Std 7 Basic Science: Chapter 03 Acids and Alkalis - Questions and Answers | Teaching Manual


Textbooks Solution for Class 7 Basic Science (English Medium) ആസിഡുകളും ആൽക്കലികളും | Text Books Solution Basic Science (English Medium) Chapter 03  Acids and Alkalis - Teaching Manual / Teachers Handbook

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Chapter 03: Acids and Alkalis - Questions and Answers Questions
1. Write a short note on hibiscus paper?
Answer: Rub thoroughly a hibiscus flower on both sides of a piece of white paper. Remove all traces of flower sticking on it after it is dried. Using scissors, cut the paper into long and narrow strips. This can be called hibiscus paper.

2. What do you know about litmus paper?
Answer: In laboratories, litmus paper is commonly used instead of hibiscus paper. They are available in red and blue colours.

3. A) Conduct an experiment with hibiscus paper on buttermilk, water, coconut oil, vinegar, soap solution, sugar solution, salt solution, tamarind
water and milk and write down the colour changes that happen in hibiscus paper in each situation?
Answer: 
Buttermilk- Changes to red colour
Water- no colour change
Coconut oil- no colour change
Vinegar-changes to red colour
Soap solution- no colour change
Sugar solution-no colour change
Salt solution- no colour change
Tamarind water- changes to red
milk- no colour change.
B) Repeat the above experiments, using blue litmus paper, instead of hibiscus paper and write down the colour changes that happen in blue
litmus paper in each situation?
Answer: 
Buttermilk- Changes to red colour
Water- no colour change
Coconut oil- no colour change
Vinegar-changes to red colour
Soap solution- no colour change
Sugar solution-no colour change
Salt solution- no colour change
Tamarind water- changes to red colour
milk- no colour change.
C) Which are the liquids that turned the hibiscus paper red?
Answer: Buttermilk, vinegar, tamarind water.
D) Did you notice that the liquids that turned hibiscus flower paper red generally have a sour taste?
Answer:  yes

4. Write a short note on acids?
Answer: Lemon juice, buttermilk, tamarind, vinegar, etc. contain certain acids Litmus paper is red in acid. All acids have a sour taste. Acids present in food substances are weak. Hydrochloric acid, nitric acid, sulphuric acid, which are used in laboratories are strong acids.

5. Write the acids present in the following food substances.
*Buttermilk *Vinegar *Tamarind *Lemon *Apple
Answer: 
6. What are the precautions that we should take while handling chemicals?
Answer: 
* Do not taste 
* Do not touch
* Do not let it fall on the body.
* Use a dropper while transferring acid from the bottle.
* While diluting an acid, pour small quantities of acid slowly into water taken in a beaker and stir well.
* Use a holder to hold a test tube. 

7. A) Write an experiment note on the reaction of dilute hydrochloric acid with zinc.
Answer: 
Aim:-
To react dilute hydrochloric acid with Zinc.
Materials required:-
Dilute hydrochloric acid, test tube, a piece of zinc.
Method
Take a little dilute hydrochloric acid in a test tube and put a piece of zinc into it. Close the mouth of the test tube with thumb for some time. Hold a lighted match stick at the mouth of the test tube, and remove the thumb.
Observation:-
A gas evolves from the test tube and burns with low noise when lighted. 
Conclusion:-
When diluted hydrochloric acid reacts with zinc, hydrogen is produced.
Hydrogen is inflammable. Hydrogen gas evolves from the test tube and
burns with low noise when lighted.
B) Do the same experiment using dilute sulphuric acid and metals like
Zinc, Magnesium and aluminium. Record the observations in your
science diary.
Answer:  When dilute sulphuric acid reacts with metals like zinc, magnesium and aluminium a lot of gas bubbles are formed. The flame goes off and gas burns in blue colour with a pop sound. Here hydrogen gas is produced by the reaction of sulphuric acid with metals like zinc, magnesium and aluminium.

