Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Matter exists in three primary states: solids, liquids, and gases. A common example is water, which can be found as solid ice, liquid , and gaseous water vapor.
A reversible change is a change that can be undone or reversed. These are usually physical changes where no new substance is formed, such as melting ice into liquid .
An irreversible change is a change that cannot be undone. New substances are created during the process, such as when wood is burned to create ash and smoke.
Heating and cooling are the main ways to cause changes in states of matter. Adding heat energy () can cause melting or evaporation, while removing heat () can cause freezing or condensation.
Mixing certain substances can cause irreversible reactions. For example, mixing vinegar and baking soda creates gas, which cannot be turned back into the original ingredients easily.
📐Formulae
💡Examples
Problem 1:
You have a bowl of liquid chocolate. You put it in the fridge for one hour and it becomes solid. Is this a reversible or irreversible change?
Solution:
Reversible change.
Explanation:
The change is caused by cooling. If you add heat to the solid chocolate, it will melt back into a liquid state. No new substance was formed.
Problem 2:
A piece of paper is set on fire and turns into grey ash. Can we turn the ash back into paper?
Solution:
No, this is an irreversible change.
Explanation:
Burning is a chemical process that changes the paper into new substances like ash and smoke (including ). These new substances cannot be combined to recreate the original paper.
Problem 3:
What happens to water () when it reaches its boiling point of ?
Solution:
It undergoes evaporation to become a gas (steam).
Explanation:
At , the liquid gains enough thermal energy to change its state from a liquid to a gas. This is a reversible physical change.