NCERT Solutions of Chapter: Acids Bases and Salts. NCERT Solutions along with worksheets and notes for Class 7.
(1) State differences between acids and bases.
(2) Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Ans- Ammonia turns red litmus blue. It shows that it is basic in nature. A basic solution turns red litmus blue.
(3) Name the source from which litmus solution is obtained. What is the use of this solution?
Litmus is a natural indicator extracted from lichens. It has mauve colour in the distilled water. It turns red when added to acidic solution and it turns blue when added to basic solution. Litmus solution is used to identify acids and bases.
(4) Is the distilled water acidic/basic/neutral? How would you verify it?
Distilled water is neutral in nature. When we put few drops of distilled water on red litmus paper no colour changed. Now, repeat it with blue litmus paper. No colour changed. It shows that distilled water is neither acidic nor basic solution.
(5) Describe the process of neutralization with the help of an example?
The process in which an acid and a base neutralize each other and salt and water produce is called neutralisation.
Acid + Base → Salt + Water (heat is evolved)
Hydrochloric acid (HCL) + Sodium hydroxide (NAOH) → Sodium chloride (NACL) + Water (H2O)
Fill ¼ of a test tube with dilute hydrochloric acid. It is a colourless acid. Take phenolphthalein solution in another test tube. It is a colourless solution in acidic and neutral solution. But phenolphthalein solution gives dark pink in basic solution. Now add few drops of indicator phenolphthalein in hydrochloric solution. Solution remains colourless. Now add a drop of sodium hydroxide solution by a dropper .Stir the tube. The colour of solution becomes dark pink as solution is basic in nature now. Add one more drop of the hydrochloric acid in the solution. Shake well test tube. The colour of the tube disappears. When an acidic solution is mixed with a base solution, both the solutions neutralise the effect of each other. Touch the tube immediately after neutrialisation, it is warm. It means that heat is always produced in the reaction.
(6) Mark “T” if 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 neutralise 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.
(i) Nitric acid turns red litmus blue. F
(ii) Sodium hydroxide turns blue litmus red. F
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. T
(iv)Indicator is a substance which shows different colours in acidic and basic solutions. T
(v) Tooth decay is caused by the presence of a base. F
(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 wants neutral drink. How will Dorji decide which drink is to be served to whom?
Dorji will taste the liquids. If drink is sour it will be acidic. If drink is bitter in taste it will be base. If drink has no taste it will be neutral. He can also check the drink by litmus paper test. Dorji will put few drops of one drink on red litmus paper. It will be basic drink if red litmus paper turns blue. He will put few drops of another drink on blue litmus paper. It will be acidic drink if blue litmus paper turns red. The third drink is neutral drink. It will be neutral drink if no colour changes on red and blue litmus paper.
(8) Explain why:
(a) An antacid tablet is taken when you suffer from acidity.
An antacid such as milk of magnesia neutralise hydrochloric acid of our stomach and we feel relief.
(b) Calamine solution is applied on the skin when an ant bites.
Ant injects formic acid into our skin which neutralizes by rubbing moist baking soda or calamine solution, which contains zinc carbonate.
(c) Factory waste is neutralized before disposing it into the water bodies.
The wastes of many factories contain acids. If they are allowed to flow into water bodies, the acid will kill fish and other organisms. So, factory waste is neutralized before disposing it into the water bodies.
(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.
Given indicator is turmeric. It will change to red colour with basic solution. Therefore, the solution that changes colour of turmeric to red colour is sodium hydroxide.
Put few drops of sodium hydroxide in second solution. If test tube becomes warm and turmeric shows no colour change. Then, second solution is hydrochloric acid because turmeric shows no colour change with acidic solution. Test tube or beaker becomes warm because sodium hydroxide neutralises hydrochloric acid and produces heat.
Put few drops of sodium hydroxide in third solution. Put few drops of this solution on turmeric. If colour changes to red it is sugar solution. Because, sugar solution is neutral in nature but sodium hydroxide (basic) gives red colour with turmeric.
(10) Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Blue litmus paper is dipped in a solution. It remains blue. It may be basic solution or neutral solution. When blue litmus paper is dipped in basic solution it remains blue. Similarly if blue litmus paper dipped in neutral solution it remains blue. Put few drops of the given solution on red litmus paper. If it will turn blue it is basic solution.
(11) Consider the following statements:
(a) Both acids and bases change colour of all indicators.
(b) If an indicator gives a colour changes 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 b
(iii) b and c
(iv) only d
(iv) Statement is correct.