NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts

These NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Acids, Bases and Salts NCERT Solutions for Class 7 Science Chapter 5

Class 7 Science Chapter 5 Acids, Bases and Salts Textbook Exercise Questions and Answers

Question 1.
State differences between acids and bases.
Answer:

Acids	Bases
1. Acid are sour to taste.	a. Bases are bitter to taste.
2. Acid turns blue litmus to red.	b. Base turns red litmus to blue.
3. Acid is a substance which contains hydrogen ion (H⁺).	c. Bases contain hydroxide ions.

Question 2.
Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Answer:
Ammonia is basic in nature.

Question 3.
Name the source from which litmus solution is obtained. What is the use of this solution?
Answer:
Litmus solution is a natural indicator which is obtained from lichens. It is used to determine whether a given solution is acidic or basic.

Question 4.
Is the distilled water acidic/basic/ neutral? How would you verify it?
Answer:
Distilled water is neutral. One can verify it by showing that neither red nor blue litmus paper changes its colour when dipped in distilled water.

Question 5.
Describe the process of neutralisation with help of an example.
Answer:
The reaction between an acid and a base to form salt and water with the evolution of heat is called neutralisation reaction. For example, when the solution of sodium hydroxide and hydrochloric acid react with each other, the acidic nature of acid and the basic nature of base are destroyed. The resulting solution is a neutral solution.

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

In this reaction, the hydrogen ions from the acid reacts with the hydroxide ions of the base to from water and salt is formed.

Question 6.
Make ‘T’ if the statement is true and ‘F’ if it is false:
a. Nitric acid turns red litmus blue. (T/F)
b. Sodium hydroxide turns blue litmus red. (T/F)
c. Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
d. Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
e. Tooth decay is caused by the presence of a base. (T/F)
Answer:
a. F,
b. F,
c. F,
d. T,
e. F.

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?
Answer:
Dorji will take red and blue litmus papers and one by one dip them into each bottle of soft drink. The drink in which red litmus paper turns blue is the bottle of basic drink. The drink in which blue litmus paper turns red is the bottle of acidic drink. The drink in which both litmus papers show no changes in colour is of the neutral drink.

Question 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.
Answer:
a. We take an antacid such as milk of magnesia to neutralise the excessive acid released in stomach during acidity.
b. Ant’s sting contains an acid called formic acid and when we apply calamine solution containing a base, it neutralises the effect of acid and prevents irritation.
c. Many factories waste contain acids. If they are allowed to flow into the water bodies, the acids will kill the aquatic life. Therefore, factory waste is neutralised before its disposal.

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.
Answer:
Dip the turmeric indicator one by one in each liquid. The liquid in which the colour of turmeric indicator changes to red is basic in natural, i.e., sodium hydroxide. Since, we already identified sodium hydroxide. We will pour the sodium hydroxide in other two liquids. The liquid which gets warm after pouring the sodium hydroxide (base) solution in it is of hydrochloric acid as heat gets evolved in the process of neutralisation. The last one which shows no effect is the liquid of sugar solution.

Question 10.
Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Answer:
There can be two cases:

The solution may be basic because blue litmus paper remains unaffected in basic medium.
The solution can be neutral. A neutral solution has no effect on any indicator.

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 change 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)
Answer:
(iv) Only (d).

NCERT Extended Learning Activities and Projects

Question 1.
Using the knowledge of acids and bases, write a secret message with the help of baking soda and beetroot. 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 beetroot over the message.)
Hint:
Beetroot acts as an indicator and changes colour upon reaction with the base, i.e., baking soda solution. Hence, message will reappear after some time.

Question 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.
Hint:
Cut the cabbage leaves into pieces. Boil water in a pan. Place the cabbage pieces in a bowl and pour the boiling water in the bowl. Keep it for thirty minutes so that the cabbage pieces get soaked. Separate the juice from the leaves by squeezing. Place the glass jars about two inches apart. Pour chemicals (lemon juice, vinegar, detergent, soda, etc.) in separate jars.

Now test whether the chemical is a base or an acid. Pour a part of your cabbage into each jar. You may have to swirl your mixture a bit. If the mixture turns pink, the chemical is an acid and if it turns blue, the chemical is a base.

Question 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.
Hint:
Litmus paper helps to determine whether the soil has the right pH balance for the plants to grow or not.

Dump a small amount of soil, about a teaspoon, into a clean container and add distilled water. The mixture should be soggier than mud but without much water, such that all the soil settles to the bottom quickly. Usually, about twice as much water as soil works well. Stir or shake the mixture to evenly distribute the soil. Dip the end of the litmus paper into the soil and water mixture. Hold it in the mixture for 1 or 2 seconds before pulling it out and then wait for a few seconds until the colour stabilises. If the paper turns red, the soil is acidic. It the paper turns blue, the soil is basic.

