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Question Answers

Q.1    Can I taste all substances to find their taste? (Page 50)

Ans.  No, you we should not taste unknown substances. They could be harmful to us.
Q.2.   Why a turmeric stain on my white shirt is turned to red when it is washed with soap?  (Page52)
Ans. It is because the soap solution is basic.
Q.3.  I am not getting the same result when using solid baking soda on dry litmus paper . Why?
 (Page 53)
Ans. Make a solution of baking soda and then try.
Q.1 State differences between acids and bases.
Ans. Differences between acids and bases:
(i) Acids are substances which contain one or more replaceable. hydrogen atoms.
(i) Bases are substances which have one or more replaceable hydroxyl group (OH).
(ii) All acids are sour in taste.
(ii) Bases are bitter in taste and slippery in touch.
(iii) All acids turn blue litmus red(iii) Bases turn red litmus blue.
(iv) Acids are compounds that contain hydrogen.
(iv) Bases are compounds which have hydroxyl group.
(v) Acids react with metal to form salts and release hydrogen gas.
(v) Bases react with acids to form salts.
Q.2 Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is it’s nature?
Ans. Basic
Q.3. Name the source from which litmus solution is obtained. What is the use of this solution?
Ans. Litmus solution is extracted from lichens. It is used as indicator.
Q.4 Is the distilled water acidic/ basic/ neutral? How would you verify it?
Ans. The distilled water is neutral.
Put a drop of distilled water on a strip of the red litmus paper with the help of a dropper. We observe that colour of the litmus paper does not change. The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.
Q.5 Describe the process of neutralisation with the help of an example.
Ans. The reaction between an acid and a base is known as neutralisation. salt and water are produced in this process with the evolution of heat.
               Acid + Base  -> Salt + Water
Example: following reaction:
Hydrochloric acid (HCl)+ Sodium Hydroxide (NaOH)
      ->Sodium Chloride (NaCl)+ Water (H2O)
Q.6 Mark ‘T’ if the statement is true and ‘F’ if it is false:
(i) Nitric acid turn 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.
Ans.(i) False (F); (ii) False (F); (iii)True (T);(iv)True (T); (v) False (F)
Q.7. 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?
Ans. He can decide by the use of indicator.
      •If the sample of drink turns red litmus blue,it is basic.
      •If it turns blue litmus red, it is acidic.
      •If it does not effect litmus, then it is neutral.
Q.8 Explain why:
    (a) An antacid tablet is taken when you suffer from acidity.
    (b) Calamine solution is applied on the skin when an ant bites.
   (C) Factory waste is neutralised before disposing it into the water bodies.
Ans. (a) Our stomach contains hydrochloric acid. It helps us to digest food .But too much of acid in the stomach causes indigestion. Sometimes, indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralises the effect of excessive acid.
(b) Ant injects an acid (Eg., formic acid) during bite which causes the burning sensation. Calamine solution is basic in Nature. It neutralises the acid and relieves us from the pain.
(c) Factory wastes contain both acidic and basic substances. If they are allowed to flow into the water bodies, then they are harmful for organisms living in water. So, the factory wastes are neutralised by adding basic substances.
Q.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.
Ans. Turmeric solution turns red in contact with bases. It is not affected by acids and neutral substances. So, first we will identify the base. Now, the base is taken and turmeric is added. It will turn red. Then one of the solutions is added to it gradually. If the solution turns yellow again ,the added liquid is hydrochloric acid because it neutralises the base. Otherwise, the added liquid is sugar solution.
Q.10. Blue litmus paper is dipped In a solution. it remains blue. What is the nature of the solution? Explain.
Ans. The nature of the solution is neutral, because the solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.
Q.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 indicator changes colour with a base, it does not change colour with an acid.
(d) Change of the colour in an acid and a base depends on the type of the indicator.
Which of these statements are correct?
Ans. (i) All four; (ii) (a) and (d);(iii) (b) and (c) ; (iv) only (d).

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Class 7  Science

  • Chapter 1. Nutrition in Plants
  • Chapter 2. Nutrition in Animals
  • Chapter 3. Fibre to Fabric
  • Chapter 4. Heat
  • Chapter 5. Acids, Bases and Salts
  • Chapter 6. Physical and Chemical Changes
  • Chapter 7. Weather; Climate and Adaptations of Animals to Climate
  • Chapter 8. Winds, Storms and Cyclones
  • Chapter 9. Soil
  • Chapter 10. Respiration in Organisms
  • Chapter 11. Transportation in Animals and Plants
  • Chapter 12. Reproduction in Plants
  • Chapter 13. Motion and Time
  • Chapter 14. Electric Current and its Effects
  • Chapter 15. Light
  • Chapter 16. Water; a Precious Resource
  • Chapter 17. Forests; Our Lifeline
  • Chapter 18. Wastewater Story