The Class 7 is an important year in a student’s life and Science is one of the subjects that require dedication, hard work, and practice. It’s a subject where you can score well if you are well-versed with the concepts, remember the important formulas and solving methods, and have done an ample amount of practice. Worry not! Home Revise is here to make your Class 7 journey even easier. It’s essential for students to have the right study material and notes to prepare for their board examinations, and through Home Revise, you can cover all the fundamental topics in the subject and the complete NCERT Class 7 Science Book syllabus.
Exercise Questions
1. State differences between acids and bases.
Solution:
| Acids | Bases |
| Acids are sour in taste | Bases are bitter in taste |
| Acids turn blue litmus paper into red color | The base does not change the colour of blue litmus paper |
| Acids does not change the colour of the red litmus | Bases turn red litmus paper to blue color |
| Acids do not change the colour of turmeric | Bases turn turmeric to red |
2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Solution:
The answer is it is basic in nature.
3. Name the source from which litmus solution is obtained. What is the use of this solution?
Solution:
Litmus solution is extracted from lichens. Litmus solution is used as an indicator to find acidic and basic nature of a solution.
4. Is the distilled water acidic/basic/neutral? How would you verify it?
Solution:
Distilled water is neutral in nature, and this can be tested by using red and blue litmus paper. In either of the cases, colour remains unchanged.
5. Describe the process of neutralisation with the help of an example.
Solution:
Neutralisation is a reaction between an acid and a base. Here, both acids and bases get neutralised. For example, when sodium hydroxide (NaOH) is added to hydrochloric acid (HCl), sodium chloride (NaCl) and water (H2 O) are obtained.
NaOH + HCl → NaCl + H2 O + Heat
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)
Solution:
i) False
ii) False
iii) True
iv) True
v) False
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 an acidic drink, another wants a basic drink, and the third one wants a neutral drink. How will Dorji decide which drink is to be served to whom?
Solution:
Dorji can taste a few drops out of soft drinks bottles; the acidic solution is sour in taste, the basic solution is bitter in taste, and the neutral solution has no taste. Along with tasting, Dorji can use litmus paper to test the nature of the soft drinks. He should use blue litmus paper to test the acidic solution. Dorji has to put a drop of solution on blue litmus. If it turns red, then the solution will be acidic in nature.
Similarly, he can use red litmus paper to test the basic solution. He has to put a drop of solution on red litmus. If it turns blue, then the solution will be basic in nature.
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.
Solution:
9. Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide, and the third is a sugar solution. How will you identify them? You have only turmeric indicator.
Solution:
The following steps are taken to test the given liquids:
10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Solution:
The above solution may be neutral or basic in nature as both will not change the colour of the 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
Solution:
(iv) Only d is correct
| Section Number | Topic |
| 1 | Acids and Bases |
| 2 | Natural Indicators around Us |
| 3 | Neutralisation |
| 4 | Neutralisations in Everyday Life |