Question 4.
Visit a doctor. Find out the medicines, he prescribes to treat acidity. Ask him how acidity can be prevented.
Hint:
The medicines prescribed by the doctors are basic in nature and are called antacids. These medicines neutralise are the effect of acids.
To avoid acidity, one should eat small meals throughout the day instead of a few large meals. Also, one should healthy and avoid junk food.

Activity 1

Objective: To test the acidic or basic nature of the given solutions using litmus as the indicator.
Materials Required: Blue litmus paper, red litmus paper, given solution and dropper.
Procedure:

Take all the given solutions in the labelled test tubes.
Take one small piece each of the blue and the red litmus papers.
Put a drop of the solution over each and observe the change in colour.
Note down your observations.
Repeat the same for all the solutions.

Observations:

Solution	Colour change with blue litmus paper	Colour change with red litmus paper
Lemon	Blue litmus turns red	No change
Vinegar	Blue litmus turns red	No change
Soap solution	No change	Red litmus bums blue
Amla juice	Blue litmus turns red	No change
Sugar solution	No change	No change
Sour milk	Blue litmus turns red	No change

Conclusion: Acidic substances turn blue litmus red and have no effect on red litmus. Basic substances turn red litmus blue and have no effect on blue litmus. Neutral substances have no effect on both the blue and the red litmus papers.

ii. Turmeric: It turns from yellow to red in alkaline (basic) solutions. While it remains yellow in acidic solutions.

Activity 2

Objective: To test the acidic or basic nature of the given substances by using turmeric paper as the indicator.
Materials Required: Filter/blotting paper, turmeric powder, brush, solutions of different
(acidic/basic) nature.
Procedure:

Make a thick paste of turmeric powder (20 g) in distilled water.
Apply this paste on a filter/blotting paper with a brush.
Allow it to dry. When dry, cut it into strips for testing the solution.
Take all the given solutions in different labelled test tubes.
Start from the first solution.
Take a strip of turmeric paper.
Put a drop of the given solution over turmeric paper and notice any change in colour.
Note down your observation.
Repeat the above three steps for all the solutions one by one.

Observations:

Solution/Sample	Colour change	Nature
1. Lemon juice	Yellow	Acidic
2. Tap water	Yellow	Neutral
3. Distilled water	Yellow	Neutral
4. Soft drink	Yellow	Acidic
5. Vinegar	Yellow	Acidic
6. Soap solution	Red	Basic
7. Curd	Yellow	Acidic
8. Soda water	Red	Basic
9. Baking powder	Red	Basic
10. Glucose solution	Yellow	Neutral
11. Sour milk	Yellow	Acidic
12. Sugar solution	Yellow	Neutral
13. Sugar solution	Yellow	Neutral
14. Amla juice	Yellow	Acidic

Conclusion: Acidic and neutral solution remain unchanged i.e., yellow, when they react with the turmeric. Basic solutions turn yellow colour of turmeric to bright red when they react with turmeric.

iii. China rose: China rose is also a natural indicator. China rose indicator is a light pink coloured solution which is extracted from the red flowers of China rose plant and mixed with water. It turns acidic solution to dark pick (magenta) and basic solutions to green colour.

Activity 3

Objective: To test the acidic or basic nature of the given solutions by using the china rose extract as the indicator.
Materials Required: China rose indicator, water, beaker, dropper, solutions of different acids and bases.
Procedure:

Collect some Chine rose petals and keep them in warm water (about 80 mL) for some time.
When water becomes coloured, fitter out the petals and use the coloured water as indicator.
Take about 2 mL of the first solution in a test tube.
Add 5 drops of the China rose indicator to the solution.
Observe the change in colour.
Repeat the experiment with other solutions one by one.

Observations: Colour of the China rose indicator is light pink.

Solution/Sample	Colour change	Nature
Lemon juice	Dark pink	Acidic
Tap water	No change	Neutral
Vinegar	Dark pink	Acidic
Amla juice	Dark pink	Acidic
Soda water	Green	Basic
Baking powder	Green	Basic
Sugar solution	No change	Neutral
Glucose solution	No change	Neutral
Lime water	Green	Basic
Sour milk	Dark pink	Acidic
Dilute sulphuric acid	Dark pink	Acidic

Conclusion: When the pink colour of China rose indicator turns magenta or dark pink, then the solution is acidic and when it turns green then the solution is basic. If there is no change in the colour, then the solution is neutral.

