Hello students and welcome to Chapter 4! We’ve provided excellent solutions to all the textbook exercise questions, extended learning – activities and projects, in-text questions and in-text activities in the chapter. We have also included additional questions, which contain a ton of information. The material is presented in a scientific manner to teach you the concepts and show you how to answer similar questions in exams. We recommend that you study the entire material in detail for maximum benefit.
Solutions to Exercise (Page No 45) of NCERT Class 7 Science Chapter 4 Acids, Bases and Salts:
1. State differences between acids and bases.
Answer:
The main differences are:
- Acids are sour in taste, while bases are bitter in taste and feel soapy when touched.
- Acids turn blue litmus paper to red, but do not have any effect on red litmus paper. Bases turn red litmus paper to blue, but do not have any effect on blue litmus paper.
Some more differences:
- Acids do not change the colour of turmeric, but bases change the colour of turmeric to red.
- Phenolphthalein remains colourless on contact with acids, but phenolphthalein turns pink on contact with bases.
- Acids cause acid rain, but bases cause no such phenomenon.
2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Answer:
Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. We know bases turn the colour of red litmus to blue. Therefore, ammonia is basic in nature.
3. Name the source from which litmus solution is obtained. What is the use of this solution?
Answer:
Litmus solution is obtained from lichens, where an algae and a fungus live together in symbiotic relationship. It is used as an indicator to identify whether a solution is acidic or basic. When added to acidic solution it turns red and when added to basic solution it turns blue.
4. Is the distilled water acidic/basic/neutral? How would you verify it?
Answer:
Distilled water is neutral is nature. You can verify that distilled water is neutral by dipping both red and blue litmus paper into it. The colour of both will not change, proving that it is neutral.
5. Describe the process of neutralisation with the help of an example.
Answer:
Neutralisation process occurs when an acid and a base react with each other to form salt and water, along with evolution of heat. For example, when the acid hydrochloric acid and the base sodium hydroxide react with each other, sodium chloride salt and water are formed. Also, heat is evolved.
6. Mark ‘T’ if the statement is true and ‘F’ if it is false.
(i) Nitric acid turns red litmus blue. (T/F)
(ii) Sodium hydroxide turns blue litmus red. (T/F)
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
(v) Tooth decay is caused by the presence of a base. (T/F)
Answers:
(i) Nitric acid turns red litmus blue. (T/F)
False. Nitric acid is acidic in nature and will have no effect on red litmus paper. However, it will turn blue litmus to red.
(ii) Sodium hydroxide turns blue litmus red. (T/F)
False. Sodium hydroxide is basic in nature and will have no effect on blue litmus paper. However, it will turn red litmus to blue.
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
True. Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water.Sodium hydroxide is a base and hydrochloric acid is an acid. Hence, they will undergo neutralisation reaction to form common salt and water.
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
True. Indicator is a substance which shows different colours in acidic and basic solutions. Hence, it is used to distinguish between acidic and basic solutions.
(v) Tooth decay is caused by the presence of a base. (T/F)
False. Tooth decay is caused by the presence of an acid and not a base.
7. Dorji has a few bottles of soft drinks 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 the third one wants neutral drink. How will Dorji decide which drink is to be served to whom?
Answer:
Dorji should decide which drink is to be served to whom by tasting each drink. The acidic drink will be sour, the basic drink bitter and the neutral drink will have tasteless.
He can also do a litmus test. The acidic drink only changes blue litmus paper to red and the basic drink only changes red litmus paper to blue. The drink which does not change the colour of both blue and red litmus paper is the neutral drink.
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.
Answers:
(a) An antacid tablet is taken when you suffer from acidity
An antacid tablet is taken when you suffer from acidity to neutralize the effect of excess acid in the stomach. Excess acid can lead to indigestion and pain and antacids such as milk of magnesia give us relief from that.
(b) Calamine solution is applied on the skin when an ant bites.
Calamine solution is applied on the skin when an ant bites because the bite injects formic acid into the skin. Calamine solution which contains the base zinc carbonate can be applied to the skin to neutralise the harmful effect of the acid.
(c) Factory waste is neutralised before disposing it into the water bodies.
Factory waste is neutralised before disposing it into the water bodies because factory waste contains acids. These acids can cause harm to fish and other aquatic organisms. Therefore, they need to be first neutralised by adding basic substances and then allowed to enter water bodies, to prevent any damage.
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.
Answer:
We know that the turmeric indicator turns from yellow to red when added to a basic solution and remains yellow in acidic and neutral solutions.
