ICSE Class 7 Chemistry · Selina Solutions Concise Chemistry Class 7 ICSE · 34 solved questions
A substance which cannot sublime is :
sugar
Reason — The substance that cannot sublime is sugar.
Iodine can sublime. It gives purple vapours on heating. Camphor also undergoes sublimation. It changes directly into vapour when heated. Dry ice sublimes directly into gas.
When you put some ice cubes in a glass, droplets of water are formed on the outer wall of the glass. This explains the phenomenon of:
condensation
Reason — When you put ice cubes in a glass, the temperature of the glass drops. As warm air comes into contact with the cold surface of the glass, the water vapour in the air cools down and condenses into liquid water droplets. This process is called condensation, where gas (water vapour) turns into liquid.
Burning is a :
fast process
Reason — Burning (combustion) is a chemical reaction that typically happens quickly, releasing heat and light. It is an exothermic process, meaning it releases energy, and is usually a rapid event unless controlled in some way.
Which one of the following is volatile in nature ?
petrol
Reason — Volatility refers to how easily a substance can evaporate at room temperature, meaning it turns into a gas quickly. Petrol evaporates readily at room temperature, so it is volatile.
Common salt is a solid and non-volatile substance. It does not evaporate easily. Water can evaporate, but not very quickly, so it is less volatile. Milk is mostly water with other substances dissolved in it; hence, it is not volatile.
An example of both physical and chemical change is :
burning of candle
Reason — When a candle burns, the wax melts and solidifies again, which is a physical change. Simultaneously, the molten wax vaporizes, burns, and forms water vapour and carbon dioxide, making it a chemical change.
Melting of ice is only a physical change as no new substance is formed. Cooking of food is a chemical change as new substances are formed, and the process is irreversible. Glowing of a bulb is a physical and reversible change, but it is not a chemical change. Hence, it is not an example of both physical and chemical change.
The compound rust is a hydrated oxide of :
iron
Reason — Rust forms when iron reacts with oxygen and water, resulting in a compound known as iron(III) oxide-hydrate (Fe₂O₃·xH₂O).
Copper forms green coating (copper carbonate), not rust. Aluminium forms a protective oxide layer, not rust. Gold does not react easily; does not form rust.
When sugar is heated, its colour changes into:
black
Reason — On heating the sugar, we get a black residue, called charcoal and some gaseous products like carbon-dioxide and water vapour. The gases immediately mix with air. This shows that its properties are completely different from those of sugar, as new compounds are formed.
Sugar does not turn red. Final residue is black, not grey.
A pop sound is heard when a lighted match stick is brought near the mouth of a jar. This indicates the release of:
hydrogen gas
Reason — The pop sound is heard when a lighted matchstick is brought near the mouth of a jar containing hydrogen gas. This sound is due to the combustion of hydrogen. Hydrogen reacts with oxygen in the air, producing a small explosion or pop sound.
Oxygen gas supports combustion but does not produce a pop sound. Nitrogen gas does not support burning; no reaction. Water vapour does not burn or produce any sound.
When we add water to the following substances, which one will show a chemical change?
quick lime
Reason — When water is added to quick lime (calcium oxide, CaO), it undergoes a chemical change. Quick lime reacts vigorously with water to form slaked lime (calcium hydroxide, Ca(OH)₂) and releases a significant amount of heat in the process.
Salt dissolves in water. This is a physical change. Oil does not mix with water, physical change only. Sugar dissolves in water. This is a physical change.
Assertion - The dissolution of salt in water is a physical change.
Reason - Salt retains its composition and properties in solution.
Both A and R are true and R is the correct explanation of A.
Reason — A is true as when salt dissolves in water, no new substance is formed. The process is reversible (salt can be recovered by evaporation).
R is true as salt does not change chemically. It still remains the same substance in solution.
Therefore, Both A and R are true and R is the correct explanation of A is the correct option.
Assertion - Naphthalene balls disappear from the cupboard after sometime.
Reason - Naphthalene balls undergo evaporation.
A is true but R is false.
