Which of the following is a non-periodic change?
- Change of day and night
- Change of seasons
- Landslides
- Full moon and new moon nights
Answer
Landslides
Reason — Changes that are not repeated at regular intervals are called non-periodic changes. They may occur randomly at any time, like landslides.
Change of day and night is a periodic change as this change occurs regularly every 24 hours due to Earth’s rotation. Seasons repeat in a fixed cycle every year due to Earth’s revolution around the Sun. Full moon and new moon nights occur in a regular cycle known as the lunar cycle.
Burning of a piece of paper into ash is a:
- periodic change
- natural change
- reversible change
- permanent change
Answer
permanent change
Reason — Burning of a piece of paper into ash is a permanent change because paper cannot be obtained back from ash. A new substance, ash, is formed and the change is irreversible.
Periodic changes occur at regular intervals (like day and night). Natural changes occur without human involvement (like rain, seasons). Reversible changes can be undone.
Which of the following is a slow change?
- Rusting of iron
- Burning of wood
- Burning of a cracker
- Switching on a fan
Answer
Rusting of iron
Reason — The changes that take longer time to complete are called slow changes. They take hours, days, months or even years to occur. Examples: Rusting of iron.
Wood burns quickly once ignited. Crackers burn and explode in seconds. The fan starts rotating almost immediately.
Which of the following is a physical change?
- Cooking of food
- Formation of curd
- A glass cut into pieces
- Boiling of eggs
Answer
A glass cut into pieces
Reason — A physical change is a change in which no new substance is formed and the chemical composition of the original substance remains the same, even though some of its physical properties like colour, state, shape, size, etc. may change. A glass cut into pieces will have different sizes and shapes, but the chemical composition remains the same.
Cooking of food is a chemical change, new substances are formed and it is irreversible. Milk changes into curd due to bacterial action. A new substance is formed, so it is chemical. The egg undergoes a permanent change due to heat. It cannot return to its original form.
In a chemical change:
- a new substance is formed
- energy is either released or absorbed
- the change is irreversible
- all of the above
Answer
all of the above
Reason — A chemical change is a permanent and irreversible change in which new substances are formed whose chemical composition and physical and chemical properties are different from those of the original substance. During a chemical change, exchange of energy takes place, depending on the nature of the reaction.
Assertion (A): The changes which result in the formation of new substances with entirely different composition and properties are called chemical changes.
Reason (R): Chemical changes are permanent and irreversible.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false
- A is false but R is true.
Answer
Both A and R are true but R is not the correct explanation of A.
Explanation — A is true as changes which result in the formation of new substances with entirely different composition and properties are called chemical changes.
R is also true because chemical changes are generally permanent and irreversible. However, R states another characteristic of chemical changes; it does not explain the definition given in A.
Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.
Assertion (A): When a change in a substance can be reversed by changing the conditions, it is said to be a reversible and temporary change.
Reason (R): Burning a piece of paper is a reversible change.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false
- A is false but R is true.
Answer
A is true but R is false
Explanation — A is true as a reversible and temporary change is one that can be undone by changing conditions (like melting and freezing). R is false as burning a piece of paper is an irreversible change as a new substance ash is formed.
Therefore, A is true but R is false is the correct option.
Assertion (A): Sublimation of camphor is a physical change.
Reason (R): No new substance is formed.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false
- A is false but R is true.
Answer
Both A and R are true and R is the correct explanation of A.
Explanation — A is true as sublimation of camphor is a physical change as no new substance is formed. When the camphor vapours cool down, they change back to solid camphor as a sublimate.
R is true as no new substance is formed in a physical change.
Therefore, Both A and R are true and R is the correct explanation of A is the correct option.
Assertion (A): For rust formation to take place, iron must be exposed to air and water for some time.
Reason (R): Rust is a reddish brown substance formed on the surface of iron.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false
- A is false but R is true.
Answer
Both A and R are true but R is not the correct explanation of A.
Explanation — Assertion is correct as iron rusts only when both oxygen (air) and water are present for some time. Reason is also true as rust is a reddish-brown substance (hydrated iron(III) oxide). However, R just describes rust; it does not explain why air and water are needed for rusting.
Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.
Assertion (A): Burning of a candle is a physical change.
Reason (R): When a candle burns, some of its wax melts and gets solidified on cooling. Also carbon dioxide and water vapour are formed which cannot be recovered easily and converted back into the candle.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false
- A is false but R is true.
Answer
A is false but R is true.
Explanation — A is false because when a candle is lit, the wax melts (a physical change), then burns to produce carbon dioxide, water vapour, light, and heat (a chemical change).
R is true as molten wax solidifying on the floor is also a physical change. Thus, melting is a physical change, while burning is a chemical change.
Therefore, A is false but R is true is the correct option.
Fill in the blanks:
(a) The process of a liquid changing into a solid is called ............... .
(b) A change which alters the composition of a substance is known as a ............... change.
(c) There is no change in the ............... of the substance during a physical change.
