Physical and Chemical Changes

By
Dr. Upendra Kant Chaubey
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icse class 7 chemistry

Solutions for ICSE Class 7 Chemistry Chapter 2

The “Physical and Chemical Changes” chapter in 7th-grade ICSE chemistry differentiates how matter can change. Physical changes alter a substance’s form or appearance (like melting ice or tearing paper) without creating new substances; the chemical makeup stays the same and is often reversible. Chemical changes, however, result in the formation of entirely new substances with different properties (like burning wood or rusting iron); the original substances are transformed, and these changes are usually hard to reverse and often involve energy changes. The chapter likely teaches how to identify these changes by looking for things like new substances forming, color changes, gas production, solid formation, or energy release/absorption. The main aim is to help students tell the difference between changes that just change how something looks versus those that create something entirely new.

Solutions for ICSE Class 7 Chemistry Chapter 2

Exercise

Question 1.
(a) Define:
(i) a physical change
(ii) a chemical change
(b) Give two examples for each of the above two changes.
Answer:

(a) Definitions:

(i) Physical Change: Alters appearance but not chemical makeup; often reversible (e.g., melting ice).

(ii) Chemical Change: Forms new substances with different properties; usually irreversible (e.g., burning wood).

(b) Examples:

Physical:

  1. Melting ice
  2. Tearing paper

Chemical:

  1. Burning wood
  2. Rusting iron

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 2.

What are reversible and irreversible changes ? Give one example for each.

Ans:

Reversible Changes: These are transformations that can be reversed, enabling the substance to return to its initial state following the change. Critically, the chemical composition of the material remains unaltered throughout the process. The change primarily affects the physical form or state of the substance, and the original substance can be recovered through an opposing physical process.

Example: Consider the dissolving of sugar in water. When sugar crystals are added to water and stirred, they disappear, forming a sugar solution. However, the sugar molecules (sucrose) still exist within the water; they have simply dispersed at a molecular level. This process is reversible: by evaporating the water, the sugar will recrystallize and return to its original solid state, with its chemical composition unchanged.

Irreversible Changes: In contrast, irreversible changes are transformations that are not easily reversed, and the original substance cannot be readily or completely recovered after the change has taken place. These changes often involve a chemical reaction where the initial substances are converted into one or more new substances with different chemical compositions and properties. Attempting to reverse these changes typically requires further chemical reactions or processes that are not simply the opposite of the initial change.

Example: Take the baking of a cake. When the various ingredients (flour, sugar, eggs, etc.) are mixed and then heated in an oven, a series of complex chemical reactions occur. Proteins are denatured, starches gelatinize, and sugars caramelize, resulting in a cake with a completely different texture, taste, and chemical composition than the original raw ingredients. You cannot simply unbake a cake to get back the separate flour, sugar, and eggs in their original forms. New chemical bonds have been formed, and the original substances have been transformed into a new, distinct entity.

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 3.

Mention a change which is always

Ans:

While some aspects of aging can be slowed or managed, the fundamental progression from birth through maturity to senescence and eventual death is a continuous and unidirectional process that cannot be reversed to a younger state. Biological systems undergo cumulative changes at cellular and molecular levels that lead to a decline in function over time.

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 4.

Is burning a physical change or a chemical change? Why?

Ans:

Burning is a chemical change because it involves a reaction with oxygen that creates entirely new substances like ash and gases, which are chemically different from the original material. This process is generally irreversible, meaning you can’t easily turn the products back into the original substance. Burning also releases energy as heat and light, a common sign of chemical reactions forming new bonds.

In essence, burning doesn’t just change how something looks; it transforms it into something new with different properties due to a chemical reaction, making it an irreversible chemical change accompanied by energy release.

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 5.

A burning candle shows both physical and chemical changes. Explain ?

Ans:

A burning candle shows a chemical change where wax reacts with oxygen to form new substances (CO₂, water, soot) and releases heat/light. It also shows physical changes as the solid wax melts to liquid, then vaporizes to gas, and the candle gets smaller – but the wax’s chemical makeup stays the same during these state/shape changes.

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 6.
State three differences between evaporation and boiling
Ans:

FeatureEvaporationBoiling
LocationIt is a surface phenomenon, occurring only at the top layer of the liquid.It is a bulk phenomenon, occurring throughout the entire volume of the liquid.
TemperatureCan occur at any temperature below the boiling point.Occurs only at a specific fixed temperature known as the boiling point.
Bubble FormationNo bubbles are formed during the process.Bubbles form throughout the liquid and rise to the surface.