Reason — A is true as naphthalene balls gradually vanish because they change directly from solid to gas.
R is false as naphthalene changes solid into gas directly, which is called sublimation, not evaporation.
Therefore, A is true but R is false is the correct option.
Assertion - During a chemical change, new substances with entirely different properties are formed.
Reason - Bursting of a cracker is a chemical change.
Both A and R are true but R is not the correct explanation of A.
Reason — A is true as in a chemical change, the original substance changes into new substances with different properties.
R is true as when a cracker bursts, new substances (gases, smoke) are formed with heat, light, and sound.
Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.
Assertion - Heating of iron with sulphur is a physical change.
Reason - A new substance, iron sulphide is formed, when iron and sulphur are heated together.
A is false but R is true.
Reason — A is false as when iron is heated with sulphur, they chemically react. A new substance is formed, so it is a chemical change, not a physical one.
R is true as when iron and sulphur are heated together, iron sulphide is formed. This new substance has different properties from iron and sulphur.
Therefore, A is false but R is true is the correct option.
Fill in the blanks.
(a) A new substance is always formed in a ............... change.
(b) Melting of ice is a ............... change.
(c) When a candle burns, wax melts. Melting of wax is a ............... change.
(d) Chemical change occurs as a result of ............... between two substances.
(e) Burning of a fuel is a ............... change.
(f) Revolution of the earth around the sun is a ............... change.
(g) Growing of a seedling into a plant is a ............... change.
(a) chemical
(b) physical
(c) physical
(d) reaction
(e) chemical
(f) periodic
(g) slow
Write true or false against the following statements :
(a) Cutting of paper into pieces is a chemical change.
(b) Rusting of iron is a chemical change.
(c) Earthquake is a desirable change.
(d) Melting of ice is a physical change.
(e) Burning of sugar is a temporary change.
(a) False
Correct Statement — Cutting of paper into pieces is a physical change.
(b) True
(c) False
Correct Statement — Earthquake is an undesirable change.
(d) True
(e) False
Correct Statement — Burning of sugar is a permanent change.
Match the following.
| Column A | Column B |
|---|---|
| (a) Evaporation of water | (i) Non-periodic change |
| (b) Milk turning sour | (ii) Periodic change |
| (c) Earthquake | (iii) Chemical change |
| (d) Change of seasons | (iv) Physical change |
| (e) Glowing of bulb | (v) Reversible change |
| Column A | Column B |
|---|---|
| (a) Evaporation of water | (iv) Physical change |
| (b) Milk turning sour | (iii) Chemical change |
| (c) Earthquake | (i) Non-periodic change |
| (d) Change of seasons | (ii) Periodic change |
| (e) Glowing of bulb | (v) Reversible change |
State whether the following are physical or chemical changes.
(a) glowing of a bulb
(b) burning of sugar
(c) heating of water
(d) growing of a piglet into a pig
(e) burning of wood
(f) passing electric current through a heater's rod
(g) water cycle in nature
(h) respiration in living beings
(i) shaping a piece of glass
(j) lightning
(k) dissolving sugar in water
(l) heating a mixture of iron filings and sulphur
(m) mixing oil with water
(n) cutting wood into small pieces
(o) photosynthesis
(p) Digestion of food
(q) Melting of wax
(r) Boiling of an egg
(s) Slaking of lime
(a) glowing of a bulb — Physical change
(b) burning of sugar — Chemical change
(c) heating of water — Physical change
(d) growing of a piglet into a pig — Chemical change
(e) burning of wood — Chemical change
(f) passing electric current through a heater's rod — Physical change
(g) water cycle in nature — Physical change
(h) respiration in living beings — Chemical change
(i) shaping a piece of glass — Physical change
(j) lightning — Chemical change
(k) dissolving sugar in water — Physical change
(l) heating a mixture of iron filings and sulphur — Chemical change
(m) mixing oil with water — Physical change
(n) cutting wood into small pieces — Physical change
(o) photosynthesis — Chemical change
(p) Digestion of food — Chemical change
(q) Melting of wax — Physical change
(r) Boiling of an egg — Chemical change
(s) Slaking of lime — Chemical change
Find the odd one out from the following.
