RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Rajasthan Board RBSE Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

RBSE Solutions for Class 7 Science

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

RBSE Class 7 Science Physical and Chemical Changes of Substances Intext Questions and Answers

Page No 35.

Question 1.
Are all of these changes stable or unstable or reversible ?
Answer:
No, all changes are not alike. These are of two types –

  1. Stable changes – These are also known as irreversible changes.
  2. Unstable Changes – These are also known as reversible changes.

Page No 36.

Question 2.
Can you solidify the water formed by melting of ice?
Answer:
Yes, we can solidify the water formed by melting of ice. Ice can be converted into water on heating and on cooling, again into ice. It is a change of a substance from one state to another state which is a temporary change.

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Page No 37.

Question 3.
On stretching a rubber band and releasing it, does it regain its original shape?
Answer:
Rubber band increases in length on stretching. If it is released loose, then it regains its original shape It is also an unstable change.

Question 4.
In the following table some changes are given. Fill in the table if these changes are stable or unstable.
Answer:
Table – Incidents of changes –
RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances-1

Page No 40.

Question 5.
What are the chemical changes ?
Answer:
Changes in which the internal chemical composition of the substance changes, are known as chemical changes.

RBSE Class 7 Science Physical and Chemical Changes of Substances Text Book Questions and Answers

Choose the correct option:

Question 1.
Example of Physical change is –
(a) Rusting
(b) Melting of ice
(c) Formation of curd
(d) Browning of apple after cutting
Answer:
(b) Melting of ice

Question 2.
Example of chemical change is –
(a) Lightening of bulb
(b) Milky change of lime water
(c) Water changes into water vapour
(d) Melting of ghee
Answer:
(b) Milky change of lime water

Question 3.
Why are doors painted in our house –
(a) To protect from sun rays
(b) To make it dust proof
(c) To prevent from rusting
(d) To protect from birds
Answer:
(c) To prevent from rusting

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 4.
The chemical formula of rust is –
(a) Fe2O3
(b) Fe
(c) FeO
(d) FeSO4
Answer:
(a) Fe2O3

Fill in the blanks:

  1. To make the solution of sugar is ………………….. Change.
  2. Generally physical change are …………………..
  3. Conversion of wheat grain into wheat flour is ………………….. change.
  4. When carbon dioxide passed in lime water, it turns milky, this is ………………….. change.

Answer:

  1. Physical
  2. Unstable.
  3. Irreversible
  4. Chemical.

Match the following Column I & Column II –
RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances-2
Answer:

  1. (b)
  2. (a)
  3. (c)

Short Answer type Questions:

Question 1.
Write the reaction of oxygen with magnesium ?
Answer:
Magnesium oxide is formed by combustion of magnesium ribbon in oxygen gas.
Magnesium (Mg) + Oxygen (O2) → Magnesium Oxide (MgO)

Question 2.
What is crystallization?
Answer:
The process of obtaining big and pure crystals from its solution is known as crystallization. It is the example of physical change.

Question 3.
Which factors are responsible for rusting ?
Answer:
Mainly two factors are responsible for rusting –

  1. Oxygen, and
  2. Moisture (Water Vapour)

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 4.
What will be the colour of a product after reaction of starch and iodine ?
Answer:
Purple Blue coloured compound is formed as a product after reaction of starch and iodine.
Starch (Potato) + Iodine → Purple Blue (New substance)

Question 5.
What kind of change tearing the paper is ?
Answer:
Tearing the paper is a physical change because on tearing the paper. only its shape and size changes.

Long Answer type Questions:

Question 1.
Describe the physical and chemical changes with examples.
Answer:
1. Physical Changes –
Those changes in which only physical properties of the substance changes, are known as physical changes. In these changes shape of substance, size, physical state, temperature, pressure, etc. changes. These type of changes are unstable and reversible in nature. New substances are not formed in such type of changes.

Example –
Melting of ice, dissolution of salt in water, lightening of bulb, formation of vapour from water, conversion of vapour into water heating of blade in flame, etc.