Question 7.
State four differences between physical and chemical changes.
Ans:
Four differences are :

FeaturePhysical ChangeChemical Change
New SubstanceNo new substance is formed. The chemical identity remains the same.One or more new substances are formed with different properties.
ReversibilityUsually reversible (e.g., melting ice back into water).Usually irreversible (e.g., you cannot “un-burn” wood).
Energy ChangeVery little or no energy (heat/light) is absorbed or released.Significant energy change occurs, often releasing heat, light, or sound.
Properties AffectedOnly physical properties (shape, size, state) change.Both physical and chemical properties of the substance change.

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 8.
What do you observe when :

  1. water is boiled
    On boiling water changes into steam (gas) physical change.
  2. a piece of paper is burnt
    On burning piece of paper produces carbon dioxide and ash is left behind. Is a chemical change.
  3. some ice cubes are kept in a glass tumbler
    Ice cubes (solid) turn into water (liquid) only state changes (physical change).
  4. solid ammonium chloride is heated
    Solid ammonium chloride on heating changes into vapours (change of state) is physical change.
  5. an iron nail is kept in tap water for few days
    We observe reddish brown coating on the nail called rust (entirely new substance) is chemical change.
  6. a spoon of sugar is heated in a pan
    When a spoon of sugar is heated in a pan, black (charred sugar) (carbon) is seen. Is a chemical change.
  7. lighted match stick is brought near the mouth of the test tube containing hydrogen gas.
    We observe that hydrogen bums at the mouth of test tube with blue flame and pop sound is heard. It is chemical change.
  8. quick lime is dissolved in water.
    The following two observations will be observed
    (i) A hissing sound is observed.
    (ii) The mixture starts boiling and lime water is obtained.
  9.  liittle ammount of curd is added to a bowl containing warm milk and kept for five hours.
    When a little amount curd is added to a bowl containing warm milk and kept for five hours, a permanent change occured.
    The milk will change to curd. On boiling water changes into steam (gas) physical change.

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 9.
Name a chemical change which takes place in presence of:
Ans:

(a) Heat: Thermal breakdown of calcium carbonate (CaCO3​) into calcium oxide (CaO) and carbon dioxide (CO2​).

(b) Light: Photochemical decomposition of silver bromide (AgBr) into silver (Ag) and bromine (Br2​).

(c) Catalyst: Catalyzed oxidation of sulfur dioxide (SO2​) to sulfur trioxide (SO3​) using vanadium(V) oxide (V2​O5​).

(d) Electricity: Electrolytic decomposition of molten sodium chloride (NaCl) into sodium (Na) and chlorine (Cl2​).

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 8.
What do you observe when :

  1. water is boiled
    On boiling water changes into steam (gas) physical change.
  2. a piece of paper is burnt
    On burning piece of paper produces carbon dioxide and ash is left behind. Is a chemical change.
  3. some ice cubes are kept in a glass tumbler
    Ice cubes (solid) turn into water (liquid) only state changes (physical change).
  4. solid ammonium chloride is heated
    Solid ammonium chloride on heating changes into vapours (change of state) is physical change.
  5. an iron nail is kept in tap water for few days
    We observe reddish brown coating on the nail called rust (entirely new substance) is chemical change.
  6. a spoon of sugar is heated in a pan
    When a spoon of sugar is heated in a pan, black (charred sugar) (carbon) is seen. Is a chemical change.
  7. lighted match stick is brought near the mouth of the test tube containing hydrogen gas.
    We observe that hydrogen bums at the mouth of test tube with blue flame and pop sound is heard. It is chemical change.
  8. quick lime is dissolved in water.
    The following two observations will be observed
    (i) A hissing sound is observed.
    (ii) The mixture starts boiling and lime water is obtained.
  9.  liittle ammount of curd is added to a bowl containing warm milk and kept for five hours.
    When a little amount curd is added to a bowl containing warm milk and kept for five hours, a permanent change occured.
    The milk will change to curd. On boiling water changes into steam (gas) physical change.

Question 9.
Name a chemical change which takes place in presence of:
Ans:

(a) Heat: Thermal breakdown of calcium carbonate (CaCO3​) into calcium oxide (CaO) and carbon dioxide (CO2​).

(b) Light: Photochemical decomposition of silver bromide (AgBr) into silver (Ag) and bromine (Br2​).

(c) Catalyst: Catalyzed oxidation of sulfur dioxide (SO2​) to sulfur trioxide (SO3​) using vanadium(V) oxide (V2​O5​).

(d) Electricity: Electrolytic decomposition of molten sodium chloride (NaCl) into sodium (Na) and chlorine (Cl2​).

