Question

Which of the following changes can be reversed by changing the temperature: (a) dew condensing on a leaf; (b) an egg turning hard when it is boiled; (c) ice cream melting; (d) a spoonful of batter cooking on a hot griddle?

Short answer

Changes (a) and (c) can be reversed by changing the temperature.

Step-by-step solution

Understand Reversible and Irreversible Changes

A change is reversible if it can be undone or returned to its original state by changing conditions like temperature. Irreversible changes cannot be undone by reversing the conditions.

Analyze Dew Condensing on a Leaf

Dew forms when water vapor in the air condenses into liquid water on a cooler surface. This process can be reversed by heating the leaf to evaporate the water back into vapor.

Examine an Egg Turning Hard When Boiled

Boiling an egg causes the proteins in the egg to denature and coagulate, resulting in a hard-boiled egg. This change cannot be reversed by cooling down the egg.

Evaluate Ice Cream Melting

Ice cream melts when it is warmed above its freezing point, turning from solid to liquid. This change can be reversed by cooling the liquid ice cream down to refreeze it.

Assess a Spoonful of Batter Cooking on a Hot Griddle

Cooking batter on a hot griddle causes a chemical change where the batter solidifies and forms a cooked product. This change cannot be reversed by cooling.

Summary

Among the given options, dew condensing on a leaf (a) and ice cream melting (c) are changes that can be reversed by changing the temperature. Hard-boiled eggs (b) and cooked batter (d) cannot be reversed by changing the temperature.

Key concepts

phase changes

Phase changes occur when a substance moves from one state of matter to another. These states can be solid, liquid, or gas. The most common phase changes include:

• Melting: Solid to liquid
• Freezing: Liquid to solid
• Condensation: Gas to liquid
• Evaporation: Liquid to gas

These changes are usually reversible. For example, ice cream melting (solid to liquid) can be reversed by refreezing it (liquid to solid). Similarly, dew (liquid) forms from water vapor (gas) condensing and can reverse when the liquid evaporates back into vapor.
Not all phase changes are reversible, especially if there are changes at the molecular level, like chemical changes during cooking.

temperature effects

Temperature changes can cause substances to change their physical states. This is because temperature affects the kinetic energy of molecules.

• Increasing temperature typically increases kinetic energy, causing solids to melt into liquids and liquids to evaporate into gases.
• Decreasing temperature reduces kinetic energy, causing gases to condense into liquids and liquids to freeze into solids.

For instance, cooling water vapor on a cool leaf forms dew (liquid), which can boil off again when heated. Similarly, warming ice cream causes it to melt, and refreezing it turns it back to solid.
However, certain changes, such as a boiled egg solidifying, are permanent as they involve altering the molecular structure through chemical reactions.

chemical changes

Chemical changes result in the formation of new substances with different properties. These changes are often irreversible because the original substances undergo a chemical reaction. Examples include:

• Boiling an egg: Heat causes the proteins to denature and coagulate, forming a solid egg white and yolk, a change that cannot be undone.
• Cooking batter on a hot griddle: Heat causes Maillard reactions and other chemical changes that produce a cooked product, which cannot revert to the original batter form.

Unlike phase changes, chemical changes alter the internal structure of the molecules, making it impossible to return to their initial state by simply reversing the temperature.
In summary, reversible changes involve physical transformations without altering the substance's chemical structure. Irreversible changes, often involving heat, result in new substances with different molecular compositions.