2. Chemical change – Changes in which chemical identity of the initial substances changes and new substances are formed are known as chemical changes. These changes can not be reversed, hence stable in nature.

Examples –
Burning of candle, curdeling of milk, digestion of food. rusting of iron. burning of types. bursting of cracker, on festivals, burning of magnesium ribbon. etc.

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 2.
Explain crystallization with the help of labelled diagram.
Answer:
Crystallization is a physical change. To understand crystallization process. perform following experiment –

Experiment –
Take a beaker and fill it half with water. Add alum powder in it. Heat the above solution and add keep on adding alum powder till it gets dissolved completely in the solution When dissolution stops. filter the solution with the help of filter paper. Let the solution cool don, Do not move the beaker at the time of cooling. Observe the solution after sometime.

We will see the crystals of alum. In this process new substance is not formed but crystals of same substance are formed. so crystallization is the example of physical change. If we want crystals of bigger size, then tie a small crystal b a thread and hang it in a way that it will touch the solution. Consequently many small size crystals stick around that small crystal and a big crystal is formed. Thus, the process of obtaining big and pure crystals from its solution is known as crystallization.
RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances-3

Question 3.
Explain the process of rusting and how could it be prevented ?
Answer:
Process of Rusting –
Iron articles such as spade, axe, hammer etc when put open in air and moisture for some days. they got rusted and their surface get converted into brownish layer known as rust. This process is known as rusting. Rust is not iron metal.

In this process iron metal transformed into a new substance called Rust (Iron oxide – Fe2O3) Process of rusting can be represented as – Iron (Fe) + Oxygen (O2 from Air) + Moisture (H2O) → Iron oxide (Fe2O3) Rust oxygen and water are two essential components for rusting of iron. Rusting destroys iron slowly.

Prevention from Rusting –
Iron is used to make the both of vehicles, like ships. trucks. cycles, bridge. etc, so it is must to protect iron from rusting. For this following tips/methods can be used –

  1. Iron must be protected from the contact of water and oxygen.
  2. Greasing and coating of paint can be applied on their surface.
  3. Apply coating of chromium and zinc on surface of iron articles to protect it from rusting.
  4. Articles made from iron should be cleaned and used frequently
  5. Mixing of carbon. manganese. nickel and chromium in iron metal gives us an hard and rust proof alloy SteeF (Stainless Steel).

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 4.
Write down four examples of physical and chemical changes from our daily life.
Answer:
Physical Changes – Following arc examples

  1. Melting of ice – We know that ice is a solid form of water. When ice is at normal temperature. it begins to melt and converts into water.
  2. Dissolution of salt in water – Salt is a mineral and exists in solid state dissolution of salt in water is also a physical change. B evaporating this solution, salt can be recovered.
  3. Heating of blade ¡n flame – It is also an example of physical change. After removing it from flame, blade cools down shortly.
  4. Conversion of vapours from water – It is also an example of physical change.

Chemical Change – Following are examples of chemical change –
1. Curdling of Milk –
Curdling of milk after adding a small amount of butter milk in milk is an example of chemical change, because in this process a new substance, curd is formed. It is a stable and irreversible change.

2. Burning of Crackers –
After burning and bursting of crackers during festivals, these changes into ash which is a new substance, It cannot be reversed. Hence it is also a chemical change and reversible in nature.

3. Burning of candle –
Candle converts into vapours after burning which cannot be recovered.

4. Burning of Magnesium ribbon –
Magnesium ribbon forms a new substance, magnesium oxide (MgO), after burning in air / oxygen.

