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Page No 85:

Question 1:

What is common to the changes mentioned below.
the setting of milk into curd; the rusting of iron; the ripening of a mango
(a) They are all reversible changes.
(b) They are slow reversible changes.
(c) They are fast reversible changes.
(d) They are slow irreversible changes.

Answer:

(d) They are slow irreversible changes.
Curd cannot be converted back into milk, ripened mango cannot turn raw, and rusting of iron cannot be reverted back. Therefore, they all are irreversible reactions.

Page No 85:

Question 2:

Which of the following is not a periodic change?
(a) formation of a rainbow
(b) the Full Moon
(c) the occurrence of seasons
(d) day and night

Answer:

(a) formation of a rainbow
The formation of rainbow is caused by the refraction of sunlight from raindrops. It is not a periodic change.



Page No 86:

Question 3:

Which of the following is a chemical change?
(a) the making of idlis
(b) grinding rice
(c) powdering of sugar crystals
(d) breaking of a window pane

Answer:

(a) the making of idlis
All other processes are physical change. Making of idlis involves chemical reaction of food ingredients.

Page No 86:

Question 4:

Which one of the following is an incorrect statement?
(a) Evaporation takes place at all temperatures.
(b) Evaporation causes a change in state.
(c) Evaporation takes place at a fixed temperature.
(d) Evaporation is a slow process.

Answer:

(c) Evaporation takes place at a fixed temperature.
It can take place at variable temperatures.

Page No 86:

Question 5:

In the table given below, one of the changes has been listed under the wrong heading. Identify this change.

Physical Changes Chemical Changes
freezing chocolate the souring of food
whipping cream making lemonade
squeezing oranges for
juice
burning petrol in a
motor car

Answer:

Whipping cream is a chemical change.

Page No 86:

Question 6:

Sunita dropped some ice cubes into a glass tumbler. After a while she saw two changes. Which of the following statements tell you what she might have observed?
A. The ice melted which is a physical change.
B. She saw droplets of water forming on the outside of the glass tumbler due to evaporation.
C. She saw droplets of water forming on the outside of the glass tumbler due to water vapour in the air condensing on it.
D. Condensation of water vapour is a physical change.
E. Formation of droplets is a chemical change as a new substance is formed.
F. The melting ice takes up heat from the surroundings.

(a) A, C, E, F are true.
(b) A, B, C, D are true.
(c) C, D, E, F are true.
(d) A, C, D, F are true.

Answer:

(d) A, C, D, F are true.
The melting ice takes up heat from the surroundings and it is a physical change. Formation of water droplets on the outside of the glass tumbler due to water vapour in the air condensing on it is a physical process.

Page No 86:

Question 7:

You find the inside of the car window getting foggy in winters. This is because of
(a) condensation.
(b) sublimation.
(c) melting.
(d) evaporation.

Answer:

(a) condensation
This is caused by condensation of water vapours present in air.

Page No 86:

Question 8:

Which of these changes involve release of energy?
(a) evaporation and melting
(b) evaporation and condensation
(c) condensation and freezing
(d) condensation and melting

Answer:

(c) condensation and freezing
In condensation and freezing, heat energy is released.

Page No 86:

Question 9:

Wax melts at 55°C. To get solid wax again the process that must take place is
(a) evaporation.
(b) melting.
(c) condensation.
(d) solidification.

Answer:

(d) solidification
The conversion of liquid to solid is called solidification.

Page No 86:

Question 10:

During evaporation
(a) solid changes into a gas.
(b) liquid changes into a gas.
(c) gas changes into a liquid.
(d) solid changes into a liquid.

Answer:

(b) liquid changes into a gas
During evaporation, a liquid changes into a gas.

Page No 86:

Question 11:

Which of the following is a physical but irreversible change?
(a) burning of matchstick
(b) melting of ice cream
(c) breaking of a flower vase
(d) lighting of an electric bulb

Answer:

(c) breaking of a flower vase
A broken flower vase cannot be reconstituted. Therefore, it is a physical but irreversible change.



Page No 87:

Question 12:

Rohan put the same volume of water in each of these open containers. He then left them in the sun for 24 hors. From which of these containers do you think was most water lost due to evaporation?
Figure

Answer:

Container C has the largest surface area; therefore, maximum water is lost from it due to evaporation.

Page No 87:

Question 13:

Changing of water to ice is a physical change because
i. a new substance is formed
ii. ice is another form of water
iii. the change is reversible
iv. heat is taken away from water

(a) i, iii, iv are correct
(b) i, ii, iv are correct
(c) ii, iii, iv are correct
(d) i, ii, iii are correct

Answer:

(c) ii, iii, iv are correct
Ice is a solid form of water and can be converted into water again upon melting.

