CBSE Class 7

NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes are part of NCERT Solutions for Class 7 Science. Here we have given NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes.

NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes

Question 1.
Classify the changes involved in the following processes as physical or chemical changes:

1. Photosynthesis
2. Dissolving sugar in water
3. Burning of coal
4. Melting of wax
5. Beating aluminium to make aluminium foil
6. Digestion of food

Answer:

1. Chemical change
2. Physical change
3. Chemical change
4. Physical change
5. Physical change
6. Chemical change

Question 2.
State whether the following statements are true or false. In case a statement is false, write the correct statement in your notebook:
(a) Cutting a log of wood into pieces is a chemical change. (True/False)
(b) Formation of manure from leaves is a physical change. (True/False)
(c) Iron pipes coaled with zinc do not get rusted easily. (True/False)
(d) Iron and rust are the same substances. (True/False)
(e) Condensation of steam is not a chemical change. (True/False)
Answer:
(a) False
Correct statement: Cutting a log of wood into pieces is a physical change, ft, False
(b) True
Correct statement: The formation of manure from leaves is a chemical change.
(c) True
(d) False
Correct statement: Iron and rust are two different substances.
(e) True

Question 3.
Fill in the blanks in the following statements:

1. When carbon dioxide is passed through lime water, it turns milky due to the formation of …….
2. The chemical name of baking soda is ………
3. Two methods by which rusting of iron can be prevented are ……. and ……….
4. Changes in which only ………. properties of a substance change are called physical changes.
5. Changes in which new substances are formed are called ……. changes.

Answer:

1. calcium carbonate
2. sodium hydrogen carbonate
3. painting or greasing, galvanization
4. physical
5. chemical

Question 4.
When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Answer:
It is a chemical change. Here, a new substance carbon dioxide gas is formed.

Question 5.
When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.
Answer:
When candle bums, both physical and chemical changes occur as given below:
(i) Physical change: Melting of wax, vapourisation of melted wax.
(ii) Chemical change: Burning of the vapours of wax to give carbon dioxide, heat, and light.
The burning of LPG in our kitchen is another such example in which a physical change occurs when LPG comes out of the cylinder and is converted to hum a liquid state to a gaseous state and a chemical change occurs when this gas burns in air.

Question 6.
How would you show that the setting of curd is a chemical change?
Answer:
When some sour substance is added to milk or kept overnight, it turns into curd. The curd in no way can be converted into milk i.e. irreversible change. Curd is a different substance than milk. So, the formation of curd is a chemical change.

Question 7.
Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer:
Burning of wood is a chemical change because in burning new substances are formed as given below:
Wood + Oxygen → Coal + Carbon dioxide + Heat + Light.
While cutting it into small pieces is physical change because by cutting we can only reduce the size of the log of wood and no change in its chemical properties occurs and no new substance is formed.

Question 8.
Describe how crystals of copper sulphate are prepared.
Answer:
A cup of water is taken in a beaker and a few drops of dilute sulphuric acid are added into it. The water is heated. When it starts boiling, copper sulphate powder is added slowly while stirring continuously. Copper sulphate powder is added continuously till no more powder can be dissolved. The solution is filtered and allowed to cool down. Crystals of copper sulphate slowly form at the bottom of the beaker.

Fig. Crystals of copper sulphate

Question 9.
Explain how the painting of an iron gate prevents it from rusting.
Answer:
We know that the process of rusting requires exposure to both oxygen and water (or moisture). Painting prevents the surface of the iron gate from coming in contact with oxygen and moisture thus prevents it from rusting.

Question 10.
Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer:
In coastal areas, there is more moisture in the air due to the presence of the sea. But in the desert, there is a scarcity of water and hence the air is almost dry there. Both air and moisture are necessary conditions for rusting. So, rusting is faster in coastal areas than in deserts.

Question 11.
The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer:
(ii) Process – B is a chemical change.

Question 12.
Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer:
(iii) Both processes A and B are chemical changes.

We hope the NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes help you. If you have any query regarding NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes, drop a comment below and we will get back to you at the earliest.

NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts are part of NCERT Solutions for Class 7 Science. Here we have given NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts.

