NCERT Exemplar Problems Class 7 Science – Acids, Bases and Salts

Question 1:
The correct way of making a solution of acid in water is to
(a) add water to acid
(b) add acid to water
(c) mix acid and water simultaneously
(d) add water to acid in a shallow container
Answer:
(b) Making solution of acid and water evolve heat. Acid should be added to water dropwise.

Question 2:
Products of a neutralisation reaction are always
(a) an acid and a base
(b) an acid and a salt
(c) a salt and water
(d) a salt and a base
Answer:
(c) Acid and base participate in neutralisation reaction and produce salt and water.

Question 3:
Turmeric is a natural indicator. On adding its paste to acid, and base separately, which colours would be observed?
(a) Yellow in both acid and base
(b) Yellow in acid and red in base
(c) Pink in acid and yellow in base
(d) Red in acid and blue in base
Answer:
(b) Turmeric paste is yellow in acidic solution and turns rod in basic solution.

Question 4:
Phenolphthalein is a synthetic indicator and its colours in acidic and basic solutions respectively are
(a) red and blue
(b) blue and red
(c) pink and colourless
(d) colourless and pink
Answer:
(d) Phenolphthalein remains colourless in acidic solution but turns pink in basic solution.

Question 5:
When the soil is too basic, plants do not grow well in it. To improve its quality, what must be added to the soil?
(a) Organic matter
(b) Quicklime
(c) Slaked lime
(d) Calamine solution
Answer:
(a) Organic matter (called manure or compost) releases acids which neutralise the excess bases present in the soil and reduce its basic nature.

Question 6:
‘Litmus’ a natural dye in an extract of which of the following
(a) China rose (Gudhal)
(b) Beetroot
(c) Lichen
(d) Blue berries (lamun)
Answer:
(c) Litmus is a mauve (purple) colour pigment obtained from Lichens. It turns red when added to acidic solution and blue when added to a basic solution.

Question 7:
Neutralisation reaction is a
(a) physical and reversible change (b) physical change that cannot be reversed (c) chemical and reversible change (d) chemical change that cannot be reversed
Answer:
(d) Neutralisation reaction is a chemical reaction between acid and base to produce salt and water. It cannot be reversed.

Question 8:
A solution changes the colour of turmeric indicator from yellow to red. The solution is
(a) basic
(b) acidic
(c) neutral
(d) None of these
Answer:
(a) Basic solution changes the turmeric indicator from yellow to red.

Question 9:
Which of the following sets of substances contains acids?
(a) Grapes, lime water
(b) Vinegar, soap
(c) Curd, milk of magnesia
(d) Curd, vinegar
Answer:
(d) Curd contains lactic acid and vinegar contains acetic acid. Lime water, soap and milk of magnesia are basic substances and grapes contains tartaric acid.

Question 10:
On adding phenolphathalein indicator to a colourless solution, no change is observed. What is the nature of this solution?
(a) Basic
(b) Either acidic or basic
(c) Either acidic or neutral
(d) Either basic or neutral
Answer:
(c) Phenophthalein is colourless in acidic and neutral medium but turns pink when introduced in basic medium.

Question 11:
Which of the following is an acid-base indicator?
(a) Vinegar
(b) Lime water
(c) Turmeric
(d) Baking soda
Answer:
(c) Turmeric paste changes it colour from yellow to red when treated with a base.
So, turmeric is an acid-base indicator.

Very Short Answer Type Questions

Question 12:
Look at the given reaction, Hydrochloric acid + Sodium hydroxide (base) —> Sodium chloride (salt) + Water; Sodium chloride formed in this reaction remains in solution form. Can we get solid sodium chloride from this solution? Suggest a method (if any).
Answer:
We can get solid sodium chloride by evaporation method. Water changes to vapour form during evaporation and solid sodium chloride is left in the container.

Question 13:
State whether the following statements are true or false. Correct the false statements.
(a) All substances are either acidic or basic.
(b) A compound if acidic will turn all indicators red.
(c) Lime water turns red litmus blue.
(d) Common salt dissolved in water turns blue litmus red.
(e) Phenolphthalein is a natural indicator.
(f) Calamine can be used to treat ant’s sting.
(g) Lemon water is basic in nature.
Answer:
(a) False, some substances, are neutral like water, table salt etc.
(b) False, acids do not turn all indicators red, e.g. China rose indicator turns to dark pink (magenta) when acidic compound is added to it.
(c) True, lime water is basic.
(d) False, common salt and water both are neutral hence, have no effect on litmus.
(e) False, phenolphthalein is synthetic indicator.
(f) True, calamine is basic in nature, while ant’s sting is acidic. Thus, calamine causes relief from the sting.
(g) False, lemon water is acidic in nature.

Question 14:
Paheli is suffering from indigestion due to acidity. Is it advisable to give her orange juice in this situation and why?
Answer:
No, because orange juice is acidic in nature. Instead she should be given an antacid (milk of magnesia), which is base and neutralise the acidity. Milk of magnesia contains a base magnesium hydroxide which reacts with the acid and cures indigestion.

