Acids, Bases and Salts Question Answers: NCERT Class 7 Science

Welcome to the Chapter 5 - Acids, Bases and Salts, Class 7 Science - NCERT Solutions page. Here, we provide detailed question answers for Chapter 5 - Acids, Bases and Salts.The page is designed to help students gain a thorough understanding of the concepts related to natural resources, their classification, and sustainable development.

Our solutions explain each answer in a simple and comprehensive way, making it easier for students to grasp key topics and excel in their exams. By going through these Acids, Bases and Salts question answers, you can strengthen your foundation and improve your performance in Class 7 Science. Whether you're revising or preparing for tests, this chapter-wise guide will serve as an invaluable resource.

Substance are categorized into three- acid, base, salt.
Substance having sour taste called as acidic.
Substance having bitter taste called as basic.
Substance which are neither acidic nor basic called as neutral.
Indicators are the substances which shows that the substances are acidic or basic in nature.
There are some natural indicators like turmeric, litmus, China rose etc.
Litmus changes into blue, when it dissolve in base.
When an acid is mixed with base, then it is called neutralization reaction.
Salt pH is 7, which is neutral.
Neutralization help in our daily life like in acidity ant bite etc.

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Exercise 1 ( Page No. : 57 )

State differences between acids and bases.

Ans:

Bases	Acids
1. These are proton acceptors/ electron pair donors.	1. These are proton donor/ electron pair acceptor.
2. Bases indicates green colour with China Rose indicator.	2. Acids indicates dark pink colour with China Rose indicator.
3. It have pH value > 7.	3. These have pH value < 7.
4. It turn red litmus paper into blue.	4. It turns blue litmus paper into red.
5. It taste bitter. 6. Bases changes into red colour whith turmeric indicator.	5. It tastes sour. 6. Acids do not change colour with turmeric indicators.
6. Example: Sodium hydroxide (NaOH)	6. Example: Hydrochloric acid (HCl)

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

Ans:

Ammonia, also known as NH₃, is a colourless gas with a distinct odour composed of nitrogen and hydrogen atoms. Ammonical smell comes from human excreta. It changes turmeric indicator into the red colour.

Ammonia in Household Cleaning Products: Ammonium hydroxide (NH₄OH) - commonly known as household ammonia, is an ingredient in many household cleaning products used to clean a variety of surfaces including tubs, sinks, toilets, counter tops and tiles.

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

Ans:

Litmus solution is obtained from the lichens. It has purple colour in distilled water. It is found as two types: Red litmus and Blue litmus paper. When we add it in the acidic solution, it turns blue litmus to red. And, when we add it in the basic solution, it turns into the blue. Litmus is available in the form of strips of paper, known as litmus paper.

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

Ans:

Distilled water is neutral by nature. We can verify this by litmus paper. Distilled water will not show any change in colour when we add it to the litmus solution. It shows that distilled water is neither acidic or nor basic.

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

Ans:

NEUTRALISATION: When an acid is mixed with a base, they neutralise the effect of each other. The products formed on mixing an acid and a base are salt and water. The reaction between acid and base which produces salt and water with the evolution or release of heat is known as neutralization reaction.

Acid + Base → Salt + Water + Heat (released)

In a neutralisation reaction, salt are formed which is neutral by nature. The salt formed during a neutralisation reaction depends on which acid and which base are reacted with each other. For example: Sodium hydroxide is a base and hydrochloric acid is an acid. So, when sodium hydroxide is treated with hydrochloric acid, then a neutralisation reaction takes place to form sodium chloride (salt) and water.

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

The salt formed in this neutralisation reaction of sodium hydroxide and hydrochloric acid is sodium chloride (NaCl). It is known as common salt.

Mark ‘T’ if the statement is true and ‘F’ if it is false:

(i) Nitric acid turn 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)

Ans:

(i) Answer: False
Nitric acid is not right to turn the red litmus paper into blue. The explanation is that the colour of the paper does not change when an acid comes into contact with a red litmus paper. Only if an acid comes into contact with blue litmus paper will the colour of the litmus paper change into red. Nitric acid is an acid so it does not turns the litmus paper into blue.

(ii) Answer: False
This statement is false because sodium hydroxide is not an acid, it is base which turns red litmus to blue.

(iii) Answer: True
When acid and base reacts with each other, they form an neutral substance, this process is called neutralisation. When the solution of sodium hydroxide (a base) and hydrochloric acid (an acid) is mixed in test tube in proper ratio, both neutralize each other.

(iv) Answer: True
This statement is true because different solution show different colours in acidic or basic medium. Indicators are chemical compounds that change Colour according to pH of the solution they are put in. They are used to determine pH of the body (acidity/basicity).

(v) Answer: False
This statement is false because tooth decay is caused by acids which is formed in our mouth when the food particles left over all night. They constantly damage tooth enamel and thus cause tooth decay.

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?

Ans:

Drinks are eatable material so Dorji can check the medium of the drinks by tasting them, acid drink will be of souring taste, basic will be bitter in taste and neutral will not contain any taste. So, just by tasting and drinking them, we can serve the drinks on the demand of the customers.

Note*: He can check the drinks medium of the drinks by using the litmus paper by putting one drop of each drinks on it. He can conclude which is acidic, basic and neutral.

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.

Ans:

(a) An antacid tablet is taken during acidity because that time excess amount of acid is produced (HCL) in the stomach. Antacid is an bases that react with excess of acids and neutralize the effect of excessive acid and brings relief. Antacids help to treat heartburn (indigestion). They work by neutralizing the stomach acid that cause heartburn. We can buy many antacids without a prescription.

(b) Calamine solution is applied on the skin when an ant bites because, it injects formic acid into the skin. calamine solution is basic in nature. So, when we applied it on the skin, it neutralize the effect of formic acid.

(c) Factory waste is neutralized before disposing it into the water bodies because it contains acids. The acidic nature of waste can harm the aquatic animals, so we add some basic chemicals to the waste to neutralize the effects of acid.

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.

Ans:

Turmeric indicator is of yellow colour, we can identify them by putting one drop of each solution to the turmeric indicator. If the liquid drop changes into red colour, then it must be a basic solution that is sodium hydroxide. If the drop will not change the colour of the turmeric indicator, then it must be an acid, that is hydrochloric acid.

Sugar solution changes the colour of turmeric indicator to red because the mixture of basic and neutral solution is basic in nature.

Turmeric indicator + Sodium hydroxide → Colour change (yellow to red)

Turmeric indicator + Hydrochloric acid (HCL) → No colour change

Turmeric indicator + Sugar solution → Colour change (yellow to red)

Q10

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

Ans:

Blue litmus paper is dipped in a solution, it remains blue; this means that the solution is either basic or neutral. Because basic and neutral solutions does not change the colour of the blue litmus. We can check it by putting a drop of the solution on red litmus paper. If the red litmus paper changes into blue then it shows the solution is basic and if it does not change the colour, then it is neutral by nature.

Q11

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 and c (iv) only d

Ans:

(iv) only d

Change of the colour in an acid or the base depends on the type of the indicator. It is so because we know that a basic solution do not change the colour of the blue litmus paper whereas the acid does. The acidic solution do not change the colour of the red litmus paper whereas the basic solution does.