Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
A change is any transition from one state or form to another. It can be classified based on whether the original substance can be recovered.
Reversible Changes: These are changes that can be undone to get back the original substance. Examples include the melting of ice () and the stretching of a rubber band.
Irreversible Changes: These are permanent changes that cannot be undone. A new substance is usually formed. Examples include the burning of paper, where wood/cellulose turns into ash and .
Expansion and Contraction: Most substances expand (increase in size) on heating and contract (decrease in size) on cooling. This is used in fixing iron rims on wooden wheels: the rim is heated to expand (), fitted, and then cooled () to grip the wheel.
Dissolving: Dissolving a solid like salt () in water is a reversible change because the solid can be recovered through evaporation or crystallization.
Physical vs. Chemical Changes: Reversible changes are often physical changes (no new substance), while irreversible changes are typically chemical changes (new substances formed).
📐Formulae
💡Examples
Problem 1:
Explain why the curdling of milk is considered an irreversible change.
Solution:
Once milk turns into curd, it cannot be converted back into milk.
Explanation:
Curdling involves a chemical reaction where the proteins in milk change their structure to form a new substance. Since we cannot obtain the original from the by any physical method, it is irreversible.
Problem 2:
A blacksmith heats an iron ring before fitting it onto a wooden handle. What principle is being used here?
Solution:
Thermal Expansion.
Explanation:
When the iron ring is heated, it undergoes expansion (), making it slightly larger in diameter. This allows it to slip onto the handle easily. Upon cooling, it contracts and fits tightly.
Problem 3:
Is the burning of a candle a reversible or irreversible change?
Solution:
It is an irreversible change (with a reversible component).
Explanation:
The burning of the wick and the combustion of wax to produce and water vapor () are irreversible. However, the melting of wax is a reversible physical change because the molten wax can solidify back into solid wax.