(d) The reaction in which energy is evolved is called ............... .
(e) The process in which a substance in solid state directly changes into ............... is called sublimation.
Answer
(a) Freezing
(b) chemical
(c) composition
(d) exothermic reaction
(e) gaseous state
Match the following:
| Column A | Column B |
|---|---|
| (a) Freezing of water | (i) Natural change |
| (b) Change of seasons | (ii) Chemical change |
| (c) Burning of wood | (iii) Physical change |
| (d) Permanent change | (iv) Sublimation |
| (e) Heating of solid camphor | (v) Fast process |
Answer
| Column A | Column B |
|---|---|
| (a) Freezing of water | (iii) Physical change |
| (b) Change of seasons | (i) Natural change |
| (c) Burning of wood | (v) Fast process |
| (d) Permanent change | (ii) Chemical change |
| (e) Heating of solid camphor | (iv) Sublimation |
Name the following:
(a) The solid formed when vapours of a sublimable substance cool down.
(b) The compound formed when magnesium burns in air.
Answer
(a) Sublimate
(b) Magnesium oxide (MgO)
Write true or false against the following statements and correct the false ones.
(a) Heating of an iron rod is a chemical change.
(b) Condensation of steam is not a chemical change.
(c) Rusting of iron is a fast change.
(d) A physical change is reversible.
(e) There is a change in the mass of a substance in a physical change.
Answer
(a) False
Correct Statement — Heating of an iron rod is a physical change.
(b) True
(c) False
Correct Statement — Rusting of iron is a slow change.
(d) True
(e) False
Correct Statement — There is no change in the mass of a substance in a physical change.
Classify the following changes as a physical or a chemical change:
(a) Drying of wet clothes
(b) Curdling of milk
(c) Butter getting rancid
(d) Boiling of water
(e) Burning of paper
(f) Melting of wax
(g) Burning of coal
(h) Formation of clouds
(i) Rusting of iron
(j) Roasting of potatoes
(k) Formation of alloys
(l) Making of a sugar solution
(m) Glowing of an electric bulb
(n) Manufacture of salt from sea water.
Answer
(a) Drying of wet clothes — Physical change
(b) Curdling of milk — Chemical change
(c) Butter getting rancid — Chemical change
(d) Boiling of water — Physical change
(e) Burning of paper — Chemical change
(f) Melting of wax — Physical change
(g) Burning of coal — Chemical change
(h) Formation of clouds — Physical change
(i) Rusting of iron — Chemical change
(j) Roasting of potatoes — Chemical change
(k) Formation of alloys — Physical change
(l) Making of a sugar solution — Physical change
(m) Glowing of an electric bulb — Physical change
(n) Manufacture of salt from sea water — Physical change
Give one example of the following changes:
(a) Natural, non-periodic change
(b) Natural, periodic change
(c) Irreversible, physical change
(d) A chemical change in which energy is absorbed.
(e) A man-made fast change.
(f) A process in which both physical and chemical changes take place simultaneously.
Answer
(a) Earthquakes
(b) Change of day and night
(c) Chopping vegetables
(d) Photosynthesis
(e) Switching on a fan
(f) Burning of candle
Define:
(a) Physical change
(b) Chemical change
Answer
(a) Physical change — A physical change is a change in which no new substance is formed and the chemical composition of the original substance remains the same, even though some of its physical properties like state, colour, shape, size, appearance etc. may change.
(b) Chemical change — A chemical change is a permanent change in which new substances are formed whose chemical composition and physical and chemical properties are different from those of the original substance.
Differentiate between Periodic and non-periodic changes.
Answer
| Periodic changes | Non-periodic changes |
|---|---|
| Changes that are repeated at regular intervals of time and not randomly are called periodic changes. | Changes that are not repeated at regular intervals are called non-periodic changes. They may occur randomly at any time. |
| Examples: Change of day and night, change of seasons, etc. | Examples: Earthquakes, landslides, an epidemic, a person becoming sick, etc. |
Differentiate between Reversible and irreversible changes.
Answer
| Reversible changes | Irreversible changes |
|---|---|
| When a change in a substance can be reversed by changing the conditions, it is said to be a reversible change. | If a substance cannot be brought back to its original state after a change, it is said to be an irreversible change. |
| Examples: Water freezing into ice on cooling and ice melting into water at room temperature. | Example: Burning a piece of paper into ash, cooking of food, formation of curd from milk, etc. |
Differentiate between Slow and fast changes.
Answer
| Slow changes | Fast changes |
|---|---|
| The changes that take longer time to complete are called slow changes. They take hours, days, months or even years to occur. | The changes that take place in a very short interval of time are called fast changes. |
| Examples: Rusting of iron, formation of curd from milk, change of seasons, a child growing into an adult, formation of fossil fuels from dead plants and animals, etc. | Examples: Bursting of a cracker, lighting of an electric bulb, switching on a fan, blinking of eyes, cutting of an apple, etc. |
Give reasons:
Freezing of water is a physical change.
