Monday, 20 June 2011

Acids, Bases and Salts: Lesson 5. CBSE NCERT Class VII (7th) Science

Exercises & Solutions :: Questions & Answer

1. Question : State differences between acids and bases.
Answer :

  1. Acids are sour in taste
  2. The chemical nature of such substances is acidic.
  3. Acid turns blue litmus red.
  4. Acids are generally found in Vinegar, Curd,Spinach, Amla, Citrus fruits, Tamarind, grapes, unripe mangoes, Citrus fruits such as oranges, lemons, et
  1. Bases are bitter in taste and soapy to touch.
  2. The nature of such substances is said to be basic.
  3. Bases turn red litmus blue
  4. Bases are generally found in lime water, soap, widow cleaner, Milk of Magnesia

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

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

Answer : Litmus is extracted from lichens . It is most commonly used as an indicator to determine the chemical nature of substance.It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper. To test a substance for an Acid or a base or a neutral, litmus test is performed in which the Acid turns blue litmus red, Bases turn red litmus blue and has no effect on neutral substance.

4. Question :  Is the distilled water acidic/basic/neutral? How would you verify it?
Answer : The Distilled water is neutral. First, it is neither sour (Acidic) nor bitter (Basic), secondly to very its chemical nature, we can perform litmus test. It neither turns blue litmus red nor red litmus blue hence we can say, distilled water is neutral substance
Answer :

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

Answer : Neutralisation is a process in which an acid solution when mixed with base solution, react with each other to produce a salt and water along with generation of heat. Salt so produced, may be acidic, basic or neutral in nature.In this process the acidic nature of the acid and the basic nature of the base are destroyed.

For Example : In the following reaction Hydrochloric acid (HCl) when added to Sodium hydroxide (NaOH) will produce Sodium chloride (NaCl) along with Water (H2O) and heat.

6. Question : Mark ‘T’ if the statement is true and ‘F’ if it is false:
(i) Nitric acid turn 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. Question : Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. 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 : An Acidic drink will have an sour taste and will turn blue litmus red. A basic drink will turn red litmus blue. A neutral drink will not change the colour of red or blue litmus. He can also use turmeric solution for checking the drinks.

8. Question : Explain why:
(a) An antacid tablet is taken when you suffer from acidity.
Answer : An antacid tablet is taken when we suffer from acidity, as it neutralise it effect.

(b) Calamine solution is applied on the skin when an ant bites.
Answer : The sting of an ant contains formic acid. When an ant bites, it injects the acidic liquid into the skin. The effect of the sting can be neutralised by rubbing some moist solution of a basic substance such as baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate

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

Answer : The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances.

9. Question : 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 : Turmeric which is yellow in colur, when exposed to neutral (Sugar Solution) or acidic substances (Hydrochloric Acid) it will retain its yellow colouration. However, if turmeric is exposed to more alkaline substances (sodium hydroxide ) it becomes a dark pink/red. So first we detect sodium hydroxide -a basic substance by a colour change from yellow to dark or red. Then will test for an acid or neutral substance with indication of no coulr change. Now out of these two, we will mix one with already tested solution for basic substance -sodium hydroxide with dark or red colur, if on mixing the colour reverses back to yellow, the liquid is an acid and the remaining third liquid is neutral.

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

11. Question : 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

Extended Learning — Activities and Projects
1. Using the knowledge of acids and bases, write a secret message with the help of baking soda and beet root. Explain how it works.
(Hint: Prepare baking soda solution in water. Use this solution to writethe message on a sheet of white paper with a cotton bud. Rub a slice of fresh beet root over the message.)
2. Prepare red cabbage juice by boiling a piece of red cabbage in water. Use it as an indicator and test the acidic and basic solutions with it. Present your observations in the form of a table.
3. Bring the soil sample of your area, find out if it is acidic, basic or neutral. Discuss with farmers if they treat the soil in any manner.
4. Visit a doctor. Find out the medicines, he prescribes to treat acidity. Ask him how acidity can be prevented.
Did you know?
Each cell in our body contains an acid, the deoxyribonucleic acid or DNA. It controls every feature of the body such as our looks, colour of our eyes, our height etc. Proteins that build part of our cells are also made of amino acids. The fats in our body contain fatty acids.


  1. please rectify the answer"7 - line 2 which reads

    "A basic drink will turn red litmus red" to "A basic drink will turn red litmus blue."

    1. Thanks...for your help..Correction has been done