8. Hydrogen gas was first discovered by .................
Answer: Henry Cavendish.

9. The meaning of the word hydrogen is ...........
Answer: Water producing

10. Who gave the name hydrogen to the inflammable gas produced while
the reaction of some metals with acids.
Answer: Lavoisier

11. Have you noticed the bottle and its stopper, used to store acid in the
laboratory. Why are the metals stoppers are not used?
Answer: Acids react with metals to form hydrogen. Hydrogen is inflammable. So, metal stoppers are not used.
12. The pickles we commonly use are acidic in nature. Are they stored in metal containers?
Answer: Pickles are acidic in nature. If they are stored in metal containers, they will react with metal. This can cause health problems.

13. Is an aluminium vessel or an earthen pot more suitable for preparing food containing buttermilk?
Answer: An earthen pot is suitable for preparing food containing buttermilk. Because buttermilk is acidic in nature which will react with metals like aluminium.

14. Write an experiment note for the reaction of dilute sulphuric acid
with pieces of eggshells.
Answer: 
Aim:-
To understand the reaction of eggshells with dilute sulfuric acid.
Materials required:-
Test tube, dilute sulphuric acid, match stick.
Method:-
Take dilute sulphuric acid in a test tube and add pieces of eggshell into it.
Bring a lighted matchstick above the test tube.
Observation:-
The flame is extinguished.
Conclusion:-
Eggshells contain calcium carbonate. When acid reacts with carbonates
carbon dioxide is produced. Carbon dioxide is a gas that extinguishes fire.

15. Write an experiment note to make a fire extinguisher
Answer: 
Aim:-
To make a fire extinguisher
Materials required:-
A plastic bottle, vinegar, baking soda, plastic tube, paper and a candle.
Method:-
Fix a tube airtight in the cap of a plastic bottle. Fill vinegar up to half of the bottle. Pack some baking soda in a paper and suspend it from the
tube at the opening of the bottle, above the level of vinegar. Take care that the baking soda does not fall into the vinegar. Close the bottle properly.
Shake the bottle thoroughly and allow the baking soda to fall into the vinegar.
Observation:-
On shaking, the baking soda fell into vinegar and gas came from the
bottle. When a lighted candle is brought to the mouth of the bottle, the flame is extinguished.
Conclusion:
Here the carbonates react with acids and carbon dioxide is formed. Carbon dioxide extinguishes fire. The fire extinguisher works on this principle.

16. Baking soda is......
Answer: Sodium bicarbonate

18. What are the general properties of acids?
Answer: 
* Colour of litmus in acid is red
* Sour in taste
* Reacts with metals and produces hydrogen.
* Reacts with carbonates and produces carbon dioxide. 

19. ‘When ants bite, we feel little pain. Why?
Answer: When ants bite, a small amount of formic acid enters our bodies. This is why it pains.

20. Dip red hibiscus paper and red litmus paper in water, buttermilk,
vinegar, soap solution, lime water, salt solution, sugar solution, milk
and clear ash suspension, and observe what happens.
A) Which are the liquids in which red hibiscus paper turns blue?
Answer:  Soap solution, lime water, clear ash Suspension.
B) which are the liquids in which red litmus paper turns blue?
Answer: Soap solution, lime water, clear ash suspension.

21) What are ‘alkalis’? Give examples. 
Answer: Substances like lime water which turn red litmus blue are called alkalis. They have a bitter taste and are slimy.
Examples:-
* Calcium hydroxide (lime water).
* Sodium hydroxide solution (caustic soda)
* Potassium hydroxide solution (caustic potash)
* Ammonium hydroxide solution (liquor ammonia dissolved in water)

22. What are indicators? Give examples.
Answer: Indicators are substances that help us to distinguish between acids and alkalis through colour change. Parts of plants like turmeric, hibiscus, beetroot etc can be used as indicators. Litmus paper is an indicator used in the laboratory.
23. How can we make red hibiscus paper?
Answer: Dip the already prepared blue hibiscus paper in acidic solution and then dried them. Then we will get red hibiscus paper.

24. Make papers of turmeric, mango leaf, beetroot, carrot, onion
and hibiscus paper. Check whether each of these papers shows different
colours in dilute acid and alkali. Record the experiments and observations in your science diary.
Answer: 

25. What are the substances that can be used to distinguish between acids and alkalis?
Answer: Turmeric, hibiscus, beetroot etc are the substances that can be used to distinguish between acids and alkalis.