Step 1: First, we add a tiny bit of each liquid to the turmeric indicator and observe what happens. The liquid which changes the colour of the indicator from yellow to red is the basic sodium hydroxide solution.
The remaining two solutions are the hydrochloric acid solution and the neutral sugar solution, both of which will not change the yellow colour of the indicator. Therefore, we must find a suitable method for distinguishing between these two solutions.
Step 2: We add basic sodium hydroxide solution to an equal amount of each of the two remaining solutions separately.
In case of hydrochloric acid solution, the following neutralisation reaction will take place, forming a neutral solution:
HCl (acid) + NaOH (base) → NaCl (neutral) + H2O (water) + Heat
In case of the neutral sugar solution, there will be no reaction and the resulting solution will turn basic:
Neutral sugar solution + NaOH (base) → Basic solution
Step 3: Now add a tiny bit of the two mixtures to the turmeric indicator and observe what happens. The mixture which changes the colour of the indicator from yellow to red is the neutral sugar solution which was made basic by addition of sodium hydroxide. The mixture which does not change the colour of the indicator is the hydrochloric acid solution which was neutralised by addition of sodium hydroxide.
Thus, the three unknown liquids are identified by following the above steps.
10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Answer:
The nature of the solution must be either basic or neutral. Acids change the colour of blue litmus paper to red. Bases and neutral solutions will not change the colour of blue litmus paper.
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 an indicator changes colour with a base, it does not change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.
Which of these statements are correct?
(i) All four (ii) a and d (iii) b, c and d (iv) only d
Answer: (iv) only d.
There are indicators such as turmeric which does not change colour for acids, it only changes colour for bases. Therefore, (a) is not true. There are other indicators such as China Rose which changes colour for both acids and bases. Therefore, (b) and (c) are not true. Change of colour in an acid and a base depends on the specific indicator. Therefore, (d) is true.
Solutions to Extended Learning – Activities and Projects (Page No 46) of NCERT Class 7 Science Chapter 4 Acids, Bases and Salts:
1. Using the knowledge of acids and bases. Write a secret message with the help of baking soda and beet root. Explain how it works. (Hint: Prepare baking soda solution in water. Use this solution to write the message on a sheet of white paper with a cotton bud. Rub a slice of fresh beet root over the message.)
Answer:
Solution to Extended Learning – Activities and Projects Problem 1
2. Prepare red cabbage juice by boiling a piece of red cabbage in water. Use it as an indicator and test the acidic and basic solutions with it. Present your observations in the form of a table.
Answer:
Solution to Extended Learning – Activities and Projects Problem 2
3. Bring the soil sample of your area, find out if it is acidic, basic or neutral. Discuss with farmers if they treat the soil in any manner.
Answer:
Solution to Extended Learning – Activities and Projects Problem 3
4. Visit a doctor. Find out the medicines he prescribes to treat acidity. Ask him how acidity can be prevented.
Answer:
Solution to Extended Learning – Activities and Projects Problem 4
Solutions to In Text Questions of NCERT Class 7 Science Chapter 4 Acids, Bases and Salts:
1. (Page 38) Complete Table 4.1.
Answer:
The completed table is shown below:
| Substance | Taste (sour/bitter/any other) |
| Lemon juice | Sour |
| Orange juice | Sour |
| Vinegar | Sour |
| Curd | Sour |
| Tamarind (imli) | Sour |
| Sugar | Sweet |
| Common salt | Salty |
| Amla | Sour |
| Baking soda | Bitter |
| Grapes | Sour |
| Unripe mango | Sour |
| Cucumber | Bland, slightly sweet or slightly bitter |
2. (Page 42) I am not getting the same result when using solid baking soda on dry litmus paper. Why?
Answer:
The same result is not obtained when using solid baking soda on dry litmus paper because baking soda is in solid form. Baking soda must be in solution form to interact with the dry litmus paper and change its colour. Solid baking soda does not soak the dry litmus paper and cannot interact with it.
3. (Page 44) Boojho added dilute sulphuric acid to lime water. Will the reaction mixture become hot or cool?
Answer:
When Boojho added dilute sulphuric acid to lime water, the reaction mixture became hot.Sulphuric acid is acidic and lime water is basic. Hence, neutralisation reaction will occur in which calcium sulphate salt and water are formed and heat is evolved. The reaction mixture will be hot due to evolution of heat.
Solutions to All Activities of NCERT Class 7 Science Chapter 4 Acids, Bases and Salts:
1. Complete Activity 4.1 (Page 40).
- Mix some water with lemon juice in a plastic cup/tumbler/test tube.
- Put a drop of the above solution on a strip of the red litmus paper with the help of a dropper. Is there any change in colour?