RBSE Class 7 Science Physical and Chemical Changes of Substances Important Questions and Answers

Objective Questions:

Question 1.
Conversion of water into water vapour is which type change –
(a) Stable
(b) Unstable
(c) Both (a) & (b)
(d) None of both
Answer:
(b) Unstable

Question 2.
In following reversible changes are-
(a) Unstable
(b) Stable
(c) Both a & b
(d) None of both
Answer:
(a) Unstable

Question 3.
Irreversible changes are –
(a) Unstable
(b) Neutral
(c) Stable
(d) None of these
Answer:
(c) Stable

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 4.
Reversible changes are called –
(a) Physical change
(b) Chemical change
(c) Neutral
(d) None of these
Answer:
(a) Physical change

Question 5.
Irreversible changes are called –
(a) Physical change
(b) Chemical change
(c) Neutral
(d) None of these
Answer:
(b) Chemical change

Question 6.
Following is example of stable change –
(a) Freezing of water
(b) Vapour formation
(c) Curdling of Milk
(d) Melting of Ghee
Answer:
(c) Curdling of Milk

Question 7.
Following is example of unstable change –
(a) Combustion of Kerosene
(b) Glowing of bulb
(c) Burning of Candle
(d) Combustion of coal
Answer:
(b) Glowing of bulb

Question 8.
Which is chemical formula of iron oxide –
(a) Fe3O2
(b) Fe2O3
(c) Fe2O2
(d) Fe3O4
Answer:
(b) Fe2O3

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 9.
When Lawsone compound comes in contact with sunlight, which coloured compound is formed –
(a) Green
(b) Red
(c) Yellow
(d) Black
Answer:
(b) Red

Fill in the blanks:

  1. Crystallization is a …………………… change. (Physical / Chemical)
  2. Combustion of magnesium ribbon is a …………………… change. (Physical / Chemical)
  3. Top prevent rusting a layer of …………………… is applied. (Iron / Zinc)
  4. In physical changes …………………… properties of the substance change. (Physical / Chemical)
  5. In chemical changes, …………………… substances are formed. (New / Same)

Answer:

  1. Physical
  2. Chemical
  3. Zinc
  4. Physical
  5. New

Match the Column I with Column II correctly –
RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances-4
Answer:

  1. (c)
  2. (e)
  3. (d)
  4. (a)
  5. (b)

Very Short Answer type Questions –

Question 1.
What is change ?
Answer:
Change means transformation. Transformation of chemical & physical properties of substances is change.

Question 2.
On the basis of nature of changes, how many types of changes ?
Answer:
On the basis of nature of changes, these are of two types –

  1. Physical change
  2. Chemical change.

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 3.
What is galvanization?
Answer:
Galvanization is a process in which iron objects are coated with Zinc to protect them from rusting.

Question 4.
What do you mean by crystallization?
Answer:
The process by which pure and big sized crystals of a compound can be obtained from its solution is known as crystallization.

Question 5.
What changes occur in substances during physical changes ?
Answer:
During physical changes, shape. size, state. temperature, pressure of the substances changes.

Question 6.
What are the physical properties of the substances ?
Answer:
Shape. Size, physical state, temperature. pressure, etc. are the physical properties of the substances.

Question 7.
What changes occur in substances during chemical changes ?
Answer:
During chemical changes, the internal chemical composition of the substance changes and new substances are formed.

Question 8.
What happens when chemical composition of the substance changes during chemical change ?
Answer:
New substances are formed.

Question 9.
Give some examples of Physical changes.
Answer:

  1. Melting of ice
  2. Dissolution of salt in water
  3. Glowing of bulb
  4. Formation of vapour from water
  5. conversion of water vapour into water.

Question 10.
Glowing bulb is an example of physical change. Why?
Answer:
Because only temperature changes during glowing of bulb, hence it is a physical change.

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 11.
Crystallization is a physical change Why?
Answer:
Because in crystallization process, crystals are formed from the same original compound.

Question 12.
Represent rusting of iron process with an equation.
Answer:
Answer:
Iron (Fe) + Oxygen (O2 from air) + Water (H2O) → Iron oxide (Fe2O3). New substance. ,

Question 13.
Do you convert chalk powder again into chalk stick ?
Answer:
Yes. it is a physical or reversible change.

Question 14.
Whose chemical formula is Fe2O3?
Answer:
Fe2O3 is chemical formula of iron oxide.

Question 15.
Name the components used in manufacturing of stainless steel.
Answer:
Manganeeze, Carbon, Nikil, Chromium, Iron, etc. are used.

