lichens. Lichen is a composite organism. Lichens consist of fungi and algae living in symbiotic relationship.

Litmus is a purple coloured liquid in distilled water. Litmus comes in the form of strips of two colours. One is called blue litmus paper and another is called red litmus paper.

Litmus liquid and litmus paper are used to detect the acidic or basic nature of a substance.

Colour of litmus paper in acid: Blue litmus paper turns into red when dipped in acidic solution.

Colour of litmus paper in base: Red litmus paper turns into blue when dipped in basic solution.

Turmeric: Turmeric is used as another natural indicator. Turmeric is of yellow colour. Turmeric paper turns into red when it is dipped into basic solution. Turmeric paper does not change its colour with acid.

China Rose: China rose is another natural indicator. China rose solution gives dark pink (magenta) colour with acid and green colour with base.

Acid Rain: Carbon dioxide, sulphur dioxide and nitrogen dioxide which are released from vehicles and chimneys mix with droplets of rain and turn the rain water acidic. When this acidic rain water falls over earth, it is known as acid rain. Acid rain damages the buildings and is harmful for plants and animals.

Taj Mahal; which is made of marble is in threat because of acid rain. Many parts of Taj Mahal and other many historical buildings and monuments have got damaged due to acid rain.

Characteristics of Acid:

• Sour in taste.
• Turns blue litmus paper red.
• Turns the solution of China rose to dark pink colour (magenta).

Characteristics of Base:

• Bitter in taste.
• Turns red litmus paper blue.
• Turns solution of China rose to green.
• Turns turmeric paper to red.

Neutralisation:

When solution of acid is mixed with the solution of base, both of them neutralize each other and a third substance; called salt; is formed. Such phenomenon is called neutralization or neutralization reaction. The solution formed because of mixing of solution of acid and base is neither acidic nor basic in nature. Such solution is known as neutral solution.

Salt formed because of neutralization reaction may be acidic or basic in nature. Acidic or Basic nature of salt depends upon the strength of acid and base. In neutralization reaction heat is evolved. Reactions in which heat is evolved are known as exothermic reactions. Thus, neutralization reaction is an exothermic reaction.

Sodium hydroxide is a base and hydrochloric acid is an acid. When solution of sodium hydroxide is mixed with the solution of hydrochloric acid, both neutralize each other and common salt (Sodium chloride) is formed. Since, it is an exothermic reaction, so reaction mixture becomes slightly hot. The reaction involved in this can be written as follows:

Sodium chloride is the chemical name of common salt which is used in household.

Neutralization Reaction in Everyday Life:

There are many uses of neutralization reaction in everyday life.

Indigestion: Our stomach releases hydrochloric acid to kill bacteria; if any; present in food. Hydrochloric acid released in our stomach also helps in the digestion of food. Sometime our stomach produces more hydrochloric acid than required. Production of more hydrochloric acid in the stomach manifests as indigestion. This condition can be painful and causes pain in the stomach.

To get rid of such symptoms, a medicine made of milk of magnesia (a base) is taken orally. Milk of magnesia, being a base neutralizes the hydrochloric acid and gives relief from pain because of indigestion.

Ant sting: Ant sting or bee sting contains methanoic acid. Methanoic acid is also known as formic acid. Bee or ant injects formic acid into our skin, while biting. Injection of acid by ant or bee results in pain at the place of bite.

Rubbing baking soda over the skin gives relief from pain due to ant or bee sting. Baking soda, which is a base, neutralizes the effect of acid injected by bee or ant. Another base, such as zinc carbonate (Calamite solution) is also used in the case of ant or bee sting.

Soil Treatment: Sometimes soil becomes acidic or basic due to excess use of fertilizers or wrong method of harvesting. Acidic or basic nature of soil affects the yield as plants do not grow properly on such soil.

Acidic soil is treated with slaked lime or quick lime. Slaked lime (Calcium hydroxide) and quick lime (Calcium oxide) are bases. Use of slaked lime or quick lime neutralizes the acidic nature of soil.

Basic soil is treated with organic matter. Organic matter releases acid and neutralizes the basic nature of soil.

Factory Waste: Wastes of most of the factories are acidic. If such acidic wastes are flushed into rivers, acid present in them kills the aquatic organisms and pollutes the water. Thus, factory wastes are treated with basic substances to neutralize the acid present in them before being flushed in the river.

Important Points:

• Acid is sour in taste.
• Acid turns blue litmus paper to red.
• Acid turns China rose solution to dark pink.
• Base is bitter in taste.
• Base turns red litmus paper to blue.
• Base turns turmeric paper or solution to red.
• Base turns China rose solution into green.
• Indicator is a special substance which shows the acidic or basic character of a substance by change in colour.
• Litmus, turmeric and solution of China rose are natural indicators.
• Litmus is obtained from lichens.
• Solutions of acid and base neutralize each other when mixed in proper ratio. This is called neutralization or neutralization reaction.
• In neutralization reaction between the solutions of acid and base, a new substance called salt is formed.
• Salt formed because of neutralization reaction may be acidic or basic in nature. The acidic or basic nature of salt depends upon the strength of acid and base used for neutralization reaction.