Neutral substances: Substances which are neither acidic nor basic are called neutral substances. These substances do not bring about any change in the colour of the indicators. Example of some neutral substances are salt, sugar, glucose, nitrogen, water, oxygen, etc.

Neutralisation: The reaction between an acid and a base to give salt and water is known as a neutralisation reaction. Heat is also evolved in this reaction.

Acids and bases react with each other to nullify the effect of each other. Let us consider a simple reaction between NaOH which is a base and dilute HCl which is an acid.

Salt: Salt is product of neutralisation reaction between an acid and a base. Salt formed because of neutralisation may be acidic or basic in nature. The acidic or basic nature of a salt depends upon the strength of the acid and the base used for neutralisation reaction.

Synthetic Indicators: The indicators prepared by humans in laboratory are called synthetic indicators. Phenolphthalein and methyl orange are some examples of chemicals which act as acid-base indicator. For example, phenolphthalein solution is colourless in acidic solution and it gives pink colour in basic solution. Similarly, methyl orange gives red colour in acidic medium and yellow colour in basic medium.

Neutralisation in Everyday Life:
i. Indigestion: Our stomach produces gastric juices which contain hydrochloric acid. Excess of HCl acid in our stomach causes indigestion and acidity. To relieve indigestion, doctors prescribe antacid tablets or syrup which contain milk of magnesia (magnesium hydroxide) and aluminium hydroxide. Since these are bases, they neutralise the effect of excessive acid in our stomach.

Therefore, while suffering from acidity, it is prescribed to take antacid which is bitter in taste, i.e., it is a base. On entering the body, the base neutralises the excess acid released into the stomach and convert it in salt and water.

ii. Ant Sting: The ant sting contains formic acid which when injected into the skin causes pain and irritation. By applying moist baking soda, which is a base, the effect of the sting can be neutralised.

iii. Soil Treatment: Plants do not grow well in excessive acidic as well as basic soil medium. When soil becomes more acidic, it is retreated with quick lime or slaked lime and when it becomes basic, organic matter is added to it.

iv. Factory Waste: Factory wastes are also treated so as to neutralise the waste. Otherwise it may kill aquatic life.

Class 7 Science Chapter 5 Acids, Bases and Salts Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What are acids?
Answer:
The chemical compounds which are sour in taste are called acids.

Question 2.
What are bases?
Answer:
The substances which are soapy in touch and bitter in taste are called bases.

Question 3.
What are indicators?
Answer:
The substances which are used to test whether the given substance is acidic or basic by indicating a colour change are called indicators.

Question 4.
What are alkalis?
Answer:
The bases which dissolve in water are known as alkalis.

Question 5.
Name the acid present in ant-sting that causes pain.
Answer:
Formic acid.

Question 6.
Why does turmeric stain on white shirt is turned to red when it is washed with soap?
Answer:
It is because the soap solution is basic and turmeric.

Question 7.
Name two synthetic indicators.
Answer:
Methyl orange and phenolphthalein.

Question 8.
What is acid rain?
Answer:
The rain containing excess of acids is called an acid rain.

Question 9.
Which acid is naturally present in our stomach?
Answer:
Hydrochloric acid (HCl).

Question 10.
What is the nature of distilled water?
Answer:
The distilled water is neutral.

Question 11.
State the nature of baking soda.
Answer:
Baking soda is basic in nature.

Question 12.
Where does the word acid come from?
Answer:
The word acid comes from the Latin word acere which means sour.

Question 13.
Why curd, lemon juice, orange juice and vinegar taste sour?
Answer:
These substances taste sour because they contain acids.

Question 14.
Which is the most commonly used natural indicator?
Answer:
The most commonly used natural indicator is litmus.

Question 15.
How acidic soil can be treated?
Answer:
When the soil is too acidic, it is treated with bases such as quick lime (calcium oxide) or slaked lime (calcium hydroxide).

Question 16.
What are salts? Give example.
Answer:
The product formed, other than water, by the neutralisation reaction of an acid a base is called salt. Salt may be acidic, basic or neutral in nature. Example: Sodium chloride.

Question 17.
Name the acid present in the following:
a. Tomato
b. Vinegar
c. Tamarind
Answer:
a. Oxalic acid
b. Acetic acid
c. Tartaric acid

Question 18.
Which base is used for removing grease and stains?
Answer:
Ammonium hydroxide is used for removing grease and stains.

Question 19.
How many types of litmus are there?
Answer:
There are two type of litmus:

Red litmus
Blue litmus

Question 20.
Name a substance which is used to neutralise the effect of excessive acid in our stomach.
Answer:
Milk of magnesia which contains magnesium hydroxide (base).

