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Chapter: 02. Physical And Chemical Changes

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EXERCISE FOR REVISION (PAGES 7-8)

A. Tick (✓) the correct options.

Changes that occur due to human effort or activities are called ______ changes.
Answer: (a) human-made
Changes that occur at fixed intervals of time are called ______ changes.
Answer: (a) periodic
Which of the following is not a slow change?
Answer: (b) burning of a matchstick
Which of the following is not a non-periodic change?
Answer: (d) swinging of the pendulum of a clock
Which of the following changes cannot be reversed?
Answer: (​c​) cooking of food

B. Assertion-Reason based Question.

Assertion: When an ice cube is kept in an open place, it starts melting. ​Reason: The heat is the cause of melting.
Answer: (a) Both A and R are true and R is the correct explanation of the assertion.
⁠

EXERCISE FOR REVISION (PAGE 11)

A. Tick (✓) the correct options.

During a physical change, the chemical properties of a substance
Answer: (b) remain the same
The process during which a solid changes into its liquid state at a fixed temperature on heating is called
Answer: (b) melting
When we put some ice cubes in a glass, droplets of water are formed on the outer surface of glass. This happens because of
Answer: (d) condensation
Kanika does the following experiments with sugar cubes. Which one of the following causes a physical change in sugar cubes?
Answer: (b) Crushing the sugar cube and dissolving it in water.

B. Assertion-Reason based Question.

Assertion: Curdling of milk is a physical change. ​Reason: Action of bacteria turns milk into curd, which is a chemical change.
Answer: (d) A is false but R is true.
⁠

EXERCISE FOR REVISION (PAGES 14-15)

A. Tick (✓) the correct options.

The change in which heat energy is absorbed is called a/an
Answer: (b) endothermic change
Which of the following is/are endothermic change(s)?
Answer: (d) all of these
Change in colour of cut fruits and vegetables kept in the open air
Answer: (​c​) is a chemical change
Which of the following is not a characteristic of a chemical change?
Answer: (b) It can be reversed by physical methods.
Which of the following substance(s) is/are not formed during the addition of zinc granules to dilute hydrochloric acid?
Answer: (d) sulphur

B. Assertion-Reason based Questions.

Assertion: When a candle burns, both physical and chemical changes take place. ​Reason: The melting of wax is a physical change, while the burning of wax is a chemical change.
Answer: (a) Both A and R are true and R is the correct explanation of the assertion.
Assertion: Coal and petroleum are used as fuels. ​Reason: Burning of coal and petroleum are endothermic changes.
Answer: (​c​) A is true but R is false. (Burning of coal and petroleum are exothermic changes as they release heat energy).
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FLOWCHART FOR REVISION

Complete the flowchart for Revision:
Blank Field 1 (counterpart to Human-made changes): Natural changes
Blank Field 2 (counterpart to Irreversible changes): Reversible changes
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COMPETENCY BASED DISCUSSION: PHYSICAL AND CHEMICAL CHANGES

