Class 7 Science Curiosity Chapter 5 MCQ – Changes Around Us: Physical and Chemical

A physical change can alter a substance’s shape, size, or state without producing a new substance. For example, water may change between liquid and solid states while remaining water. A change in state is possible but is not required.

Condensation changes water vapour into liquid water. The state changes, but the substance is still water, so the change is physical. The chapter also explains that evaporation can be reversed by collecting the vapour through condensation.

The conversion of milk into curd is treated as a chemical change because a substance with different properties is formed. It is more than a change in shape or size, unlike folding paper or melting a piece of ice.

Water, ice, and steam are different states of the same substance. Changing between these states does not produce a new substance, so these are physical changes. Heating or cooling can bring about these changes in state.

A chemical change produces one or more new substances through a chemical reaction. The chapter uses examples such as rusting and the formation of a white substance in lime water to show how this differs from a physical change.

Bursting changes the shape and condition of the balloon, but its rubber does not become a new substance. This is a physical change even though the original balloon cannot easily be restored. Reversibility and formation of new substances are different ideas.

Carbon dioxide reacts with the calcium hydroxide in lime water to form calcium carbonate. This white substance is insoluble in water and makes the mixture look milky. Its formation shows that a chemical change has taken place.

The chapter uses the milky appearance of lime water to test for carbon dioxide. This gas reacts with lime water and forms an insoluble white substance. Observing that change helps identify the gas produced in the activity.

Vinegar and baking soda react to release carbon dioxide, producing the observed bubbles and fizzing. Passing the gas into lime water makes it milky. Since new substances form during the reaction, the process is a chemical change.

Rusting produces a new brown substance on iron, identified in the chapter as iron oxide. The formation of this substance is the reason for calling rusting a chemical change, rather than simply a change in appearance or position.

Burning magnesium reacts with oxygen to produce magnesium oxide, along with heat and light. The product is a new substance, so the process is a chemical change. It also illustrates the type of reaction called combustion.

Wood is listed as a combustible substance because it can undergo combustion. Paper, cotton, and kerosene are other examples in the chapter. Being combustible means a material can burn under suitable conditions, not that it burns all the time.

Ignition temperature is the lowest temperature at which a substance catches fire. The chapter explains that paper can be present in air without burning because it still needs enough heat to reach the temperature required for ignition.

The covered candle has only a limited supply of air available. As it burns, the conditions no longer provide enough oxygen to keep combustion going. The uncovered candle continues receiving air, which explains the difference between the two observations.

Combustion is a chemical reaction in which a substance reacts with oxygen and releases heat, light, or both. The chapter gives burning magnesium as an example and also discusses familiar combustible materials such as wood and paper.

Combustion requires a combustible substance, oxygen, and enough heat to reach the fuel’s ignition temperature. The fire triangle groups these three requirements. Having fuel and air alone does not mean burning will begin without the necessary heating.

Wax melts and evaporates as the candle burns, and these are physical changes. The wax vapour then burns and forms new substances, which is a chemical change. Both kinds of change therefore occur in the same overall process.

Michael Faraday used the candle to introduce scientific study in his lectures called “Chemical History of a Candle”. Through this familiar object, he discussed processes such as melting, vapourisation, and combustion, connecting physical and chemical changes with everyday observations.

Weathering includes physical changes that break rocks into smaller pieces and chemical changes that alter their composition. The chapter explains that temperature changes, growing roots, water, and other substances can contribute to these changes and eventually to soil formation.

Erosion involves rock material, soil, and sediments being worn down and moved by natural forces such as wind and flowing water. The chapter describes it as a physical change and explains how transported material may later settle elsewhere.

Rolling changes the shape of the dough but does not produce a new substance. The question concerns only the rolling step, not cooking. This distinction helps us classify a particular part of a process using the chapter’s definition of physical change.

Melting and solidifying wax change its state while leaving the substance as wax. Cooling can reverse the melting in this situation. Because no wax vapour is burned, this example involves the physical part of the candle processes discussed.

Cutting changes the paper’s size and shape, but the pieces remain paper. Difficulty in restoring the original sheet does not by itself prove a chemical change. The chapter distinguishes changes in physical form from changes that create new substances.

The chapter explains that decomposition can spoil stored food but can also turn food waste into useful compost. The process remains a chemical change, while our judgement of its usefulness depends on where and why it occurs.

Vinegar and baking soda react to produce new substances, including carbon dioxide, so the reaction is chemical. The collected gas stretches the balloon without changing its rubber into a new substance. That stretching is a physical change in shape.

Focused sunlight supplies heat to a small area of the paper. If that area reaches the paper’s ignition temperature while oxygen is available, burning can begin. The chapter uses this to show that an existing flame is not always needed.

The fire triangle includes fuel, oxygen, and sufficient heat. Removing any one of these requirements can stop combustion. In this case, the fuel supply has stopped, so the remaining air cannot by itself keep the flame burning.

Bubbles alone can result from air moving through a liquid and do not prove that a new substance has formed. Lime water becoming milky indicates formation of calcium carbonate, providing the clearer evidence of a chemical change in this comparison.

Growing roots can force a rock apart and change its size without necessarily forming a new substance. The chapter includes root growth among the causes of rocks breaking physically, which is one part of the broader process of weathering.

The chapter explains that materials carried by wind or flowing water settle when the movement slows. A river entering a lake provides such a situation. The settling changes where the sediments are located without requiring a new substance to form.