Page No 87:

Question 14:

Under which of the following conditions will evaporation best occur?
(a) large surface area, windy day, cloudy
(b) sunny day, small surface area, windy day
(c) sunny day, very high humidity, high temperature
(d) high temperature, dry, windy day

Answer:

(d) high temperature, dry, windy day
Evaporation occurs maximum when the temperature is high and air has less humidity, i.e., conditions of dryness in atmosphere, on a windy day.

Page No 87:

Question 15:

If you wanted to get a wet shirt dried as quickly as possible which of the following are the best ways of doing so?
(a) Stretch the shirt out in a sunny place.
(b) Roll it into a ball.
(c) Pin it onto a clothes line on a windy day.
(d) Stretch it out under a fan.
(e) Put it on a clothes line on a wet day.
(f) Stretch it over a warm radiator.
(g) Put it in a cool shady place.

Answer:


To get a wet shirt dried as quickly as possible, following are the best ways:
(a) Stretch the shirt out in a sunny place.
(c) Pin it onto a clothes line on a windy day.
(d) Stretch it out under a fan.
(f) Stretch it over a warm radiator.

Page No 87:

Question 16:

Choose words from the list below and fill in the blanks.
water, condenses, vaporisation, boiling, steam, cold, reversible
The water in the kettle is _____________.
As it boils, it changes into ____________.
This process is known as _____________.
If you hold a _____________ metal plate near the spout of the kettle, the steam ___________ into droplets of ___________. The water can then be collected in a beaker. Hence, this is an example of __________ change.
Figure

Answer:


The water in the kettle is boiling.
As it boils, it changes into steam.
This process is known as vaporisation.
If you hold a cold metal plate near the spout of the kettle, the steam condenses into droplets of water. The water can then be collected in a beaker. Hence, this is an example of reversible change.

Page No 87:

Question 17:

Identify the process that is involved in the following changes. Choose from the words given below.
freezing, evaporation, melting, sublimation, condensation, vaporisation
(a) The juice that you left in the freezer to cool last evening is a frozen treat today.
(b) You forgot to switch off the tea kettle and all the water is gone.
(c) The cold hard butter that you spread on your warm toast is now soft and easy to spread.
(d) The big ice cubes you put in your water are now tiny.
(e) When you put a lid on hot soup, the inside of the lid gets wet.
(f) The wet jeans that you hung out to dry last night when the temperature dropped below freezing are now cold, hard and stiff.
(g) When you breathe onto a cold mirror it turns foggy.
(h) The perfume from the bottle that you left open last evening was gone when you looked at it this morning.
(i) Puddles of water turn into ice on a very cold night.
(j) The solid piece of freshener that your mother left on the bathroom shelf disappeared in a few weeks.

Answer:

(a) Freezing
(b) Vaporisation
(c) Melting
(d) Melting
(e) Condensation
(f) Freezing
(g) Condensation
(h) Evaporation
(i) Freezing
(j) Sublimation



Page No 88:

Question 18:

A salt solution was heated and after a while substance X was formed on the inner wall of the funnel. What do you think it is? Explain your answer.
Figure

Answer:

Substance X is salt. Here, water gets evaporated and salt is left behind. Some salt mixed with water vapours rises up to the neck of the bottle as shown in the diagram, but later, settles down and sticks to the neck of the bottle, whereas water escapes in the form of vapours.

Page No 88:

Question 19:

Give one word for the following.
(a) a solid becoming a liquid
(b) a vapour becoming a liquid
(c) a change which occurs again and again
(d) a change in which the original substance can be got back easily
(e) a liquid becoming a solid
(f) a change in which a new substance is formed
(g) a change in which a solid directly changes into vapour
(h) a liquid changing to vapour
(i) changes that happen over and over again in a certain pattern

Answer:

(a) Melting
(b) Condensation
(c) Periodic change
(d) Reversible change
(e) Freezing
(f) Chemical change
(g) Sublimation
(h) Evaporation
(i) Periodic change

Page No 88:

Question 20:

State the conditions under which evaporation takes place quickly.

Answer:

Evaporation takes place quickly during a sunny day. Other favourable conditions for evaporation are dry weather, high temperature, wind and more surface area of exposure.

Page No 88:

Question 21:

What would you do in the following cases? Explain why.
(a) A pile of newspapers gets wet in the rain.
(b) A cotton shirt has to be dried in a hurry.
(c) A bag of salt is accidentally dropped in a little water.
(d) The plastic cap of a bottle of perfume gets cracked and breaks.

Answer:

(a) Dry the newspapers under sun. Sun rises the temperature due to which the rate of evaporation increases and newspapers get dried.
(b) Dry the cotton shirt by spreading it under a moving fan. This will expose all the surface area of the shirt to the fan and it will dry faster.
(c) Dry the bag under sun. This will allow the water to evaporate and salt will be left behind.
(d) Replace the cap or fix the cap by an adhesive. Without a cap, all the perfume will get evaporated.

