NCERT Exemplar Class 7 Science Chapter – 5 -Acids, Bases and Salts- (Solved with detailed explanation)
MULTIPLE CHOICE QUESTIONS
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) add acid to water.
Explanation- Adding acid to water is the correct method because it allows for the heat generated by the exothermic reaction to be absorbed by the larger amount of water, minimising the risk of splashing or boiling.
Also Check – What is Neutralisation? – The Basics Explained
2. Products of a neutralisation reaction are always
- (a) an acid and a base.
- (b) an acid and a salt.
- (c) salt and water.
- (d) a salt and a base.
Answer- (c) salt and water.
Explanation- Neutralisation reactions between an acid and a base typically produce salt and water, following the general equation- Acid + Base → Salt + Water.
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) Yellow in acid and red in base.
Explanation- Turmeric paste remains yellow in acidic solution and turns red in basic solution.
Also Check – Indicators for Acid and Base- A Guide for Students
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) colourless and pink.
Explanation- Phenolphthalein is colourless in acidic solutions and turns pink in basic solutions.
Also Check – Rapid Revision – Class 7 Science – Chapter 5-Acids, Bases and Salts
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) Quick lime
- (c) Slaked lime
- (d) Calamine solution
Answer- (a) Organic matter.
Explanation- Organic matter releases acids which can neutralise the basic nature of the soil.
6. ‘Litmus’, a natural dye, is an extract of which of the following?
- (a) China rose (Gudhal)
- (b) Beetroot
- (c) Lichen
- (d) Blueberries (Jamun)
Answer- (c) Lichen.
Explanation- Litmus is obtained from lichens and is used as a pH indicator.
Also Check – Class 7 science -Chapter 5-Acids, Bases and Salts- Complete Notes
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) chemical change that cannot be reversed.
Explanation- Neutralisation is a chemical change because new substances are formed, and it is not reversible under normal conditions.
Also Check – What is a Neutralisation Reaction?- Explained
8. A solution changes the colour of the turmeric indicator from yellow to red. The solution is
- (a) basic
- (b) acidic
- (c) neutral
- (d) either neutral or acidic
Answer- (a) basic.
Explanation- Turmeric turns red in the presence of a basic substance.
9. Which of the following sets of substances contain acids?
- (a) Grapes, lime water
- (b) Vinegar, soap
- (c) Curd, milk of magnesia
- (d) Curd, vinegar
Answer- (d) Curd, vinegar.
Explanation- Both curd and vinegar contain acids. Curd contains lactic acid and vinegar contains acetic acid.
Also Check – Class 7- Chapter 4 – Acids, Bases and Salts – 4 Worksheets Solved and Unsolved
10. On adding phenolphthalein 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) Either acidic or neutral.
Explanation- If no colour change occurs upon adding phenolphthalein to a solution, it indicates that the solution is either acidic or neutral because phenolphthalein is colourless in both cases.
11. Which of the following is an acid-base indicator?
- (a) Vinegar
- (b) Lime water
- (c) Turmeric
- (d) Baking soda
Answer- (c) Turmeric.
Explanation- Turmeric acts as a pH indicator changing colour in the presence of an acid or a base. Vinegar and lime water are substances that can be tested by indicators, and baking soda is a base.
VERY SHORT ANSWER QUESTIONS
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- Yes, we can get solid sodium chloride from this solution. The method to obtain solid sodium chloride from its solution is called evaporation.
Explanation- When sodium hydroxide reacts with hydrochloric acid, sodium chloride (salt) and water are formed. This reaction is an example of a neutralisation reaction. The sodium chloride is dissolved in the water, creating a saline solution. To recover the solid sodium chloride, the solution can be heated to allow the water to evaporate. As the water evaporates, the solution becomes more concentrated until the salt begins to crystallise out of the solution. Once all the water has evaporated, the solid sodium chloride remains and can be collected. This method is widely used in salt production industries where sea water or saline water is evaporated to obtain salt.
Question 13- State whether the following statements are true or false. Correct the false statements.
(a) All substances are either acidic or basic.
Answer- False
Corrected Statement- Some substances are neutral, meaning they are neither acidic nor basic.
Explanation- Substances can be acidic, basic, or neutral. The pH scale, which ranges from 0 to 14, classified substances as acidic (less than 7), neutral (at 7), or basic (more than 7).
(b) A compound if acidic will turn all indicators red.
Answer- False
Corrected Statement– An acidic compound may turn some indicators red, but not all indicators have the same colour change in acidic conditions.
Explanation- Different indicators have different colours in acidic solutions; for instance, phenolphthalein remains colourless in acidic solutions, while methyl orange turns red.
(c) Lime water turns red litmus blue.
Answer- True
Explanation- Lime water is a basic solution and basic solutions turn red litmus paper blue, confirming the nature of the substance as basic.
