Test: Changes Around Us - Class 6 MCQ

Chemical changes involve the formation of new substances. In the options given, the burning of a matchstick is a chemical change where the matchstick combines with oxygen to produce new substances like ash and smoke.

When cement gets wet due to rain, it undergoes a chemical change known as hydration. This change is not easily reversible as the properties of cement are altered by the absorption of water. Sunlight may dry the surface, but the chemical changes are irreversible.

Physical and chemical changes often occur due to the absorption of energy, which can be in the form of heat, light, or electrical energy.

• Physical changes like melting, boiling, or dissolving usually require energy to break or form physical bonds (e.g., heat or light).
• Chemical changes (like reactions) often require energy (such as heat or light) to break bonds between atoms or molecules and form new ones.

Thus, the energy involved in both types of changes can be of different forms, not just heat or light alone.

A chemical change involves both a change in composition (where new substances with different chemical properties are formed) and sometimes a change of state (like gas formation, precipitation, or color change).

For example, when iron rusts, it undergoes a chemical change, forming iron oxide, and this process often involves a change in state (from solid iron to solid iron oxide, and also the release of gas).

Hence, chemical changes typically result in the formation of new substances, not just a physical change like a change of state or color.

The germination of a seed is a slow, irreversible, desirable, chemical change. During germination, the seed undergoes a complex process of chemical changes where stored food is broken down to provide energy for the growing plant. This is an irreversible process, as once a seed has germinated, it cannot return to its original dormant state. This change is also desirable because it leads to the growth of a new plant.

Sugar dissolving in water is a physical change. The sugar can be obtained back by evaporation of water and the water by condensation of water vapour.

• Option A (Heating of water to steam) is a reversible change because water can condense back into liquid water by cooling.
• Option B (Cooking of egg to scrambled eggs) is not a reversible change. Once an egg is cooked, its chemical structure is permanently altered, making it impossible to return to its raw form.
• Option C (Heating of wax to molten wax) is a reversible change because molten wax can solidify back into its original solid form when cooled.
• Option D (Cooling a cake) is a reversible change in the sense that it can return to its solid state after cooling.

Thus, Option B is the correct answer because cooking an egg is an irreversible chemical change.

Iron adopts changes slowly and gradually through the process of rusting. Rusting is a chemical reaction where iron reacts with oxygen and moisture from the air, forming iron oxide (rust) over time. This process occurs slowly and leads to the gradual degradation of iron.

• Galvanization (Option B) is the process of coating iron with a layer of zinc to prevent rusting.
• Crystallization (Option C) is the process of forming solid crystals from a solution and is not related to the gradual change of iron.

Therefore, the correct answer is Option A: rusting.

Changes in the physical and chemical properties of matter can occur due to the application of various forms of energy, including heat, light, and pressure. These energies can alter the arrangement, motion, or composition of the particles in a substance, leading to a physical or chemical change.
For example:

• Heat energy can cause phase changes (solid to liquid or liquid to gas) and chemical reactions.
• Light energy can initiate chemical changes like photosynthesis or photochemical reactions.
• Pressure can change the state of matter or facilitate chemical reactions (e.g., in high-pressure conditions like inside the Earth's crust).

Therefore, energy in any form can lead to alterations in the properties of matter.

Burnt crackers and burnt pages are examples of irreversible changes. When crackers or pages are burnt, a chemical reaction occurs, producing new substances like ash and gases. These changes are permanent, and the original substances cannot be restored.

Thus, the correct answer is Option B: irreversible change.