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

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

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

Question 3.
Name the source from which litmus solution is obtained. What is the use of this solution?
Answer:
The extraction of litmus solution takes place from Lichens (a symbiotic association of algae and fungi). It is used to determine whether the given solution is acidic or basic.

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

Question 5.
Describe the process of neutralization with the help of an example.
Answer:
The process in which an acid reacts with a base to form salt and water is known as a neutralization reaction. For example,

Question 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 neutralize 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)
Answer:
(i) F
(ii) F
(iii) T
(iv) T
(v) 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 a neutral drink. How will Dorji decide which drink is to be served to whom?
Answer:
He can decide by the use of an indicator. If the sample of drink turns red litmus blue, it is basic. If it turns blue litmus red, it is acidic. If it does not affect litmus, it is neutral.

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) To relieve indigestion, we take an antacid such as milk of magnesia. It neutralizes the effect of excessive acid.
(b) When an ant bites, it injects the acidic liquid into the skin. This acidic liquid is formic acid, which leads to inflammation to the skin. The effect of the sting can be neutralised by rubbing calamine solution which contains zinc carbonate. The zinc carbonate is a weak base and causes no harm to the skin.
(c) The wastes of many factories usually contain acids. If they are allowed to flow into the water bodies without neutralising it properly, then the acids will kill fish and other aquatic organisms. So factory wastes must be neutralised by adding some basic substances into the wastes before disposing these into 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 a turmeric indicators.
Answer:

Question 10.
Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Answer:
The solution may be neutral or basic. Both types of substances have no effect on blue litmus. Put a drop of this solution on a red litmus paper. If it turns blue, then the above solution is basic in nature and if no colour change occurs, then it is neutral.

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 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:
(ii) (a) and (d)

We hope the NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts help you. If you have any query regarding NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts, drop a comment below and we will get back to you at the earliest.

NCERT Solutions for Class 7 Science Chapter 4 Heat are part of NCERT Solutions for Class 7 Science. Here we have given NCERT Solutions for Class 7 Science Chapter 4 Heat.

NCERT Solutions for Class 7 Science Chapter 4 Heat

Question 1.
State similarities and differences between the laboratory thermometer and the clinical thermometer.
Answer:
Similarities:

1. Both contain mercury in bulbs.
2. Both use the Celsius scale on a glass tube.

Differences:

1. The temperature on the laboratory thermometer falls by itself, but not so in the clinical thermometer.
2. The range of a laboratory thermometer is much higher than a clinical thermometer.

Question 2.
Give two examples each of conductors and insulators of heat.
Answer:

• Conductors of heat: Copper, iron.
• Insulators: Plastic, wood (dry).

Question 3.
Fill in the blanks:

1. The hotness of an object is determined by its …….
2. The temperature of boiling water cannot be measured by a ………. thermometer.
3. Temperature is measured in degree …………
4. No medium is required for transfer of heat by the process of ………..
5. A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of ………
6. Clothes of ……….. colours absorb heat better than clothes of light colours.

Answer:

1. temperature
2. clinical
3. Celsius
4. radiation
5. conduction
6. dark

Question 4.
Match the following:

Answer:

Question 5.
Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Answer:
If there are more than one layer of clothes, air would be trapped in between them. Air is a poor conductor of heat. This increases the insulation and thus comfortability of the clothes as a whole.

Question 6.
Look at Fig., mark where the heat is being transferred by conduction, by convection, and by radiation.

Fig. Heat transferred by conduction, by convection and by radiation
Answer:
In this figure, the heat is transferred at three points in different ways:

• by radiation: from the flame to the bottom of the pan.
• by conduction: from lower surface to upper surface of the pan.
• by convection: from the base to the upper region of pan water.

Question 7.
In places of the hot climate, it is advised that the outer walls of houses be painted white. Explain.
Answer:
Objects with white surfaces reflect more heat and absorb less. So, less heat from outside would enter the house.

Question 8.
One liter of water at 30°C is mixed with one liter of water at 50°C. The temperature of the mixture will be
(a) 80°C
(b) more than 50°C but less than 80°C
(c) 20°C
(d) between 30°C and 50°C
Answer:
(d) between 30°C and 50°C.

