CBSE Class 7 Science Chapter 05 Changes Around Us Physical And Chemical MCQs Set 01

Answer: (B) The paper's shape changes but it remains paper with no new substance formed

Explanation:
Folding paper only alters its shape or size, but the material remains paper throughout the process. No new substance is created, which is the defining feature of a physical change.

Answer: (B) The lime water changes colour to milky and a solid settles at the bottom

Explanation:
Blowing exhaled air into lime water produces a cloudy or milky appearance, and over time a white solid (calcium carbonate) settles down. This appearance of a new substance is what marks the change as chemical, unlike plain water where only bubbles form with no new substance.

Answer: (B) The gas produced is carbon dioxide

Explanation:
Lime water turning milky is a specific test for carbon dioxide gas, since carbon dioxide reacts with the calcium hydroxide in lime water to form insoluble calcium carbonate. The fizzing seen when vinegar meets baking soda produces exactly this gas.

Answer: (B) Rusting produces a new substance called iron oxide

Explanation:
Rust is not simply iron in a different shape; it is a distinct brown-coloured substance, iron oxide, formed through a chemical reaction with air and moisture. Since a new substance forms, rusting qualifies as a chemical change.

Answer: (B) Heat and light

Explanation:
When magnesium burns in air, it combines with oxygen to form magnesium oxide, and this reaction releases both heat and light as visible outputs. This is a hallmark of combustion reactions in general.

Answer: (B) The covered candle has no continuous supply of oxygen from outside air

Explanation:
Once the glass tumbler is placed over the candle, fresh air can no longer reach the flame. Since oxygen supports combustion, using up the trapped oxygen without replacement causes the flame to die out, unlike the open candle that has unlimited air access.

Answer: (B) A substance can start burning once it reaches its ignition temperature through heat alone

Explanation:
Focusing sunlight raises the paper's temperature until it reaches the ignition temperature, at which point it begins to burn on its own, showing that an existing flame is not strictly necessary — sufficient heat is enough to trigger combustion.

Answer: (B) Fuel, oxygen, and heat to reach ignition temperature

Explanation:
• Combustion needs a combustible substance, referred to as fuel
• It also requires oxygen from the surrounding air
• Finally, enough heat must be supplied so the fuel reaches its ignition temperature
Without any one of these three, burning cannot begin or continue.

Answer: (B) That a simple everyday object like a candle can be used to explain both physical and chemical processes

Explanation:
Faraday believed the candle was an ideal subject for introducing scientific study because burning a candle displays multiple processes — melting, vaporisation, and combustion — that together demonstrate the difference between physical and chemical changes.

Answer: (A) Melting of ice (reversible) and chopping of vegetables (irreversible)

Explanation:
Ice that melts into water can be refrozen back into ice, making it a reversible change. Chopped vegetables, however, cannot be reassembled into their original whole shape, so that change cannot be undone.

Answer: (B) Rotting food in storage is wasteful, but decomposing food waste into compost is beneficial for plants

Explanation:
Food decaying while being stored is generally unwanted since it spoils something meant to be eaten. Yet, that very same decomposition process becomes useful when applied deliberately to convert vegetable and fruit waste into compost for growing plants.

Answer: (B) Iron in the rock reacts with water or moist air to form iron oxide, giving a red colour

Explanation:
Basalt rock contains iron, and when its surface stays exposed to water or water vapour in the air for long periods, the iron undergoes a chemical change to form iron oxide. This new reddish substance is what gives the sediment its distinct colour.

Answer: (B) Weathering breaks down rocks in place through physical and chemical changes, while erosion moves the broken material elsewhere

Explanation:
Weathering refers to the breaking down of large rocks into smaller pieces through both physical processes like temperature changes and chemical processes like reactions with water. Erosion, on the other hand, is specifically about wind or water carrying that broken material away to a new location.

Answer: (B) This is a physical change caused by erosion wearing down the rock surface

Explanation:
Constant movement of water against rocks and pebbles gradually smooths their surfaces without creating any new substance — the rock composition itself does not change, only its shape and texture. This makes it a physical change linked to the erosion process.

Answer: (C) Bioluminescence

Explanation:
The chapter mentions that fireflies produce light through a chemical change without generating heat, and this phenomenon in living organisms is termed bioluminescence.

Answer: (B) It cuts off the air supply needed for combustion to continue

Explanation:
Wrapping a blanket around a person stops fresh air, and therefore oxygen, from reaching the flames, so the fire dies out for lack of one of the three essentials of combustion.

Answer: (B) Ignition temperature

Explanation:
Ignition temperature is the minimum temperature a substance must reach before it starts burning; the focused sunlight kept heating the paper until it crossed this threshold.

Answer: (C) No, but it is still a physical change since the chalk material itself has not changed

Explanation:
Crushing changes only the size and appearance of the chalk; the substance remains chalk throughout, which is why the activity is used to illustrate a physical change even though the original shape cannot be restored.

Answer: (D) That even though the balloon bursts suddenly, the rubber material itself remains unchanged

Explanation:
Whether the air is let out slowly or the balloon is burst with a pin, the rubber sheet is still the same substance afterward — only its shape has changed, confirming this is a physical change, not a chemical one.

Answer: (A) Calcium hydroxide + Carbon dioxide gives Calcium carbonate + Water

Explanation:
Lime water is calcium hydroxide, and when carbon dioxide passes through it, the reaction produces insoluble calcium carbonate along with water — this is the exact equation given in the text for that change.

Answer: (B) Wood, paper, cotton, and kerosene

Explanation:
The text lists wood, paper, cotton and kerosene as examples of combustible substances because they react with oxygen and release heat and/or light during burning.

Answer: (A) Sediments

Explanation:
These accumulated fragments of rock, soil, and sand are described as sediments, formed as large rocks are gradually broken down by physical and chemical processes.

Answer: (B) Because the corn undergoes an irreversible transformation into a new form

Explanation:
Popcorn formation is listed among changes where the original substance cannot be recovered, unlike melting ice or boiling water which can be reversed by cooling.

Answer: (B) A desirable chemical change

Explanation:
Curdling of milk is grouped along with ripening of fruits and cooking as useful, desirable changes that occur in daily life.

Answer: (B) The matchstick flame is already hotter than the paper's ignition temperature, so it ignites it right away

Explanation:
Since the flame of a matchstick is already well above the ignition temperature of paper, contact causes near-instant burning; sunlight, however, must first gradually raise the paper's temperature up to that same ignition point, which takes longer.