(a) Burning of wood, cooking of food, glowing of bulb, formation of curd.
(b) Water turning into ice, cutting a glass into pieces, dissolving sugar into water, boiling an egg.
(c) Drying of clothes, ripening of fruits, photosynthesis, rusting of iron.
(a) glowing of bulb
Reason — Burning of wood, cooking of food, and formation of curd are all chemical changes as new compounds are formed. The glowing of a bulb, however, is a physical process as it involves electricity turning into light and heat without changing the substance itself.
(b) boiling an egg
Reason — Water turning into ice, cutting a glass into pieces, and dissolving sugar into water are physical changes (no new substances are formed). Boiling an egg is a chemical change because it involves the permanent transformation of the egg’s proteins.
(c) Drying of clothes
Reason — Drying of clothes is a physical change as water evaporates, but the clothes do not undergo a chemical change. Ripening of fruits, photosynthesis, and rusting of iron are all chemical changes as they involve the formation of new substances.
Mention a change which is always
(a) desirable
(b) undesirable
(c) periodic
(a) Photosynthesis
(b) Rusting of iron
(c) Change of day and night
Name a chemical change which takes place in the presence of:
(a) Heat
(b) Light
(c) Electricity
(a) Cooking of food
(b) Photosynthesis
(c) Decomposition of water to hydrogen and oxygen
What do you observe when:
(a) Water is boiled
(b) a piece of paper is burnt
(c) some ice cubes are kept in a glass tumbler
(d) solid ammonium chloride is heated
(e) an iron nail is kept in tap water for few days
(f) a spoon of sugar is heated in a pan
(g) a lighted match stick is brought near the mouth of the test tube containing hydrogen gas
(h) quick lime is dissolved in water
(i) little amount of curd is added to a bowl containing warm milk and kept for five hours?
(a) When water is boiled it converts into vapours.
(b) When a piece of paper is burnt, it changes into ash.
(c) When some ice cubes are kept in a glass tumbler, water droplets are seen on the outer wall of the tumbler and after some time the ice cubes change into water.
(d) When solid ammonium chloride is heated, it directly changes into vapours.
(e) When an iron nail is kept in tap water for a few days, a reddish-brown coating is seen on the nail.
(f) When a spoon of sugar is heated in a pan, it first melts and changes its colour to reddish-brown. Finally, it turns black and gets charred.
(g) When a lighted matchstick is brought near the mouth of the test tube containing hydrogen gas, a pop sound is heard and the flame of the matchstick gets extinguished.
(h) When quicklime is dissolved in water, a large amount of heat energy is evolved and a hissing sound is produced.
(i) The milk changes into curd.
Is burning a physical change or a chemical change? Why?
Burning is a chemical change because a substance reacts with air or oxygen to form a new substance i.e. oxide, with evolution of energy in form of heat and light.
(a) Define:
(i) a physical change
(ii) a chemical change
(b) Give two examples for each of the above two changes.
(a)(i) A physical change is a temporary 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.
(ii) 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.
(b) Examples of physical changes:
Dissolution of salt in water.
Water freezing into ice.
Examples of chemical changes:
Rusting of iron.
Ripening of fruits.
What are reversible and irreversible changes? Give one example for each.
When a change in a substance can be reversed by changing the conditions, it is said to be a reversible change.
Example — Vaporisation of water.
If a substance cannot be brought back to its original state after a change, it is said to be an irreversible change.
Example — Grinding of wheat into flour.
A burning candle shows both physical and chemical changes. Explain.
When a candle is lighted, the wax vapour burns in oxygen and forms new substances, mainly carbon dioxide and water vapour. Heat and light are also produced. This shows a chemical change, and the candle becomes smaller and smaller. At the same time, some wax melts and changes into the liquid state. When the molten wax falls on the floor or table, it solidifies again. This shows a physical change. Thus, burning of a candle involves both physical and chemical changes.