- Repeat the same exercise with the blue litmus paper.
Note down if there is any change in colour. Perform the same activity with the following substances: Tap water, detergent solution, aerated drink, soap solution, shampoo, common salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, lime water. If possible make solutions in distilled water. Record your observations as in Table. 4.2. In your Table, are there any substances on which litmus had no effect? Name those substances.
Answer:
2. Complete Activity 4.2 (Page 40).
- Take a tablespoonful of turmeric powder. Add a little water and make a paste.
- Make turmeric paper by depositing turmeric paste on blotting paper/filter paper and drying it. Cut thin strips of the yellow paper obtained.
- Put a drop of soap solution on the strip of turmeric paper. What do you observe?
Similarly test the solutions listed in Table 4.3 and note down your observations. You may try solutions of other substances also.
Answer:
3. Complete Activity 4.3 (Page 41).
Collect some China rose (Gudhal) petals and place them in a beaker. Add some warm water. Keep the mixture for some time till water becomes coloured. Use the coloured water as an indicator. Add five drops of the indicator to each of the solutions given in Table 4.4. What is the effect of the indicator on acidic, basic and neutral solutions?
Answer:
4. Complete Activity 4.4 (Page 42). The teacher is requested to get the dilute solution of the following chemicals from his/her school laboratory or from a nearby school: hydrochloric acid, sulphuric acid, nitric acid, acetic acid, sodium hydroxide, ammonium hydroxide, calcium hydroxide (lime water). Demonstrate the effect of the three indicators on each of thesesolutions. Record your observations in Table 4.5.
Answer:
Extra Questions to Complement Solutions to NCERT Class 7 Science Chapter 4 Acids, Bases and Salts:
Very Short Answer Type Questions:
1. How do bases like baking soda feel when touched?
Answer:
Bases likebaking soda feel soapy when touched.
2. If we cannot taste every substance how do find its nature?
Answer:
We can test the nature of substances using indicators which change colours when added to an acidic or a basic substance.
3. Name a neutral substance that we commonly drink.
Answer:
Water is a neutral substance that we commonly drink.
4. Which acids are found in citrus fruits such as oranges, lemons etc?
Answer:
Citric acid and ascorbic acids are found in citrus fruits such as oranges and lemons.
5. What is ascorbic acid commonly known as?
Answer:
Ascorbic acid is commonly known as Vitamin C.
6. Are cleaning liquids such as soaps and detergents acidic or basic in nature?
Answer:
Soaps and detergents are basic in nature.
7. Which indicator is used to determine whether an acid-base neutralisation reaction is complete?
Answer:
The colour change of phenolphthalein indicator indicates when an acid-base neutralisation reaction is complete.
8. Name one neutral substance formed during an acid-base reaction?
Answer:
Salt is a neutral substance formed during an acid-base reaction.
Multiple Choice Questions:
1. On adding phenolphthalein indicator to a colourless solution, no change is observed. What is the nature of this solution? (NCERT Exemplar)
(a) Basic
(b) Either acidic or basic
(c) Either acidic or neutral
(d) Either basic or neutral
Answer: (c) Either acidic or neutral. Phenolphthalein indicator changes to pink colour only when a solution is basic. In case of an acidic or neutral solution it remains colourless.
2. Neutralisation reaction is a (NCERT Exemplar)
(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
3. The correct way of making a solution of acid in water is to (NCERT Exemplar)
(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. When water is added to acid a lot of heat is evolved in the reaction because acid is present in much greater amount in the mixture, which can cause the mixture to spill outwards and burn you. But when acid is added to water not much heat is evolved because water is present in much greater amount in the mixture and danger can be averted.
Short Answer Type Questions:
1. How can you convert a neutral solution into an acid or base?
Answer:
A neutral solution is neither acidic nor basic. So, if you add an acid to a neutral solution then the resulting mixture will have the nature of an acid. Similarly, if you add a base to the neutral solution then the resulting mixture will have the nature of a base.
2. Why is it good to use distilled water when preparing a solution for testing for acidity or basicity?
Answer:
Distilled water is neutral – neither acidic nor basic. Therefore, if we use distilled water it will not change the level of acidity or basicity of the solution as shown below:
Neutral distilled water + base → Same level of basicity
Neutral distilled water + acid → Same level of acidity
Therefore, we can accurately determine how acidic or basic a solution is using distilled water.
3. Suppose you identify an acid by soaking an entire litmus paper in it. Can you use that same litmus paper to identify whether a second solution is acidic or basic?