Short Answer Type Questions

Question 1.
What is litmus? Give uses of litmus.
Answer:
The most commonly used natural indicator is named as litmus. It is a purple coloured dye extracted from lichens. Uses: Litmus helps in the identification of acids and bases. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue.

Question 2.
Define organic acids?
Answer:
The acids which are found in plants and animals are called organic acids. For example, vinegar contains acetic acid, lemon juice contains citric, etc., are all organic acids.

Question 3.
What do you mean by soil treatment?
Answer:
When a farmer uses excessive chemical fertilisers in the soil, the soil becomes acidic. Plants do not grow well when the soil is either too acidic or too basic. When soil is too acidic, it is treated with bases. If the soil is basic, organic matter is added to it. Organic matter releases acids which neutralise the basic nature of the soil. This process of maintaining soil pH balance is called soil treatment.

Question 4.
Give an example each of acidic salt, basic salt and neutral salt.
Answer:
Salts are the compounds generally formed by neutralisation of an acid with base. They can be acidic, basic as well as neutral. Example of acidic salt is sodium bicarbonate, basic salt: magnesium chloride and neutral salt is sodium chloride.

Question 5.
State few properties of acids.
Answer:
Few properties of acids are:

Acids are sour in taste.
Acids turns blue litmus red.
Acids do not react with ammonium salts.
Acids are generally found in citrus fruits like lemon, vinegar, curd, amla etc.

Question 6.
State few properties of bases.
Answer:
Few properties of bases are:

Bases are bitter in taste and soapy in touch.
Bases turn red litmus blue.
Bases do react with ammonium salts to give ammonium gas.
Bases are generally found in soaps, window cleaner, milk of magnesia etc.

Question 7.
What is the effect of ant bite? How is its cured?
Answer:
When an ant bites, it injects the acidic liquid (formic acid) into the skin. It causes irritation and burning effect on the skin. The effect of the acid can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution which contains zinc carbonate. Being bases, they neutralise the effect of formic acid.

Question 8.
How do you prepare lime water?
Answer:
Dissolve some lime (chuna) in water in a bottle. Stir the solution and keep it for some time. Pour a little from the top. This is lime water.

Question 9.
What are the uses of China rose petals as indicators?
Answer:
China rose petals, when added to warm water, form coloured solution which may be used as an indicator. This indicator turns acidic solutions to dark pink and basic solution to green.

Question 10.
What do you understand by synthetic indicators?
Answer:
Indicators which are prepared artificially are called synthetic indicators. Phenolphthalein and methyl orange are examples of synthetic indicators. Phenolphthalein turns colourless in acidic solution and turns pink in basic solutions. Methyl orange turns red in acidic solutions and yellow in basic solutions.

Question 11.
What do you mean by a neutral solution? Give examples.
Answer:
The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. Examples- sugar solution, distilled water, salt, etc.

Question 12.
How does rain become acidic?
Answer:
The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in raindrops to form carbonic acid, sulphuric acid and nitric acid, respectively, hence making the rain acidic.

Long Answer Type Questions

Question 1.
Mention two uses each of the following :
a. Ammonium hydroxide
b. Hydrochloric acid
c. Magnesium hydroxide
Answer:
a. Uses of ammonium hydroxide:

To remove grease from window panes.
To remove ink spots from clothes.

b. Uses of hydrochloric acid:

It is used for the purification of common salt.
It is used for cleaning of metals, before soldering, tinning and for galvanisation.

c. Uses of magnesium hydroxide:

It is used in making antacids.
It is used as a reagent in the chemistry laboratory.

Question 2.
Common indicators is cannot determine how acidic or basic a solution is. Explain.
Answer:
Common indicators, such as litmus, methyl orange and phenolphthalein can easily tell us whether a solution is acidic or alkaline but they cannot easily tell how much a given acidic solution is stronger than another acidic solution. It means they cannot really tell us the degree of acidic or basic nature of a solution.

Question 3.
How will you obtain China rose indicator solution?
Answer:
China rose is a natural indicator. First collect some China rose petals and put them in beaker. Add some warm water and keep the china rose petals immersed in water for some time till the water in the beaker turns into light pink colour. Remove the petals by filtration. This light pink coloured solution is the china rose indicator used for testing acids and bases. This indicator turns acidic solutions to dark pink (magenta) and basic solution to green.

Picture-Based Questions

Question 1.
a. Identify the indicator shown below.

b. What colour does it produce in acidic and basic solutions.
Answer:
a. It is China rose indicator.
b. It gives dark pink colour in acids and green colour in bases.

Question 2.
a. Identify the image shown below.

b. Which indicator is extracted from this source?
Answer:
a. Lichens.
b. Litmus solution is obtained from lichens that is used as indicator.