(d) Common salt dissolved in water turns blue litmus red.
Answer- False
Corrected Statement- Common salt dissolved in water does not change the colour of blue litmus paper because it is neutral.
Explanation- Sodium chloride solution is neutral and will not affect the colour of litmus paper.
(e) Phenolphthalein is a natural indicator.
Answer- False
Corrected Statement- Phenolphthalein is a synthetic indicator.
Explanation- Phenolphthalein is a chemical compound created in a laboratory and is not found naturally.
(f) Calamine can be used to treat an ant’s sting.
Answer- True
Explanation- Calamine lotion, which often contains zinc oxide or zinc carbonate, can help neutralise the formic acid from an ant sting and relieve irritation.
(g) Lemon water is basic in nature.
Answer- False
Corrected Statement- Lemon juice contains citric acid, making it acidic in nature.
Explanation- Lemon water is acidic due to the presence of citric acid, and this acid contributes to the sour taste of lemons.
Question- Paheli is suffering from indigestion due to acidity. Is it advisable to give her orange juice in this situation and why?
Answer- No, it is not advisable to give her orange juice.
Explanation- Orange juice is acidic because it contains citric acid. Consuming orange juice when suffering from indigestion due to acidity can exacerbate the condition by increasing the acid content in the stomach, leading to more discomfort. It is generally recommended to consume alkaline or neutral substances, like water or milk, to neutralise the excess stomach acid during indigestion.
SHORT ANSWER QUESTIONS
Question- Look at Figure 5.1 which shows solutions taken in test tubes A (Milk in water), B (Baking powder in water), C (Vinegar in Water) and D (Glucose in water). 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– For Test Tube A (Milk in water)- No change. Milk is generally neutral, so red litmus paper should remain red. For Test Tube B (Baking powder in water)- Blue. Baking powder is basic when dissolved in water. For Test Tube C (Vinegar in Water)- Red. Vinegar is acidic, but since the paper is already red, there will be no change. For Test Tube D (Glucose in water)- No change. Glucose solution is neutral, so red litmus paper should remain red.
Explanation-
- Litmus paper is used as an indicator to determine the acidity or alkalinity of a solution. Red litmus paper turns blue in basic conditions but remains red in acidic or neutral solutions.
- Milk is a neutral substance, meaning it does not change the colour of red litmus paper.
- Baking powder contains sodium bicarbonate, which reacts with water to form a basic solution, thus turning red litmus paper blue.
- Vinegar contains acetic acid, which is acidic, but red litmus paper is used to test for basicity, so it does not change colour when placed in an acidic solution.
- Glucose is a sugar and does not affect the pH of the water significantly, so it remains neutral and does not change the colour of red litmus paper.
Question 16- While playing in a park, a child was stung by a wasp. Some elders suggested applying baking soda and lemon juice as a remedy. Which remedy do you think is appropriate and why?
Answer- Applying a paste of baking soda is the appropriate remedy.
Explanation- A wasp sting injects an alkaline venom into the skin. Baking soda, or sodium bicarbonate, is a mild base and can neutralise the effects of the wasp’s alkaline venom when made into a paste with water and applied to the sting. On the other hand, lemon juice is acidic and may not help in neutralising an alkaline sting, and in some cases, it may even irritate the skin further.
Question 17- Form a sentence using the following words – baking soda, ant bite, moist, effect, neutralised, rubbing.
Answer- When I got an ant bite, I quickly made a moist paste of baking soda and applied it to the affected area, rubbing it gently, which neutralised the stinging effect.
Explanation- This sentence demonstrates how to use a common household item, baking soda, as a neutralising agent to alleviate the painful effect of an ant bite. The words ‘moist’ and ‘rubbing’ indicate the method of applying the baking soda paste to the skin.
Question 18- Match the substances in Column I with those in Column II.
| Column I | Column II |
| (a) Tartaric acid | (i) soap |
| (b) Calcium hydroxide | (ii) curd |
| (c) Formic acid | (iii) unripe mangoes |
| (d) Sodium hydroxide | (iv) ant’s sting |
| (e) Lactic acid | (v) lime water |
Matched Column Answers with Explanation-
(a) Tartaric acid – (iii) unripe mangoes Explanation- Tartaric acid is naturally found in unripe mangoes, giving them their sour taste.
(b) Calcium hydroxide – (v) lime water Explanation- Calcium hydroxide is another name for lime water, which is a solution of calcium hydroxide in water.
(c) Formic acid – (iv) ant’s sting Explanation- Formic acid is the chemical that ants inject when they sting, causing pain and irritation.
(d) Sodium hydroxide – (i) soap Explanation- Sodium hydroxide is used in the production of soap; it reacts with fatty acids to form soap through a process called saponification.