Solutions for ICSE Class 7 Chemistry Chapter 2

Question 1.
Fill in the blanks.

1. A new substance is always formed in a———– change.

Ans : chemical

2. Melting of ice is a——— change.

Ans : physical

3. When a candle burns, wax melts. Melting of wax is a ——– change.

    Ans : physical

    4. Chemical change occurs as a result of ——– between two substances.

      Ans : rearrangement of atoms

      5. Burning of a fuel is a ——— change.

      Ans : chemical

      6. Revolution of the earth around the sun is a ———– change.

      Ans : natural periodic

      7. Growing of a seedling into a plant is ——— change.

      Ans : chemical

      Question 2.
      State whether the following are physical or chemical changes.

      1. glowing of a bulb — physical change,
      2. burning of sugar — chemical change,
      3. heating of water — physical change,
      4. growing of a piglet into a pig — chemical change,
      5. burning of wood — chemical change,
      6. passing electric current through a heater rod — physical change.
      7. water cycle in nature — physical change,
      8. respiration in living beings — chemical change,
      9. shaping a piece of glass — physical change,
      10. lightning — chemical change,
      11. (energy produced) dissolving sugar in water — physical change,
      12. heating a mixture of iron filings and sulphur—chemical change.
      13. mixing oil with water — physical change.
      14. cutting wood into small pieces — physical change,
      15. photosynthesis — chemical change.
      16. Digestion of food — chemical change.
      17. Melting of wax — physical change.
      18. Boiling of an egg — chemical change.
      19. Slaking of lime — chemical change.

      Solutions for ICSE Class 7 Chemistry Chapter 2

      Question 4.
      Write true or false against the following statements :

      1. Cutting of paper into pieces is a chemical change.
        Ans :         False
      2. Rusting of iron is a chemical change.
        Ans :         True
      3. Earthquake is a desirable change.
        Ans :         False
      4. Melting of ice is a physical change.
        Ans :         True
      5. Burning of sugar is a temporary change.
        Ans :         False

      Solutions for ICSE Class 7 Chemistry Chapter 2

      Multiple Choice Questions

      Tick (√) the correct alternative from the choice given for the following statements :
      Question 1.
      A substance which can not sublime is
      Ans:

      1. iodine
      2. camphor
      3. sugar
      4. dry ice

      Question 2.
      When you put some ice cubes in a glass, droplets of water are formed on the outerwall of the glass. This explains the phenomenon of
      Ans:

      1. melting
      2. freezing
      3. condensation
      4. Evaporation

      Question 3.
      Burning is a
      Ans:

      1. slow process
      2. fast process
      3. natural process
      4. none of the above

      Question 4.
      Which one of the following is volatile in nature ?
      Ans:

      1. common salt
      2. petrol
      3. water
      4. Milk

      Question 5.
      An example of both physical and chemical change is
      Answer:

      1. burning of candle
      2. melting of ice
      3. cooking of food
      4. blowing of bulb

      Question 6.
      The compound rust is a hydrated oxide of
      Answer:

      1. copper
      2. aluminium
      3. iron
      4. Gold

      Question 7.
      When sugar is heated, its colour changes into
      Answer:

      1. red
      2. brown
      3. black
      4. Grey

      Question 8.
      A pop sound is heard when a lighted match stick is brought
      near the mouth of a jar. This indicates the release of gas.
      Answer:

      1. oxygen
      2. hydrogen
      3. nitrogen
      4. Water-vapour

      Question 9.
      When we add water to the following substances, which one will show a chemical change ?
      Answer:

      1. salt
      2. sugar
      3. oild
      4. quick lime

      Solutions for ICSE Class 7 Chemistry Chapter 2

      FAQ’s

      What is ICSE Class 7 Chemistry Chapter 2 about?

      ICSE Class 7 Chemistry Chapter 2 explains physical and chemical changes, helping students understand how substances change in daily life and how these changes differ from each other.

      What is a physical change in ICSE Class 7 Chemistry?

      A physical change is a change in which no new substance is formed. Only the shape, size, or state of matter changes, such as melting of ice or tearing of paper.

      What is a chemical change according to ICSE Class 7 Chemistry?

      A chemical change is a change in which a new substance is formed. These changes are usually irreversible, like rusting of iron or burning of paper.

      What is the main difference between physical and chemical changes?

      In a physical change, the substance remains the same, while in a chemical change, a new substance with different properties is formed.

      Why is ICSE Class 7 Chemistry Chapter 2 important for exams?

      This chapter forms the foundation of chemistry and helps students answer objective, short-answer, and reasoning-based questions effectively in ICSE exams.