Answer:
When the acid is soaked in blue litmus paper the colour will change to red. Now when you add the second solution it will either remain red in case of an acid or turn blue in case of a base. Using this logic we can identify whether the second solution is an acid or a base.
4. Why did the walls of the Taj Mahal deteriorate?
Answer:
The walls of the Taj Mahal deteriorated due to acid rain. Pollutants such as carbon dioxide, sulphur dioxide and nitrogen dioxide dissolve in raindrops to form carbonic acid, sulphuric acid and nitric acid respectively. The walls of the Taj Mahal are made of calcium carbonate. When rain containing these acids fell on the Taj Mahal they reacted with the basic calcium carbonate, thereby causing degradation and loss of colour.
5. Why is not wise to wash turmeric clothes stained clothes using soap?
Answer:
Sopas are basic in nature and turmeric (which is actually used as an indicator) turns red on contact with soap. So, the stain on the clothes turns red and persists even after washing with soap.
6. Paheli is suffering from indigestion due to acidity. Is it advisable to give her orange juice in this situation and why? (NCERT Exemplar)
Answer:
Orange juice contains citric acid, ascorbic acid, tartaric acid and is acidic in nature. Therefore, giving orange juice to Paheli will increase the acids in her stomach and make her indigestion worse.
7. How would you convert a basic solution into an acidic solution?
Answer:
Keep adding acid to the given basic solution. Neutralisation reaction between the acid and base will be completed after a certain point to form a neutral solution.
Acid + Base → Neutral solution
If you keep adding the acid after the solution becomes neutral, you will have an acidic solution.
Acid + Neutral solution → Acidic solution
You can test the solution using a blue litmus paper. The acidic solution will turn blue litmus paper red.
8. How would you convert an acidic solution into a basic solution?
Answer:
Keep adding base to the given acidic solution. Neutralisation reaction between the acid and base will be completed after a certain point to form a neutral solution.
Acid + Base → Neutral solution
If you keep adding the base after the solution becomes neutral, you will have a basic solution.
Base + Neutral solution → Basic solution
You can test the solution using a red litmus paper. The acidic solution will turn red litmus paper blue.
9. Why should we wear gloves while handling acids and bases?
Answer:
Laboratory acids and bases, especially the stronger ones, are corrosive and can cause irritation and harm to the skin. Therefore, they should be handled while wearing gloves.
10. What are the uses of acids?
Answer:
The uses of acids are as follows:
(i) The industrial manufacturing of many different chemicals.
(ii) Used to manufacture fertilizers.
(iii) Used as electrolytes in many different kinds of batteries.
(iv) Acids are used as ‘catalysts’ to increase the speed of a reaction.
(v) Used in various kinds of drugs and shampoos.
(vi) Used as cleaning agents.
11. What are the uses of bases?
Answer:
The uses of bases are as follows:
(i) They are used to make soaps.
(ii) It is used in the pharmaceutical industry to make medicines such as antacids.
(iii) Bases are used to synthesize many different kinds of chemicals.
(iv) Used as household cleaning agents.
12. Do all acids and bases function the same?
Answer:
No, different acids and bases vary in strength. Some acids are stronger than other acids and function differently. Similarly, some bases are stronger than other bases and differ in their function.
13. If you add a little bit of acid to a large amount of base, will the solution be acidic, basic or neutral?
Answer:
If you add a little bit of acid to a large amount of base, the resulting solution will still remain basic due to the much larger amount of base present.
14. Name three acids in our body.
Answer:
Three acids in our body are:
(i) Deoxyribonucleic acid (DNA) present in the cells.
(ii) Amino acids present in proteins in cells.
(iii) Fatty acids present in fats.
Long Answer Type Questions:
1. Explain neutralisation reaction in detail with the help of an example.
Answer:
When an acidic solution and a basic solution are mixed in suitable amounts, the acidic nature of the acid and the basic nature of the base destroy each other. The resulting solution is neither acidic nor basic and is said to be neutral. In the neutralisation reaction the acid and the base form salt and water, along with the evolution of heat.
If you carry out the neutralisation reaction in a test tube, you will find that it becomes hot due to evolution of heat during the reaction. If you test the resulting solution with red and blue litmus paper, you will find that the colour of both will not change, proving that it is neutral.
The general neutralisation reaction can be represented as follows:
Acid + Base → Salt + Water (Heat is evolved)
For example, when hydrochloric acid (HCl) and the base sodium hydroxide (NaOH) are mixed together, sodium chloride (NaCl), salt and water are produced. Heat is evolved which makes the resulting reaction mixture hot. This neutralisation reaction can be represented as follows:
HCl (acid) + NaOH (base) → NaCl (salt) + H2O (water) + Heat
2. How can you verify when an acid-base neutralisation reaction is complete?
Answer:
You can verify that an acid-base neutralisation is complete using the following experiment:
Aim: To determine when an acid-base neutralisation is complete.