26. What could be the reason for the red colour appearing on clothes when the turmeric stain on them is washed with soap?
Answer: Soap solution is alkaline. On washing with soap, a turmeric stain on the cloth turns red. Turmeric is a natural indicator to identify alkali.

27. Besides litmus, what other indicators are used in the laboratory?
Answer: Besides litmus, Phenolphthalein, Methyl orange etc are the other
indicators used in the laboratory

28. See the table given below:
A) What are the substances that can be used as indicators to identify acid?
Answer:
*Blue litmus paper
*Blue hibiscus paper
*Methyl orange
B) What are the indicators that can be used to identify alkali?
Answer:
*Red litmus paper
*Phenolphthalein
*Turmeric
*Methyl orange 

29. What is known as a universal indicator?
Answer: Universal Indicator is a mixture of different indicators. Depending on the strength of the acidic or the alkaline nature of the solution, the universal indicator gives different colours. These colours can be compared with the colour chart affixed outside the bottle.

30. What is called neutralisation?
Answer: When definite amounts of acid and alkali are mixed, their acidic and alkaline natures are lost and salt and water are formed. This is called
neutralisation.
Acid + Alkali = Salt +Water

31. Which are the products obtained when hydrochloric acid and sodium
hydroxide are combined?
Answer: Sodium chloride and water.

32. Why are indicators like phenolphthalein used while conducting neutralisation reactions?
Answer: Phenolphthalein is often used as an indicator while conducting
a neutralization reaction. Because it will appear pink in alkali solutions
and colourless in acidic solutions.
33. Write a short note On PH paper.
Answer:
* OH paper is used to examine whether a substance is acidic or alkaline.
* If the pH value is 7, it indicates a neutral substance. It means the substance is neither acidic nor alkaline.
* The pH value of pure water is 7.
* When pH value is greater than 7, the Substance is alkaline and when it is less than 7, it is acidic.

34. How can we find the pH of soil?
Answer:
* Take a glass and fill it half with soil.
* Pour water till the soil is immersed completely and stir well.
* Tilt the glass and collect the oozing water.
* When the water clears, dip a pH paper into it.
* Compare the colour change of the pH paper with the colour chart and find the pH of the soil.

35. What is called acidity? What kind of medicine will the doctor prescribe to overcome this?
Answer: Acid is produced in the stomach to support the digestion of food. At times, when food is not taken properly or when enough water is not consumed and unhealthy food habits are followed, the number of acids increases in the stomach. This is called acidity. Doctors prescribe medicines containing alkali to overcome this. 

36. Find the instances when acids and alkalis are used in daily life.
Answer:
37. Write a short note on the production of soaps Materials required for the production of soap:-
Answer:
Sodium hydroxide (caustic soda)- 180 g,
Coconut oil- 1 kg, Water -350 ml,
sodium silicate- 100 g, Stone powder
(talcum powder)- 100 g.
Procedure:-
Dissolve caustic soda in water taken in a steel vessel. Heat is liberated when caustic soda dissolves in water. It takes 3-4 hours for the solution to get cooled. When the solution cools, pour it slowly into coconut oil taken
in a plastic or steel vessel and stir it. Add stone powder and sodium silicate. This is to increase the hardness and quantity of soap. Stir well till the mixture solidifies. Perfumes and dyes may be added for fragrance and
attractive colour. Pour the above mixture into moulds. In three to four days, the mixture becomes soap. The soap taken from moulds should be used only after two weeks.

Let us asses

1. The pH values of different soil samples are given. Which of the soils should be treated with lime?
Answer: 5 

2. When a magician rubbed white paper with a kerchief, red letters appeared. Which of the following materials might have been used by him?
Answer: Methyl orange, sulphuric acid

3. Classify the substances in the following table
Answer:
A- alkali
B- acid
C- acid
D- Neutral
E- alkali
F- Neutral 

4. When buttermilk spills on a marble floor, a stain appears after some time. Why?
Answer: Marble contains calcium carbonate and buttermilk contains lactic acid. Calcium carbonate reacts with lactic acid in buttermilk and produces carbon dioxide. That is why, when buttermilk spills on a marble floor, a stain appears after some time.






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