(e) Lactic acid – (ii) curd Explanation- Lactic acid is produced by the fermentation of lactose in milk products, which is what gives curd its tangy flavour.
19. Fill the blanks in the following sentences
Question (a)- Lemon juice and vinegar taste ___________ because they contain ___________.
Answer- Lemon juice and vinegar taste sour because they contain acid.
Explanation- Lemon juice contains citric acid, and vinegar contains acetic acid. Both acids impart a sour taste to these substances.
Question (b)- Turmeric and litmus are _________ acid-base indicators.
Answer- Turmeric and litmus are natural acid-base indicators.
Explanation- Turmeric and litmus are substances extracted from natural sources that change colour in response to the acidity or basicity of a solution. Litmus turns red in acid and blue in base, while turmeric turns reddish-brown in base and yellow in acid.
Question (c)- Phenolphthalein gives _________ colour with lime water.
Answer- Phenolphthalein gives pink colour with lime water.
Explanation- Phenolphthalein is an acid-base indicator that is colourless in acidic solutions and turns pink in basic solutions. Lime water is a basic solution of calcium hydroxide, so phenolphthalein would turn pink in its presence.
Question (d)- When an acidic solution is mixed with a basic solution, they _________ each other forming _________ and water.
Answer- When an acidic solution is mixed with a basic solution, they neutralise each other forming salt and water.
Explanation- Neutralisation is the reaction between an acid and a base to form a salt and water, with the effect of the acid and base cancelling each other out. This reaction often releases heat and results in a pH closer to neutral (7).
LONG ANSWER QUESTIONS
Question- Boojho, Paheli and their friend Golu were provided with a test tube each containing a China rose solution which was pink in colour. Boojho added two drops of solution ‘A’ in his test tube and got dark pink. 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- The different colours observed after adding the solutions to the China rose indicator can be attributed to the varying nature (acidic, basic, or neutral) of the solutions added.
Explanation-
- Boojho’s result (Dark Pink Colour)- The change in colour to dark pink suggests that solution ‘A’ added by Boojho was slightly acidic in nature. China rose solution is known to turn to a darker pink or magenta shade when it becomes more acidic. Hence, the solution ‘A’ was probably a weak acid.
- Paheli’s result (Green Colour)- The green coloration indicates that solution ‘B’ was basic. China rose, also known as Hibiscus, is a natural pH indicator that turns green in the presence of a basic substance. Therefore, solution ‘B’ must have been a base like sodium hydroxide or any other alkaline solution.
- Golu’s result (No Colour Change)- The absence of a colour change in Golu’s test tube indicates that solution ‘C’ was neutral, neither acidic nor basic. This is why it did not affect the pink colour of the China rose solution. Solution ‘C’ could be pure water or a salt solution that is neutral in nature.
21. Fill in the cross word given as Figure 5.2 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) The 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 a pink colour in the basic solution. (5) Nature of ant’s sting. (6) It is responsible for increase in temperature during a neutralisation reaction.
Answer –
Across 2. Neutral- A solution that does not change the colour of red or blue litmus paper.
- 4. Basic- Phenolphthalein gives a pink colour in this type of solution.
- 7. Red- The colour blue litmus paper turns when it is put in lemon juice, which is acidic.
Down
- 1. Indicator- It is used to test whether a substance is acidic or basic.
- 3. Litmus- A natural indicator that turns pink in a basic solution.
- 5.Acidic- The nature of an ant’s sting because it contains formic acid.
- 6. Heat- This is released and is responsible for the increase in temperature during a neutralisation reaction.
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 for the farmer to neutralise the soil?
Answer- To neutralise acidic soil, the farmer should add a base like quicklime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is too basic, adding organic matter (compost) is advisable because it contains acids that can neutralise the alkalinity.
Explanation- Acidic soils can hinder plant growth by affecting the availability of essential nutrients and the activity of soil bacteria. Bases like quicklime or slaked lime can neutralise the excess acidity. Conversely, basic or alkaline soils can also be detrimental to plants. Organic matter, when decomposed, releases acids that can help balance the soil pH to a more neutral level, which is ideal for most crops.
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- To determine the nature of each solution, you can perform a simple test using red and blue litmus paper, which is an acid-base indicator.
- Sugar solution- Dip red and blue litmus paper into the solution. If there is no colour change, the solution is neutral.
- Baking soda solution- Dip red and blue litmus paper into the solution. The red litmus paper should turn blue, indicating that the solution is basic.
- Tamarind solution- Dip red and blue litmus paper into the solution. The blue litmus paper should turn red, indicating that the solution is acidic.
- Salt solution- The nature of the salt solution depends on the salt. For common table salt (sodium chloride), dip red and blue litmus paper into the solution. If there is no colour change, the solution is neutral.