Short Answer Type Questions

Question 15:
Look at the figure which shows solutions taken in test tubes A, B, C and D. What colour is expected, when a piece of red litmus paper is dropped in each test tube? Nature of the solutions is given in the table for your help.

Answer:
Change in colour of red litmus is given below:

Question 16:
While playing in a park, a child was stung by a wasp. Some elders suggested applying paste of baking soda and others lemon juice as remedy. Which remedy do you think is appropriate and why?
Answer:
Wasp sting inject a liquid in the skin which is acidic in nature. Hence, baking soda is the appropriate remedy as it is basic in nature and neutralises the acid.

Question 17:
Form a sentence using the following words :
baking soda, ant bite, moist, effect, neutralised, rubbing
Answer:
The effect of an ant bite can be neutralised by rubbing moist baking soda.

Question 18:
Match the substances in Column I with those in Column II.

Answer:
The correct matching is as given
(a) -(iii), (b)—(v), (c)—(iv). (d)—(i), (e)-(ii)

Question 19:
Fill in the blanks.
(a) Lemon juice and vinegar taste…………… because they contain ..
(b) Turmeric and litmus are…………. acid-base indicators.
(c) Phenolphthalein gives……………. colour with lime water.
(d) When an acidic solution is mixed with a basic solution, they each other forming……………. and water.c
Answer:
(a) sour, acids
(b) natural
(c) pink
Note Lime water is basic.
(d) neutralise, salt
Note This reaction is called neutralisation reaction.

Long Answer Type Questions

Question 20:
Boojho, Paheli and their friend Golu were provided with a test tube, each containing China rose solution which was pink in colour. Boojho added 2 drops of solution ‘A’ in his test tube and got dark pink colour. Paheli added 2 drops of solution ‘B’ to her test tube and got green colour. Golu added 2 drops of solution ‘C but could not get any change in colour. Suggest the possible cause for the variation in their results.
Answer:
China rose (gudhal) is an acid-base indicator.
China rose solution in different medium shows following changes:
China rose + acid —> give dark pink colour, hence A is an acidic solution.
China rose + base —> give green colour, hence 6 is a basic solution.
China rose + neutral —> no change in colour, hence C is a neutral solution.

Question 21:
Crossword Puzzle
Fill in the crossword given as figure with the help of the clues provided.
Across
2. The solution which does not change the colour of either red or blue litmus.
4. Phenolphthalein gives pink colour in this type of solution.
7. Colour of blue litmus in lemon juice.
Down
1. It is used to test whether a substance is acidic or basic.
3. It is a natural indicator and gives pink colour in basic solution.
5. Nature of ant’s sting.
6. It is responsible for increase in temperature during a neutralisation reaction.
Answer:

Question 22:
A farmer was unhappy because of his low crop yield. He discussed the problem with an agricultural scientist and realised that the soil of his field was either too acidic or too basic. What remedy would you suggest the farmer to neutralise the soil?
Answer:
The soil may be acidic or basic naturally. The excessive use of fertilisers in the fields also makes the soil too acidic. When the soil is too acidic, it is treated with base such as quicklime (calcium oxide) or slaked lime (calcium hydroxide) which neutralises the excess acid present in the soil and reduces its acidic nature.
If the soil is too basic, then decaying organic matter (called manure or compost) is added it. The decaying organic matter releases acid which neutralises the excess bases present in the soil and reduces its basic nature.

Question 23:
You are provided with four test tubes containing sugar solution, baking soda solution, tamarind solution, salt solution. Write down an activity to find the nature (acidic/basic/neutral) of each solution.

Answer:
We can use both red and blue litmus solutions to find the nature (acidic/basic/neutral) of each solution. In acidic media, blue litmus solution turns to red and in basic media, red litmus solution turns to blue.

Question 24:
You are provided with three test tubes A, B and C as shown in figure with different liquids. What will you observe when you put
(a) a piece of blue litmus paper in each test tube?
(b) a piece of red litmus paper in each test tube?
(c) a few drops of phenolphthalein solution to each test tube?

Answer:

Question 25:
Paheli observed that most of the fish in the pond of her village were gradually dying. She also observed that the waste of a factory in their village is flowing into the pond which probably caused the fish to die.
(a) Explain why the fish were dying?
(b) If the factory waste is acidic in nature, how can it be neutralised?
Answer:
(a) If the wastes of a factory flow into waterbodies, it can cause a threat to the lives of aquatic creatures and to anybody who drink the water. Since, the acids present in the factory waste will kill the fish and other aquatic organisms.
(b) If the factory waste is acidic in nature, it can be neutralised by adding basic substances.

Question 26:
Explain two neutralisation reactions related to daily life situations.
Answer:
(a) Ant bite When an ant bite 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 caiamine soiution, which contains zinc carbonate.
(b) Indigestion 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.