Class 7 Science Chapter 5 Notes

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Class 7 Science Chapter 5 Notes

Chapter Name: - Acids, Bases and Salts

Class 7 Science Chapter 5 Notes

Section - A (Keywords)

Explain the following Keywords

A chemical substance that is sour in taste and turns blue litmus paper red.

The chemical property, shown by the acid substances.

A chemical substance that is bitter in taste, soapy in touch and turns red litmus paper blue.

The chemical property, shown by the base substances.

The substances used to test the acidic or basic nature of the chemical substances. Indicators can be natural or artificial.

Some substances that are neither acid nor base are called neutrals. They do not change the colour of either blue or red litmus paper.

A chemical reaction between an acid and a base substances that produce water and salt is called a neutralisation reaction.

Acid + Base → Salt + water (Heat is evolved)

A new substance produced in the neutralisation reaction is called salt.

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Class 7 Science Chapter 5 Notes

Section - B (Text Questions - Answer)

Question 1

State differences between acids and bases.

AcidsBases
1. They are sour in taste.1. They are bitter in taste.
2. It turns blue litmus paper into red.2. Blue litmus paper remains unchanged in bases.
3. Red litmus paper remains unchanged in acids.3. It turns red litmus paper into blue.
4. Turmeric indicator remains unchanged in acids.4. Turmeric indicator turns reddish in bases.
5. China rose indicator turns acidic solutions into dark pink (magenta).5. China rose indicator turns basic solutions into green. 

6. Examples of acids:

Hydrochloric(HCL) acid, vinegar, lemon juice

6. Examples of bases:

Sodium hydroxide (NaOH), Baking Soda (NaHCO3

Question 2

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

Ammonia is basic in nature because the bases turn red litmus paper into blue. Also, bases are soapy in nature and are used in cleansing objects.

Question 3

Name the source from which litmus solution is obtained. What is the use of this solution?

Litmus solution is obtained from lichens. 

Uses:

Litmus is a natural indicator and is used to detect acids and bases. It has a mauve (purple) colour in distilled water. It turns red when added to an acidic solution, and turns blue when added to the basic solution.

Question 4

Is the distilled water acidic/basic/neutral? How would you verify it?

The distilled water is neutral in nature. We would use indicators to verify it. Distilled water does not change the colour of any indicator.

Question 5

Describe the process of neutralisation with the help of an example.

A chemical reaction between an acid and a base substances that produce water and salt is called a neutralisation reaction.

Acid + Base → Salt + water (Heat is evolved)

Example: –

Hydrochloric acid (HCL) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H2O)

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Class 7 Science Chapter 5 Notes

Question 7

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?

Dorji can decide the natures of his soft drinks with the help of the litmus test.

Results:

if it turns blue → the drink is basic

if it turns red → the drink is acidic

if it does not change colour → the drink is neutral

Question 8

Explain why:

Acidity is the symptom of excessive acid (HCL) in the stomach. An antacid tablet contains magnesium hydroxide, which is a base. It neutralises the effect of excessive acid.

Ant injects an acidic liquid (formic acid) on biting. To neutralise its effect basic substance calamine is used. Calamine contains zinc carbonate which is a base.

The factory waste contains acidic substances. It can kill fishes and other aquatic organisms. Therefore it is neutralised before disposing it into the water bodies.

Question 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.

Identifying liquids with turmeric indicator

Sodium hydroxide: Sodium hydroxide is a base and the turmeric indicator turns red on reacting with the bases. This way we will identify the Sodium hydroxide liquid.

To identify the remaining liquids, take a few drops from these liquids in two separate test tubes and add sodium hydroxide one by one in both of them.

Hydrochloric acid: On adding sodium hydroxide to the hydrochloric acid, a neutralisation reaction will occur in one of the test tubes.

Acid (HCL) + Base (Sodium hydroxide) → Salt + Water (Heat is evolved)

On testing this solution with turmeric indicator, it will show no change in colour. Also, the test tube will become a little hot as heat is evolved in a neutralisation reaction.

Sugar solution: On adding sodium hydroxide to the sugar solution, the whole solution will become basic in nature.

Sugar solution + Base (Sodium hydroxide) → Base

The turmeric indicator will turn red in this solution.

Question 10

Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.

A blue litmus paper is dipped in a solution. It remains blue. This can happen if the solution is basic or neutral in nature.

Reason:

Base changes a red litmus paper into blue, but a blue litmus paper remains unchanged in bases. The solution can be neutral as the solutions which do not change the colour of either red or blue litmus are known as neutral solutions.

Question 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

Option: (iv) only d is correct.

Comment below if you want to know why “only d” is correct.

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Class 7 Science Chapter 5 Notes

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