Question 16.
Rancidity of food materials ¡s a chemical change. Why ?
Answer:
Chemical composition of the substances changes by rancidity process. so it is a chemical change.

Question 17.
Rancidity of food materials is what type of change ?
Answer:
Chemical change.

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 18.
Colour of peeled surface of potato change when it is kept in open atmosphere. Which type of change is this ?
Answer:
It is a chemical change.

Short Answer type Questions:

Question 1.
Explain the reversible and irreversible changes with examples.
Answer:
Changes are two types based on their stability –

  1. Reversible changes – Changes which can be reversed in back direction. arc reversible changes. For example – conversion of water into ice and conversion of ice into water.
  2. Irreversible changes – Changes which can not be reversed in opposite direction, are known as irreversible changes. For example – Curdling of milk (curd can not be changed back into milk)

Question 2.
Represent the conversion of various phases of water by a diagram.
Answer:
RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances-5

Question 3.
Why iron substances get rusted sometimes ?
Answer:
If we put an iron piece in open air. its surface get converted into brownish layer known as rust. This is a chemical process called rusting. Oxygen and water arc two essential components for rusting of iron. It is converted into iron oxide (Fe2O3) when comes in contact with open air and moisture. It is know as rusting of iron.

Question 4.
State the methods of preventing rusting of iron.
Answer:

  1. Iron must be protected from the contact of water and oxygen.
  2. Greasing and coating of paint can be applied on their surfaces.
  3. Apply coating of Chromium and zine on the surface of iron articles to protect them from rusting.
  4. Articles made from iron should be cleaned and used frequently.
  5. Mixing of carbon, manganese, nickel and chromium in iron metal gives us a hard and rust proof alloy Steel (Stainless Steel).

Question 5.
What is stainless Steel ? State its characteristics.
Answer:
Mixing of carbon, manganese, nickel and chromium in iron metal gives us an hard and rust proof alloy ‘Stainless Steel’. It is rust proof, light in weight and easy to use. It is used in domestic kitchenware, other domestic items and machines to prepare.

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 6.
State some changes occurring in nearby surrounding.
Answer:

  1. Curdling of milk
  2. dissolution of salt in water
  3. souring of curd
  4. conversion of water into vapour
  5. happening of day & night
  6. conversion of water into ice
  7. Rusting of iron
  8. running a farm
  9. glowing a bulb
  10. lightening in sky
  11. digestion of food.

Question 7.
What happens when iodine solution drops are put on surface of an unpeeled potato? Which type of change is this ?
Answer:
The unpeeled surface of potato becomes purple blue after adding a few drops of iodine solution on it. Potato (starch) + iodine solution ⇒ Purple blue substance (New substance). In this process, new substance is formed. Hence it is a chemical change.

Question 8.
Write chemical equations of some examples of chemical changes ?
Answer:

  1. Wax + Oxygen (O2) Burning ⇒ carbon dioxide (CO2) + Water vapour (New substance)
  2. Lime water [Ca(OH)2] + Carbon dioxide (CO2) → Milky water (CaCO3) – New substance
  3. Starch (Potato) + iodine ⇒ Purple blue substance (New substance)
  4. Magnesium (Mg) + Oxygen (O2) → magnesium oxide (MgO) (Ne substance)
  5. Iron (Fe) + Oxygen (O2 from air) + water (H2O) → Fe2O3 (iron oxide) – New substance.

In all above reactions, new substances are formed as products, hence these are chemical changes.

Question 9.
Why colour of apple changes when left ‘cut’ in air ?
Answer:
Apple contains iron (Fe). due to this when it comes in contact with air. iron reacts with oxygen producing iron oxide (Fe2O3) which is reddish brown in colour. This is a chemical change.

Question 10.
Based on which property, henna dyes on hands ?
Answer:
Heena contains chemical compounds such as quinine, napthoquinone and Lawsone. Lawsone is colourless but when it comes in contact with air or sunlight it forms a red coloured compound. Thus henna dyes and it is a chemical change.