Question 9.
An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will
(a) flow from the iron ball to water.
(b) not flow from iron ball to water or from water to iron ball.
(c) flow from water to iron ball.
(d) increase the temperature of both.
Answer:
(b) not flow from iron ball to water or from water to iron ball.

Question 10.
A wooden spoon is dipped in a cup of ice cream. Its other end
(a) becomes cold by the process of conduction.
(b) becomes cold by the process of convection.
(c) becomes cold by the process of radiation.
(d) does not become cold,
Answer:
(d) does not become cold.

Question 11.
Stainless steel pans are usually provided with copper bottoms. The reason for this could be that
(a) copper bottom makes the pan more durable.
(b) such pans appear colourful.
(c) copper is a better conductor of heat than stainless steel.
(d) copper is easier to clean than stainless steel.
Answer:
(c) copper is a better conductor of heat than stainless steel.

We hope the NCERT Solutions for Class 7 Science Chapter 4 Heat help you. If you have any query regarding NCERT Solutions for Class 7 Science Chapter 4 Heat, drop a comment below and we will get back to you at the earliest.

NCERT Solutions for Class 7 Science Chapter 3 Fibre to Fabric are part of NCERT Solutions for Class 7 Science. Here we have given NCERT Solutions for Class 7 Science Chapter 3 Fibre to Fabric.

NCERT Solutions for Class 7 Science Chapter 3 Fibre to Fabric

Question 1.
You must be familiar with the following nursery rhymes:
(i) ‘Baa baa black sheep, have you any wool.
(ii) ‘Mary had a little lamb, whose fleece was white as snow.
Answer the following:
(a) Which parts of the black sheep have wool?
(b) What is meant by the white fleece of the lamb?
Answer:
(a) The hairy skin – called fleece, has wool in black sheep.
(b) White fleece means hairy skin which is white in colour.

Question 2.
The silkworm is (a) a caterpillar, (b) a larva. Choose the correct option.
(i) (a)
(ii) (b)
(iii) both (a) and (b)
(iv) neither (a) nor (b)
Answer:
(iii) both (a) and (b)

Question 3.
Which of the following does not yield wool?
(i) Yak
(ii) Camel
(iii) Goat
(iv) Woolly dog
Answer:
(iv) Woolly dog

Question 4.
What is meant by the following terms?

1. Rearing
2. Shearing
3. Sericulture

Answer:

1. Rearing: Bringing up and looking after the sheep is called rearing.
2. Shearing: To obtain wool, the fleece of the sheep along with a thin layer of skin is removed from its body. This process is called shearing.
3. Sericulture: The rearing of silkworms for obtaining silk is called sericulture.

Question 5.
Given below is a sequence of steps in the processing of wool. Which are the missing steps? Add them.
Shearing, ……., sorting ………, ………., ……….
Answer:
Shearing, scouring, sorting, picking out burrs, colouring, rolling.

Question 6.
Make sketches of the two stages in the life history of the silk moth which are directly related to the production of silk.
Answer:

Question 7.
Out of the following, which are the two terms related to silk production?
Sericulture, floriculture, moriculture, apiculture and silviculture.
Hints:
(i) Silk production involves the cultivation of mulberry leaves and rearing silkworms.
(ii) Scientific name of mulberry is Morus alba.
Answer:
Sericulture, Moriculture.

Question 8.
Match the words of Column I with those given in Column II:

Answer:

Question 9.
Given below is a crossword puzzle based on this lesson. Use hints to fill in the blank spaces with letters that complete the words.

Down
(D) 1 : Thorough washing
2 : Animal fibre
3 : Long thread like structure
Across
(A) 1 : Keeps warm
2 : Its leaves are eaten by silkworms
3 : Hatches from egg of moth
Answer:

We hope the NCERT Solutions for Class 7 Science Chapter 3 Fibre to Fabric help you. If you have any query regarding NCERT Solutions for Class 7 Science Chapter 3 Fibre to Fabric, drop a comment below and we will get back to you at the earliest.