Answer
Freezing of water is a physical change because the change is reversible and the chemical composition of water remains the same. No new substance is formed. Only change of state occurs i.e. water (liquid) is converted to ice (solid) in freezing.
Give reasons:
Evaporation of water is a physical change.
Answer
Evaporation of water is a physical change because the change is reversible and the chemical composition of water remains the same. No new substance is formed. Only change of state occurs i.e. water (liquid) is changed to vapour (gaseous) state.
Give reasons:
Burning of paper is a chemical change.
Answer
When a piece of paper is burnt a new substance ash is produced which cannot be changed back into paper. This change is permanent and irreversible, so it is a chemical change.
Give reasons:
Cutting of a cloth piece is a physical change, although it cannot be reversed.
Answer
Cutting a cloth piece does not produce any new substances. There is no change in the chemical composition of the cloth piece after cutting. Hence, even though it is irreversible, still it is a physical change.
Give reasons:
An inflated balloon bursts when brought near a lit bulb.
Answer
The inflated balloon bursts when brought near a lighted bulb. This is because, the air inside the balloon becomes hot, the kinetic energy of its particles increases the pressure on the inner walls of the balloon and it bursts. It is a physical change.
Give reasons:
Sublimation of camphor is a physical change.
Answer
Sublimation of camphor is a physical change that is reversible. When the evaporated vapours of camphor cool down, they change back to solid camphor as a sublimate. This shows that no new substance is formed, and the change is temporary and reversible.
State the characteristics of physical changes.
Answer
The characteristics of a physical change can be summarised as follows :
- No new substance is formed.
- There is no change in the chemical composition of the original substance.
- The change is temporary and it can be reversed by reversing the conditions.
- The change is only in the state, size, shape, colour, texture or the smell of some or all of the substances that undergo physical change.
- There is no change in the mass of the substances involved in a physical change.
- There is usually no loss or gain of energy as a result of physical change.
State the characteristics of chemical changes.
Answer
The characteristics of a chemical change can be summarised as follows :
- New substance(s) is/are formed.
- The composition of the original substance changes completely.
- The change is permanent and irreversible.
- There is a change in the mass of the original substance.
- There is an exchange of energy during a chemical change which means that energy in the form of heat and light may be released or absorbed.
Why is burning of a candle an example of both a physical and a chemical change ?
Answer
When a candle is lighted, wax near the wick burns to produce new substances such as carbon dioxide and water vapour, along with the evolution of heat and light. This shows a chemical change. At the same time, some wax melts and the molten wax solidifies again on cooling. This is a physical change. Hence, burning of a candle is an example of both a physical and a chemical change.
Give four differences between physical and chemical changes.
Answer
| Physical Change | Chemical Change |
|---|---|
| In a physical change, no new substance is formed and the chemical composition of the substance remains the same. These are changes only in physical properties and state. | In a chemical change a new substance with entirely different chemical composition and properties is formed. |
| The change can be reversible or irreversible. | The change is permanent. |
| The change can be reversed by simple physical methods. | The change cannot be reversed by simple physical methods. |
| Heat or light energy may or may not be released or absorbed. | Heat or light energy are given out or absorbed. |
Name the terms marked as A, B, C, D and E in the following and state the type of changes.
In which of the above changes does a gain in mass take place?
Answer
A → Sublimation
B → Deposition
C → Solid camphor / sublimate
D → Oxygen / air
E → Heat
The change from solid camphor to white vapours and back to solid camphor is a physical change because no new substance is formed.
The change in which magnesium combines with oxygen to form magnesium oxide is a chemical change because a new substance is formed.
Gain in mass takes place in the second case, where magnesium combines with oxygen from air to form magnesium oxide.
During a power cut, Vihaan's mother lit a candle. Vihaan observed that :
- The solid wax near the flame melted and later resolidified, when it dripped onto the table.
- The molten wax at the wick burnt with a flame, producing light, smoke and heat.
Answer the following questions :
(a) Classify the changes below as physical or chemical with justification :
(i) Solid wax melting and resolidifying.
(ii) Molten wax at the wick burning to produce light, heat and smoke.
(b) Name the gases produced when molten wax is burnt.
(c) Burning of wax is an exothermic reaction releasing energy in the form of heat and light. Can you think of an exothermic reaction occurring in living beings?
Answer
(a) The changes below are :
(i) Solid wax melting and resolidifying is a physical change because no new substance is formed. Only the state of wax changes, and the change is reversible.
(ii) Molten wax at the wick burning to produce light, heat and smoke is a chemical change because new substances such as carbon dioxide and water vapour are formed, and energy is released in the form of heat and light.
(b) The gases produced when molten wax is burnt are carbon dioxide and water vapour.
(c) Respiration is an exothermic reaction occurring in living beings. During respiration, glucose reacts with oxygen to form carbon dioxide and water, releasing energy in the form of heat.