State three differences between evaporation and boiling.
| Evaporation | Boiling |
|---|---|
| Evaporation is a slow process. | Boiling is a fast process. |
| Evaporation takes place from the surface of the liquid. | Boiling takes place from all parts of the liquid. |
| Evaporation takes place at all temperatures below its boiling point. | Boiling takes place at a fixed temperature i.e. at its boiling point. |
State four differences between physical and chemical changes.
| Physical changes | Chemical changes |
|---|---|
| Change is temporary. | Change is permanent. |
| No new substance is formed. There is change only in physical properties. | New substances are formed with entirely different properties. |
| Change can be reversed by simple methods. | Change cannot be reversed by simple methods. |
| Heat or light may or may not be given out or consumed. | Heat or light or both are given out or consumed. |
Give reasons :
Burning is a chemical change.
Burning is a chemical change in which a substance reacts with oxygen or air to produce a new substance called oxide with the evolution of energy in the form of heat and light.
Example: Burning of fuel, candle, etc.
Give reasons :
Cutting of a mango into pieces is a physical change.
It is a physical change because, although the original mango cannot be obtained back once it is cut, no new substance is formed.
Give reasons :
Rusting of iron is an undesirable change.
Rusting of iron is an undesirable change because it weakens the material, making it brittle and prone to breaking. Rusting shortens the lifespan of iron products, and it can interfere with the functionality of machinery, causing jams or malfunctions. Overall, rusting decreases the durability, safety, and appearance of iron objects.
The diagram given ahead shows an experiment in which a few pieces of quicklime are added to water.
(a) What do you observe when quicklime is added to water ?
(b) Why does the beaker become hot ?
(c) What type of change is depicted here ?
(d) What conclusion can be drawn on the basis of your observation ?
(a) The quick lime pieces first crumble and then dissolve in water. A new white solid substance, calcium hydroxide, is formed which partially dissolves in water to produce a solution called lime water. The remaining white solid, called slaked lime, settles at the bottom of the beaker.
(b) The beaker becomes hot as quick lime's reaction with water is an exothermic reaction, and a large amount of heat is evolved.
(c) The given experiment depicts a chemical change.
(d) The given experiment proves that dissolution of quick lime in water is a chemical change in which energy change also takes place.
(a) Draw a well-labelled diagram to show the process of sublimation.
(b) Is sublimation a physical change or a chemical change ? Explain.
(c) Name two substances which can sublime.
(a) A well-labelled diagram to show the process of sublimation of ammonium chloride is given below:
(b) Sublimation is a physical change. When the solid sublimes, it changes directly into vapours, but when the vapours are cooled down, it again changes into the original solid substance. Since no new substance is formed, sublimation is a physical change.
(c) Camphor and Iodine
Solve the crossword puzzle given below with the help of the following clues:
Down :
Across :
2. It is a physical change in which a liquid changes into vapour.
3. An object which attracts iron filings.
Down :
Across :
2. EVAPORATION
3. MAGNET
Ravi had brought some iron nails to fix pictures on the walls of his room. However, he got busy and he left those nails in the courtyard of his house to work with later. Upon returning to the courtyard after a few days, he saw that a reddish-brown substance was deposited on the nails. His father told him that this is a common phenomenon on iron articles, especially during rainy season.
(a) Name the reddish-brown substance formed on the iron nails.
(b) Is it a physical change ? Justify your answer.
(c) What will you observe when a magnet is brought near the nails ?
(d) State two conditions under which the above phenomenon takes place. Why is this faster during rainy season ?
(a) The reddish-brown substance formed on the iron nails is rust (hydrated iron oxide).
(b) No, it is a chemical change because a new substance (rust) is formed and the process is irreversible. The properties of iron change completely.
(c) The rusted nail will not be attracted towards the magnet because rust is a new substance and is not magnetic in nature. This shows that rusting is a chemical change.
(d) Two conditions under which the above phenomenon takes place are :
This is faster during rainy season because there is more moisture (humidity) in the air, which provides water and speeds up rusting.
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