Q1. Fill in the blanks.
i. Change of colour in an acid and a base depends on the type of the indicator.

ii. Acid turns blue litmus red.

iii. Bases turn red litmus blue.

iv. Litmus has a mauve (purple) colour in distilled water.

v. In neutralisation reaction a new substance is formed. This is called salt.

vi. Lemon juice is acidic in nature.

Q2. True/False
i. Nitric acid turns red litmus blue. False

ii. Sodium hydroxide turns blue litmus red. False

iii. Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. True

iv. Indicator is a substance which shows different colours in acidic and basic solutions. True

v. Tooth decay is caused by the presence of a base. False

vi. If an indicator changes colour with a base, it does not change colour with an acid. False

Q3. What is acid rain?
Ans. The rain containing excess of acids is called an acid rain.

Q4. Which acid naturally present in our stomach?
Ans. Hydrochloric acid (HCl)

Q5. What is the role of hydrochloric acid in the stomach?
Ans. Hydrochloric acid (HCl) helps us to digest food.

Q6. Which acid is present in an ant sting?
Ans. Formic acid is present in an ant sting.

Q7. What is the nature of soap solution?
Ans. Soap solution is basic in nature.

Q8. What is the nature of distilled water?
Ans. The Distilled water is neutral.

Q9. State the nature of baking soda.
Ans. Baking soda is basic in nature.

Q10. Where does the word acid come from?
Ans. The word acid comes from the Latin word acere which means sour.

Q11. Why curd, lemon juice, orange juice and vinegar taste sour?
Ans. These substances taste sour because they contain acids.

Q12. Which is the most commonly used natural indicator?
Ans. The most commonly used natural indicator is litmus.

Q13. Tom rubs a solution between fingers and feels soapy, what is the nature of that solution?
Ans. Basic

Q14. Name the acid which is present in each cell of our body.
Ans. Each cell in our body contains an acid, the deoxyribonucleic acid or DNA.

Q15. When red litmus paper is dipped in a solution; it remains red, what is the nature of the solution?
Ans. Neutral

Q16. What are the effects of acid rain?
Ans. Acid rain can cause damage to buildings, historical monuments, plants and animals.

Q17. What does organic matter do in soil?
Ans. If the soil is basic, organic matter is added to it. Organic matter releases acids which neutralises the basic nature of the soil.

Q18. Why solid baking soda does not change colour of dry litmus paper?
Ans. The solid baking soda does not change colour of dry litmus paper because in solid states ions are not free to move.

Q19. How acidic soil can be treated?
Ans. When the soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide).

Q20. What are salts? Give example.
Ans. In neutralisation reaction a new substance is formed. This is called salt. Salt may be acidic, basic or neutral in nature. Example: Sodium chloride.

Q21. What is the role of DNA in the cell of the human body?
Ans. It controls every feature of the body such as our looks, colour of our eyes, our height etc.

Q22. Give examples of some acids and bases.
Ans. Acids - Curd, lemon juice, vinegar, orange juice etc.

Base - baking soda, lime water etc.

Q23. What are indicators?
Ans. Solutions of substances that show different colour in acidic, basic and neutral solutions are called indicators.

Q24. What is the effect of the China rose indicator on acidic and basic solutions?
Ans. China rose indicator turns acidic solutions to dark pink (magenta) and basic solutions to green.

Q25. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Ans. Ammonia is basic in nature as it turns the red litmus blue.

Q26. Why a turmeric stain on my white shirt is turned to red when it is washed with soap.
Ans. A turmeric stain on my white shirt is turned to red when it is washed with soap because the soap solution is basic in nature.

Q27. How to prepare lime water?
Ans. To prepare limewater, dissolve some lime (chuna) in water in a bottle. Stir the solution and keep it for some time. Pour a little from the top. This is lime water.

Q28. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Ans. The solution could be a base or neutral solution because blue litmus paper doesn’t change its colour in the neutral as well as in basic solution.

Q29. What do you mean by neutral solution? Give examples.
Ans. The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. Examples- sugar solution, distilled water, salt etc.

Q30. Is the distilled water acidic/basic/neutral? How would you verify it?
Ans. Distilled water is neutral in nature. This can be verified by using red and blue litmus papers. Neither will show change in colour with distilled water. This proves that distilled water is neutral.

Q31. How does rain become acidic?
Ans. The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively.

Q32. Why factory waste is neutralised before disposing it into the water bodies?
Ans. 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.

Q33. Calamine solution is applied on the skin when an ant bites. Give reason.
Ans. When an ant bites, it injects the acidic liquid (formic acid) into the skin. The effect of the acid can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate.

Q34. 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 most commonly used natural indicator. 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.

Q35. Why we take an antacid tablet when we suffer from acidity?
Ans. 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.

Q36. Explain 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

(Heat is evolved)

The following reaction is an example:

Q37. 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. Dorji can use red and blue litmus paper in order to test the drink.

i.        The drink which does not change the colour of either red or blue litmus is the neutral drink.

ii.        The drink which turns red litmus blue is the basic drink.

iii.        The drink which turns blue litmus red is the acidic drink.

Q38. Complete the following table.

Ans.

Found in Name of acid
Vinegar Acetic acid
Ant’s sting Formic acid
Citrus fruits such as oranges, lemons, etc. Citric acid
Curd Lactic acid
Spinach Oxalic acid
Amla, Citrus fruits Ascorbic acid (Vitamin C)
Tamarind, grapes, unripe mangoes, etc. Tartaric acid

 

Ans. Properties of base

1. Bases are bitter in taste and soapy to touch.

2. Bases turn red litmus blue.

3. China rose indicator turns basic solutions to green.

4. Turmeric indicator turns basic solutions

5. Odorless (except ammonia).

6. Bases are slippery.

7. When the solution is basic, phenolphthalein gives a pink colour.

Q40. State few properties of acids.

Ans. Properties of acids

1. Acids are sour in taste

2. Acid turns blue litmus red.

3. China rose indicator turns acidic solutions to dark pink (magenta).

4. Turmeric indicator does not change its colour with acid

5. Gives burning sensation.

6. Acids are usually sticky.

7. When the solution is acidic, phenolphthalein remains colourless.

Q41 State the differences between acids and bases.

Ans.  Difference between acids and bases

Acids Bases
1. Acids are sour in taste 1. Bases are bitter in taste and soapy to touch.
2. Acid turns blue litmus red. 2. Bases turn red litmus blue.
3. China rose indicator turns acidic solutions to dark pink (magenta). 3. China rose indicator turns basic solutions to green.
4. Turmeric indicator does not change its colour with acid 4. Turmeric indicator turns basic solutions
5. Gives burning sensation. 5. Odorless (except ammonia).
6. Acids are usually sticky. 6. Bases are slippery.
7. When the solution is acidic, phenolphthalein remains colourless. 7. When the solution is basic, phenolphthalein gives a pink colour.

 

Q42. 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. Put a drop of each of the three liquids i.e. hydrochloric acid, sodium hydroxide and sugar solution on the turmeric indicator, the liquid which changes the colour of turmeric indicator to red is basic in nature, that is, sodium hydroxide.

Now, put a drop of sodium hydroxide on a drop of each of the other two liquids separately i.e. hydrochloric acid and sugar solution. After that, put the drops of these mixtures on turmeric indicator. The drop which will change colour of the turmeric indicator to red contains sugar solution. This is because the mixture of basic and neutral solution is basic in nature. On the other hand, the drop will not change the colour of turmeric indicator contains hydrochloric acid. This is because hydrochloric acid reacts with sodium hydroxide to form a neutral solution.

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