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 11.
How does magician produce white dense fumes by magic ?
Answer:
Magician has one glass ammonium hydroxide solution and in other hydrochloric acid solution. When he mixes the two solution. The reaction between ammonium hydroxide and hydrochloric acid produces ammonium chloride and water. Ammonium chloride produced as white fumes. Ammonium hydroxide + Hydrochloric acid → Ammonium chloride + Water (White fumes)

Question 12.
How a magician lit fire over a glass full of water?
Answer:
In this trick, the magician uses a sodium metal which he tricks into the glass of water. Sodium metal reacts rapidly with water to form a colourless solution of sodium hydroxide (NaOH) and hydrogen gas (H2). The reaction is exothermic. During the reaction, the sodium metal may well become so hot that it catches fire and bums.
2Na (S) + 2H2O → 2NaOH (1) + H2(2) + Heat

Question 13.
Give examples of chemical changes and physical changes occurring in nearby surrounding.
Answer:
We observe different changes in our environment daily. Some of these are physical changes while some are chemical changes. Example of these changes are –

  1. Physical change – Freezing of water into ice. Going of bulb. dissolution of salt in water. vaporization of water,
  2. Chemical changes – Burning of candle, curdling of milk. digestion of food. rusting of iron. etc

Essay type Questions:

Question 1.
What type of changes are ? Describe.
Answer:
Based on nature of changes. these can be classified in two categories –
1. Physical changes – Shape. size. physical state. temperature. pressure etc are physical properties of the substances. Changes in which only physical properties are changed. are know as physical changes. These are reversible and temporary. No new substance is formed during physical change.

Example –
Melting of ice. dissolution of salt in water, glowing of bulb. heating blade in flame. formation of vapour from water, condensation of water vapour into water. etc. are physical changes. Crystallization also a physical change.

2. Chemical changes –
Changes in which chemical corn position of the substance changes, i.e. substance converts into new substance from its original form. are know as chemical changes. After a chemical change substance can not be changed into previous form. Hence these changes arc permanent and irreversible in nature.

Example –
Burning of candle. curdling of milk. burning of type. digestion of food. rusting of iron. combustion of magnesium ribbon. etc. arc chemical changes.

RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances

Question 2.
Justify that phase transformation is a temporary change.
(OR)
Describe phase transformation of substances with example and diagram.
Answer:
Conversion of solid into liquid or liquid into solid is phase transformation, Phase transformation can be understood by following experiment.

Experiment –
Take some wax in one bowl and heat it gently. Remove the bowl from the flame and let it cool down, After sometime we observe that the wax present in the bowl solidifies again Hence we can say that conversion of solids into liquids or vice-versa is called change of state of matter. Change of state of matter is an unstable change.
RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances-6

Question 3.
Turning of Lime water into milky is which type of change ? Explain with diagram.
Answer:
Light a candle and keep it on the table. Take a funnel and join its one end with a rubber tube and the second end of rubber tube with a glass tube Dip the glass tube in beaker filled with lime water cover the flame of candle with funnel so that the gas formed by burning candle can go to the beaker containing lime water. After some time we observe that the lime water turns milky.
RBSE Solutions for Class 7 Science Chapter 4 Physical and Chemical Changes of Substances-7
in above reaction. it is clear that hen carbon dioxide is passed into lime water. a new substance CaCO3 is formed. hence it is a chemical change.

Question 4.
Comment on iron tower of Mehroli with its characteristics.
Answer:
In Delhi near Qutub Minar in Mehroli. there is an ancient iron tower situated which was built about 400 B.C. by Indian black smiths. Its height is 8 meter and weight is 6 ton. So many centuries has been passed but this iron pillar is still intact and not rusted at all, It is known by researches that the Meh roll Towers Surface is coated with a thin layer of iron oxide (Fe2O3).

At the time of synthesizing it. phosphorus must have been mixed in it. It must have been painted with a mixture of  various chemicals and it must have been heated up at a high temperature. This is an evidence of advanced knowledge of our ancestors. Even at that time they had developed the technique of keeping. iron rust free.

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