Explanation- Litmus paper is an easy-to-use indicator for acids and bases. Acids turn blue litmus paper red, and bases turn red litmus paper blue. Neutral substances do not cause a colour change in litmus paper. Sugar and table salt solutions are usually neutral, baking soda is a base, and tamarind contains acids, hence its solution is acidic.
Question- You are provided with three test tubes A- (Solution of tartaric acid), B (Soap Solution), and C (pure water) as shown in Figure 5.3 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-
(a) Blue litmus paper-
- In test tube A (tartaric acid), the blue litmus paper will turn red because tartaric acid is acidic.
- In test tube B (soap solution), the blue litmus paper will remain blue as soap solutions are basic.
- In test tube C (pure water), there will be no change in the colour of the blue litmus paper since pure water is neutral.
(b) Red litmus paper-
- In test tube A (tartaric acid), the red litmus paper will remain red because tartaric acid is acidic.
- In test tube B (soap solution), the red litmus paper will turn blue indicating the basic nature of the soap solution.
- In test tube C (pure water), the red litmus paper will remain red because pure water is neutral and does not affect the litmus paper.
(c) Phenolphthalein solution-
- In test tube A (tartaric acid), no colour change will occur and the solution will remain colourless since phenolphthalein is colourless in acidic solutions.
- In test tube B (soap solution), the solution will turn pink indicating the basic nature because phenolphthalein turns pink in a basic solution.
- In test tube C (pure water), the solution will remain colourless because phenolphthalein is also colourless in neutral solutions.
Explanation- Litmus paper and phenolphthalein are common indicators used to test the pH level of a solution. Acidic solutions turn blue litmus paper red and do not affect red litmus paper. Basic solutions turn red litmus paper blue and have no effect on blue litmus paper. Neutral solutions like pure water do not change the colour of either red or blue litmus paper. Phenolphthalein is colourless in acidic and neutral solutions but turns pink in basic solutions. These tests are a simple way to determine whether a solution is acidic, basic, or neutral.
Question- 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) The fish were likely dying because the waste from the factory could be polluting the pond water, making it toxic for the fish to survive. If the waste contains harmful chemicals or is acidic, it can lower the pH of the pond water. A lower pH, which indicates acidity, can disrupt the biological processes of fish and other aquatic life, leading to their death.
(b) If the factory waste is acidic in nature, it can be neutralised by treating the pond water with a basic substance. One common method is to add lime (calcium carbonate, CaCO3, or calcium hydroxide, Ca(OH)2) to the water. This substance will react with the acid in the waste to form a neutral salt and water, thus raising the pH of the water back to a level that is safe for aquatic life.
Explanation- The death of fish in the pond could be due to acidification of the water caused by the acidic waste from the factory. Acidic water can damage or kill aquatic life by causing significant changes in the water chemistry, affecting the availability of nutrients, and disrupting physiological processes in fish. Neutralising the acid with a base like lime is a process of neutralisation, where an acid reacts with a base to form salt and water, which is typically less harmful to the environment.
Question- 26. Explain two neutralisation reactions related to daily life situations.
Answer-
- Treating Bee Stings- Bee venom contains formic acid, which can cause pain and swelling when someone is stung by a bee. A common household remedy for a bee sting is to apply baking soda (sodium bicarbonate) to the affected area. The baking soda, which is a base, reacts with the formic acid in the bee venom, neutralising it and relieving some of the pain and swelling. The reaction produces water and carbon dioxide as byproducts.
- Curing Indigestion– Indigestion is often caused by excess stomach acid. Antacids, which are basic, can be taken to neutralise the excess stomach acid. For example, magnesium hydroxide reacts with hydrochloric acid in the stomach, neutralising it and forming magnesium chloride (a salt) and water. This reaction helps to relieve the symptoms of heartburn and indigestion.
Explanation- Neutralisation reactions in daily life involve the reaction between an acid and a base to produce salt and water. These reactions are important for managing pH levels in various situations, such as medical treatments and environmental cleanups. Neutralisation can provide relief from acid-induced discomfort or counteract environmental pollutants.
Also Check – Chapter 5- Acids, Bases and Salts-Class 7 Question and Answer (Short)
Also Check – NCERT Solutions For Class 7 Science Chapter 5 – Acids, Bases and Salts
Also Check – Acid Rain – Class 7 -Chapter 5-Acids, Bases and Salts
Also Check – Class 7 science -Chapter 5-Acids, Bases and Salts – Definition and Explanation of Important Keywords
Also Check – Chapter 5- Acids, Bases and Salts-Class 7 Question and Answer (Fill in the Blanks)
Also Check – Chapter 5- Acids, Bases and Salts-Class 7 Question and Answer (MCQ)