Page No 88:

Question 22:

Differentiate between the following, giving at least one point of difference.
(a) periodic and non-periodic changes
(b) irreversible and reversible changes
(c) vaporisation and sublimation
(d) physical and chemical changes
(e) evaporation and boiling

Answer:

(a)

Periodic changes Non-periodic changes
They are cyclic changes that occur at regular intervals of time and have a fixed pattern. They do not have a fixed pattern and do not occur at fixed intervals of time.
Example: Seasons Example: Souring of milk

(b)
Irreversible changes Reversible changes
The changes which can not be reverted back are called irreversible change. The changes which can be reverted back are reversible changes.
Example: Photosynthesis Example: Melting of ice

(c)
Vaporisation Sublimation
The conversion of liquid into vapours is called vaporisation.  The conversion of solid into gas is called sublimation.

(d)
Physical changes Chemical changes
The changes that do not involve a change in the properties and identity of a substance are called physical changes. The changes that involve changes in the properties of a substance or formation of a new substance is called chemical change.

(e)
Evaporation Boiling
It is a slow process. It is a faster process.
Bubbles are not formed. Bubbles are formed.

Page No 88:

Question 23:

In these columns, indicate whether the given changes are periodic or non-periodic; reversible or irreversible; a physical or chemical. Use the letters given below for your answer.

Change: P or NP  R or IR  P or C

(a) sharpening a pencil
(b) breaking a cricket bat
(c) the burning of gas in a stove
(d) dissolving salt in water
(e) the burning of a matchstick
(f) the stitching of cloth into a shirt
(g) heating butter strongly to make ghee
(h) cutting cucumber for salad

Answer:

(a) P, NP and IR
(b) P, NP and IR
(c) C, NP and IR
(d) P, NP and R
(e) C, NP and IR
(f) P, NP and IR
(g) P, NP and R
(h) P, NP and IR



Page No 89:

Question 24:

State whether the following statements are true or false. Rewrite the false ones correctly.
(a) All irreversible changes are chemical changes.
(b) A liquid can sublime.
(c) The evaporation of water take place only at 100°C. when it is boiling.
(d) The melting of ice is a physical change.
(e) The souring of milk is a periodic change.
(f) The flowering of trees is a periodic change.
(g) Solidification and condensation are the same.
(h) The breaking of glass tumbler is a chemical change because it is irreversible.
(i) Boiling, evaporation and condensation are physical changes.

Answer:

(a) False. Some physical reactions are also irreversible like breaking of bat.
(b) False. Liquid vaporises.
(c) False. Evaporation can take place at variable temperatures.
(d) True.
(e)False. It is a non-periodic change.
(f) True
(g)False. They are different. Solidification is the conversion of liquid into solid, whereas condensation is the conversion of gas into liquid.
(h)False. It is a physical change.
(i)True.

Page No 89:

Question 25:

Explain the differences that you would see if you separately heat 5 g of iodine and 5 g of water.

Answer:

On heating iodine, it will directly turn into vapours without having a liquid state. This is called sublimation.
Water will boil to form vapours. This is called vaporisation.

Page No 89:

Question 26:

Define sublimation. Draw a neat labelled diagram to show the apparatus used to demonstrate the sublimation of ammonium chloride.

Answer:

Sublimation is a process of conversion of solid into gas without having an intermediate liquid state. This happens when heat is absorbed by the solid.
Figure: Sublimation of ammonium chloride

Page No 89:

Question 27:

Explain why:
(a) A dhobi spreads wet bed sheets on the ground in a sunny area.
(b) Dewdrops are found on the surface of lawns early in the morning.

Answer:

(a) A dhobi spreads wet bed sheets to increase their surface area exposed to the sun to allow them to dry quickly.
(b) Dewdrops are formed early in the morning because of the condensation of water vapours present in the air.

Page No 89:

Question 28:

(a) During photosynthesis, green plants prepare glucose with the help of carbon dioxide from the air and water from the soil. Is photosynthesis a physical change or a chemical change? Justify your answer.
(b) Copper sulphate is a blue coloured chemical. When it is heated, the blue colour disappears and it turns white. After is cools, if a few drops of water are put into the test tube, the copper sulphate turns blue again. Is this a physical or a chemical change?

Answer:

(a) Photosynthesis is a chemical change. During photosynthesis, a new substance glucose is formed from raw materials like carbon dioxide and water. This process is irreversible.
(b) Heating of copper sulphate is a physical change. It does not change the properties of copper sulphate and only evaporates water present in it. It is a reversible change. 



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