A. Short Answer Questions:

What is a non-periodic change? Give two examples.
Answer: Changes that do not occur at fixed intervals of time are called non-periodic changes. Examples are landslides and the eruption of volcanoes.
What is melting? Is it reversible?
Answer: The process during which a solid changes into its liquid state at a fixed temperature on heating is called melting. Yes, it is a reversible change because the liquid can be frozen back into a solid on cooling.
What is meant by slow and fast changes? Classify the following into slow and fast changes: blinking of eyes, formation of curd from milk, rusting of iron and explosion of crackers.
Answer: Changes that take a longer time to complete (hours, days, months, or years) are called slow changes. Changes that take a shorter time to complete (seconds or minutes) are called fast changes.
Slow changes: Formation of curd from milk, rusting of iron.
Fast changes: Blinking of eyes, explosion of crackers.
Some changes are both desirable and undesirable. Explain giving example.
Answer: A change can be desirable or undesirable depending on the context in which it is viewed. For example, the cutting down of trees is desirable for making essential products like paper and furniture, but it is undesirable because it leads to environmental issues like soil erosion, habitat loss, and flooding. Similarly, rainfall is desirable for farmers during sowing but undesirable during the harvesting period.
Differentiate between reversible and irreversible changes.
Answer:
Reversible Changes: Changes that can be easily reversed by altering the conditions to recover the original substance (e.g., melting of ice, stretching of a rubber band).
Irreversible Changes: Changes that cannot be reversed, even by altering the conditions, meaning the original substance cannot be obtained back (e.g., burning of paper, curdling of milk).
How is evaporation different from boiling?
Answer:
Evaporation is a slow process that takes place only from the surface of the liquid, occurs at all temperatures, and causes a cooling effect.
Boiling is a fast process that takes place throughout the entire bulk of the liquid, occurs only at a specific temperature (the boiling point), and does not cause a cooling effect.
How is digestion of food a chemical change?
Answer: Digestion is a chemical change because it involves the permanent breakdown of complex food substances into simpler substances by the action of enzymes. New products are formed, and the raw food cannot be recovered, making the process irreversible.
How do we know that a chemical change has taken place?
Answer: A chemical change is typically characterized by distinct signs such as a permanent change in colour, the evolution of a gas (bubbling), the formation of new products (such as a precipitate), and the absorption or release of energy in the form of heat, light, or sound.
What is the difference between endothermic and exothermic changes?
Answer:
Endothermic changes absorb heat energy from the surroundings, causing the surrounding temperature to drop (e.g., vaporization of perfume, melting of ice on the palm).
Exothermic changes release/evolve heat energy to the surroundings, causing the surrounding temperature to rise (e.g., burning of fuels, dissolving quicklime in water).
How is the burning of a candle simultaneously a physical and chemical change?
Answer: When a candle burns, the heat of the flame melts the solid wax into liquid wax, which can freeze back into solid wax upon cooling. This melting is a physical change. Simultaneously, some of the molten wax rises up the wick, vaporizes, and burns in the presence of oxygen to produce new substances (carbon dioxide and water vapour) along with heat and light. This burning is an irreversible chemical change.
Give reasons for the following:
(a) Burning of wood is a chemical change.
Answer: When wood burns, it reacts with oxygen to form entirely new substances like ash, carbon dioxide, and water vapour. Since we cannot get the wood back from these products, and energy is released in the form of heat and light, it is a permanent and chemical change.
(b) An earthquake is a natural and undesirable change.
Answer: It is a natural change because it occurs through geological processes that are beyond human control. It is an undesirable change because it causes immense destruction to life and property.
(​c​) Sublimation of ammonium chloride is a temporary and reversible change.
Answer: Upon heating, solid ammonium chloride changes directly into gas without forming any new chemical substance, and on cooling, the gas deposits directly back into the solid form. Because the original substance is easily recovered by changing the temperature, it is a temporary, physical, and reversible change.
(d) Growing of seedling into a plant is a slow and irreversible change.
Answer: It is a slow change because it takes months or years to complete. It is an irreversible change because a fully grown plant can never be turned back into a tiny seedling.
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B. Long Answer Questions:

List the importance of chemical changes.
Answer: Chemical changes are essential for human life and industrial development:
Cooking: Hard raw food items like rice and pulses undergo chemical changes during cooking to become soft, edible, and easily digestible.
Industrial Products: Useful products such as soaps, detergents, plastics, paints, and pesticides are prepared via chemical processes.
Extraction of Metals: Metals like iron, copper, and aluminium are extracted from their natural ores using various chemical methods.
Life Processes: Vital life functions such as digestion (breaking down complex food) and respiration (reacting glucose with oxygen to produce cellular energy) are chemical changes.
Energy Generation: Burning fuels (like wood, coal, CNG, and petrol) produces heat energy used for transport, manufacturing, and cooking.
Differentiate between physical and chemical changes.
Answer:
Parameter
Physical Change
Chemical Change
New Substances
No new substance is formed.
One or more new substances are formed.
Nature of Change
Temporary and generally reversible.
Permanent and generally irreversible.
Properties
Only physical properties (size, shape, state, colour) change.
Both physical and chemical properties change.
Chemical Composition
Chemical composition remains identical.
Chemical composition changes completely.
Energy Involvement
Generally, no overall change in energy occurs.
Overall energy is changed (absorbed or released).
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How is the mixing of iron and sulphur a physical change? Why does it become a chemical change on heating? Explain giving suitable reasons.
Answer:
Mixing of iron and sulphur (Physical change): Simply mixing iron filings and sulphur powder is a physical change because no chemical reaction occurs. The constituent elements retain their individual properties; the iron remains magnetic and can be easily separated from the yellow sulphur powder using a hand magnet.
Heating the mixture (Chemical change): When the mixture is heated strongly, a chemical reaction occurs to form a brand new compound called iron sulphide (
math
⁠
). This new compound is dark grey and does not get attracted to a magnet because the iron has lost its magnetic properties. This change is permanent, irreversible, and results in a substance with entirely unique chemical properties, making it a chemical change.
Identify the following changes and categorise them in different types. A change may be of more than one type.
(a) Stainless steel pot filled with boiling liquid on a gas stove:
Answer: Identification: Boiling of water. Categorization: Fast, desirable, reversible, human-made, physical change.
(b) Two orange pencils with smoke rising from their tips:
Answer: Identification: Burning of wood / pencils. Categorization: Fast, undesirable, irreversible, human-made, chemical change.
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C. Application/Skill-based Questions:

Categorise the following in different types of changes you have read about in this chapter. Formation of dew, baking of bread, melting of gold, burning of hydrogen gas, folding of paper to make an airplane, drawing on paper, breaking of a sheet of glass.
Answer:
Formation of dew: Slow, natural, physical, reversible change.
Baking of bread: Slow, human-made, chemical, irreversible change.
Melting of gold: Fast, human-made, physical, reversible change.
Burning of hydrogen gas: Fast, human-made, chemical, irreversible change.
Folding of paper to make an airplane: Fast, human-made, physical, reversible change.
Drawing on paper: Fast, human-made, physical change (can be reversible if drawn with pencil, or irreversible if permanent ink is used).
Breaking of a sheet of glass: Fast, human-made, physical, irreversible change.
Jai was very fond of collecting coins of different metals. One day, he observed that the colour of silver and copper coins had changed. Explain reasons for his observation.
Answer: The change in colour is due to a chemical process called corrosion, where metals react with gases and moisture in the air:
Silver coins turn black because silver reacts with hydrogen sulphide gas in the air to form a layer of black silver sulphide (
math
⁠
).
Copper coins develop a green coating because copper reacts with moist carbon dioxide and oxygen in the air to form green basic copper carbonate (
math
⁠
).
Priya placed an iron nail in a beaker of tap water. She observed that after a few days, the nail was covered with a brown substance.
(a) What kind of change (physical or chemical) has the nail undergone?
Answer: Chemical change (rusting).
(b) Is it a reversible change? Why?
Answer: No, it is an irreversible change because rust is a completely new substance and the original iron cannot be recovered from it.
(​c​) What is the brown substance formed?
Answer: Rust (hydrated iron oxide).
(d) How can Priya prevent the formation of the above substance on the nail?
Answer: Priya can prevent rusting by:
Applying a coat of paint, grease, or oil to block air and moisture.
Galvanizing the nail (coating it with a protective layer of zinc).
Kartik just finished a 100m race. He wanted to have a glass of glucose water. He mixed a few spoons of glucose in a glass of water. The glass felt cooler.
(a) Is the mixing of glucose in water, a physical change or chemical change?
Answer: It is a physical change (dissolving of a substance).
(b) Explain why the glass felt cooler after dissolving the glucose?
Answer: Dissolving glucose in water is an endothermic change. The process absorbs heat energy from the water and the glass container, leading to a drop in temperature, which makes the glass feel cooler to the touch.
(​c​) How can the glucose be recovered from the water? Explain.
Answer: Glucose can be recovered by the process of evaporation. By heating the solution, the water converts into vapour and escapes into the air, leaving behind the solid glucose residue in the beaker.
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