Acids, Bases and Salts | Note | ClassVII

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

Acids: Taste of acid is sour. There are many substances that contain acid 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 acids.
The word acid comes from Latin ‘acere’ which means sour.

Uses of Acids:

Hydrochloric Acid (HCl)

  • Used for removing deposits from the inside of the boilers.
  • Used for cleaning sinks.

Sulphuric Acid

  • Sulphuric acid is used in car batteries
  • It is used in the manufacture of paints, drugs, dyes, and to produce fertilizers.

Nitric Acid (HNO3)

  • It is used by goldsmiths for cleaning gold and silver ornaments.
  • It is also used for the production of fertilizers such as ammonium nitrate.

Acetic Acid (CH3COOH)

  • Acetic acid is used directly to enhance the flavour of food. In fact, we commonly know acetic acid as vinegar.

Bases: 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.

Uses of Bases:

Indicator: A substance which detects the acidic or basic nature of another substance by change in colour is called acid-base indicator. It is not advisable to taste any substance in laboratory because it can be harmful. Hence, acid-base indicators are used to check if a given substance is acid or base.

Types of Indicator:

Indicators can be divided into two types.

Natural Indicator: Indicators that are obtained from naturally occurring substance are called Natural indicators. Example: litmus, turmeric, China rose, etc.

Synthetic Indicator: Indicators that are made in laboratory are called Synthetic indicators. 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 also used as 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.

The Taj Mahal; which is made of marble; is under threat because of acid rain. Many parts of Taj Mahal and many other 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.

Neutralization

When a 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.

Acid + Base → Salt + Water

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:

HCl + NaOH → NaCl + H2O

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. Some examples are as follows:

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 abdomen and burning sensation in chest.

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 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 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.

NCERT Solution

Question: 1. State differences between acids and bases.

Answer:

Acid Base
Sour in taste. Bitter in taste.
Turns blue litmus paper to red. Turns red litmus paper to blue.
No effect on turmeric paper. Turns turmeric paper to red.
Turns China rose solution to magenta. Turns China rose solution to green.

Question 2: Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?

Answer: Since window cleaner turns red litmus paper to blue, it is basic in nature.

Question 3: Name the source from which litmus solution is obtained. What is the use of this solution?

Answer: Litmus solution is obtained from lichens. Litmus solution is used to detect the acidic and basic characteristic of a substance.

Question 4: Is the distilled water acidic/basic/neutral? How would you verify it?

Answer: Distilled water is neutral in character, i.e. it is neither acidic nor basic. Neutral nature of distilled water can be verified by the use of blue and red litmus paper. Distilled water does not change the colour of either blue or red litmus paper.

Question 5: Describe the process of neutralization with the help of an example.

Answer: When the solution of acid is mixed with the solution of base in proper ratio, both of them neutralize the effect of each other and a new substance; called salt; is formed; along with water. This is called neutralization or neutralization reaction.

Example: When the solution of sodium hydroxide (a base) is mixed with the solution of hydrochloric acid (an acid) in proper ratio, both neutralize each other and the reaction mixture so obtained is neutral in character. In this reaction, a new substance sodium chloride (common salt) is formed. The reaction involved in this can be written as follows:

HCl + NaOH → NaCl + H2O

Question 6: Mark ‘T’ if the statement is true and ‘F’ if it is false:

(i) Nitric acid turns red litmus blue.
(ii) Sodium hydroxide turns blue litmus red.
(iii) Sodium hydroxide and hydrochloric acid neutralize each other and form salt and water.
(iv) Indicator is a substance which shows different colours in acidic and basic solutions.
(v) Tooth decay is caused by the presence of a base.

Answer: (i) F, (ii) F, (iii) T, (iv) T, (v) F

Question 7: Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labeled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom?

Answer: Dorji would dip blue and red litmus paper in the sample of each of the bottles.

  • Sample of solution of bottle which turns blue litmus paper red is acidic.
  • Sample of solution of bottle which turns red litmus paper blue is basic.
  • Sample of solution of bottle which does not change the colour of either blue or red litmus paper is neutral in nature.
  • After detecting the acidic, basic and neutral nature of soft drink, Dorji would serve the drink to the customers according to their requirement.

Question: 8. Explain why:

(a) An antacid tablet is taken when you suffer from acidity.

Answer: Antacid means substance that works against acid. Antacid tablet is taken in the case of acidity to neutralize the excess acid produced in the stomach. Antacid tablets contain basic substances and hence neutralize the acid produced in the stomach and give relief from acidity.

(b) Calamine solution is applied on the skin when an ant bites.

Answer: Calamine solution, which is zinc carbonate, is a base. In the case of ant bites, ant injects an acid; called formic acid; in the skin which causes pain and irritation. By applying calamine solution, it neutralizes the effect of acid inject in the course of ant bite and gives relief from pain.

(c) Factory waste is neutralized before disposing it into the water bodies.

Answer: Most of the factory wastes contain acid. It they are flushed into water as they are, then acid present in them would kill the aquatic organisms; along with creating pollution. Thus, it is necessary to neutralize the factory waste before disposing it into the water bodies.

Question 9: Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator.

Answer: Label the test tubes as A, B and C.

  • Dip the turmeric indicator in test tube A. If the turmeric paper changes to red then the liquid in the test tube is sodium hydroxide; which is a base. Otherwise, it is either hydrochloric acid or sugar solution; which is neutral.
  • If the liquid in the test tube A is not sodium hydroxide then keep it aside and repeat above procedure with the test tube B. If it is not sodium hydroxide then the liquid in test tube C must be sodium hydroxide.
  • Take aside the test tube with sodium hydroxide and pour sodium hydroxide in remaining two test tubes. The test tube which becomes warm contains acid. When acid and base neutralize each other, heat is released in the process.

Question 10: Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.

Answer: If blue litmus paper is dipped in a solution and it would remain blue then the solution may be basic or neutral in character.

Blue litmus paper does not change its colour with basic solution and neutral solution.

Question 11: Consider the following statements:

(a) Both acids and bases change colour of all indicators.
(b) If an indicator gives a colour change with an acid, it does not give a change with a base.
(c) If an indicator changes colour with a base, it does not change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.

Which of these statements are correct?
(i) All four (ii) a and d (iii) b and c (iv) only d

Answer: (iv) only d

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