Materials Required: Hydrochloric acid, sodium hydroxide, phenolphthalein indicator, test tube.
Procedure:
Step 1: Fill one fourth of a test tube with dilute colourless hydrochloric acid. Add 2-3 drops of colourless phenolphthalein indicator to the solution and shake the tube gently.
Observation: You will see that the solution remains colourless.
Conclusion: The colour of phenolphthalein does not change on contact with acid, therefore we confirm that the medium is acidic.
Step 2: Now add basic sodium hydroxide solution to the acidic solution containing phenolphthalein drop by drop, while stirring the tube gently. Continue this until you see pink colour beginning to appear.
Observation: The solution will turn slightly pink after adding a few drops of sodium hydroxide. Also, the test tube will feel hot when touched.
Conclusion: Since after adding a few drops of sodium hydroxide the colour is just beginning to change to pink, we can conclude that the medium is just starting to turn basic and the acid-base neutralisation reaction has just been completed. Also, heat is evolved in the neutralisation reaction which makes the test tube hot. Step 3: Once you see the first signs of pink colour, add a drop of hydrochloric acid.
Observation: The solution will turn colourless again.
Conclusion: Addition of only one drop of hydrochloric acid to the slightly pink basic solution turned the solution to colourless neutral. This confirms that the neutralisation reaction in Step 2 was completed just before the pink colour appeared.
Fill in the Blanks:
turmeric, limewater, acidic, water, ammonium hydroxide
(a) _________ is used as window cleaning liquids.
(b) _________ is a natural indicator.
(c) Chemical fertilisers should be used excessively because they make the soil too _________.
(d) _________ is prepared by adding chuna to water.
(e) Acids and bases and soluble in _________.
Answers:
(a) Ammonium hydroxide is used as window cleaning liquids.
(b) Turmeric is a natural indicator.
(c) Chemical fertilisers should be used excessively because they make the soil too acidic.
(d) Limewater is prepared by adding chuna to water.
(e) Acids and bases and soluble in water.
Match and Pair:
1. Match the items of Column I with suitable items in Column II:
| Column I | Column II |
| (i) Vinegar | (a) Zinc carbonate |
| (ii) Calamine | (b) Oxalic acid |
| (iii) Crops | (c) Acetic Acid |
| (iv) Sulphur dioxide in water | (d) Neutral soil |
| (v) Spinach | (e) Acidic |
Answer:
The correct table is shown below:
| Column I | Column II |
| (i) Vinegar | € Acetic Acid |
| (ii) Calamine | (a) Zinc carbonate |
| (iii) Crops | (d) Neutral soil |
| (iv) Sulphur dioxide in water | € Acidic |
| (v) Spinach | (b) Oxalic acid |
++++++++++++++
Frequently Asked Questions (FAQs) on NCERT Solutions to Class 7 Science Chapter 4 Acids, Bases and Salts:
Our team of Indian and foreign-educated teachers, engineers and scientists have prepared these materials focusing on the basics, so that beginner students are able to develop a solid foundation. These concepts will be studied in much greater detail later, so it is essential that you have a clear understanding of them from the very beginning. All the exercise questions, extended learning projects, in-text questions and in-text activities have been answered, making this the most complete solutions package for Chapter 4.
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The following topics are covered:
4.1 – Acids and Bases
4.2 – Natural Indicators Around Us
4.3 – Neutralisation
4.4 – Neutralisation in Everyday Life
Here are the number of problems for the chapter:
(i) 7 Short Answer Type Questions (Questions 1, 2, 3, 4, 5, 7, 10)
(ii) 1 True/False Type Question (Question 6)
(iii) 2 Long Answer Type Questions (Questions 8, 9)
(iv) 1 Multiple Choice Question (Question 11)
Yes! You can download the free PDF versions of educationroundtheworld.com’s NCERT Solutions for Class 7 Science Chapter 4 Acids, Bases and Salts anytime! You will find all different types of additional questions also included in the PDF version! Please look towards the top of the page!
We recommend that you read the chapter and then study our solutions. The most important parts are how different indicators respond to acids, bases and neutral solutions and how acid-base neutralisation takes place. The solutions and extra questions we have included approach these topics from all angles and will teach you the concepts and how to answer questions. If you understand the reasoning we have used to answer questions, you will learn to think in the right way when you encounter similar questions in your exams.
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