Which of the following describes a chemical change involving water? a) H₂O(l) → H₂O(s) b) H₂O(s) → H₂O(g) c) H₂O(l) → H₂(g) + O₂(g) d) H₂O(g) → H₂O(l)
The Correct Answer and Explanation is :
The correct answer is c) H₂O(l) → H₂(g) + O₂(g).
This represents a chemical change where water (H₂O) is decomposed into hydrogen (H₂) and oxygen (O₂) gases. This process is known as electrolysis of water. Here’s why it’s a chemical change:
Explanation:
- Chemical change involves the transformation of one substance into a different substance, often with the formation of new products and a change in chemical composition. This is distinct from a physical change, where the substance undergoes a change in state or appearance without changing its molecular structure.
- Electrolysis of Water (c):
- Electrolysis involves passing an electric current through water, causing it to break apart into hydrogen and oxygen gases. This change is chemical because the water molecules (H₂O) are chemically altered, resulting in the production of two different substances: hydrogen (H₂) and oxygen (O₂). The molecular structure of water is broken down, making it a clear example of a chemical change.
- Physical Changes:
- a) H₂O(l) → H₂O(s): This is the freezing of water, a physical change where water changes from liquid to solid (ice), but no new substance is formed.
- b) H₂O(s) → H₂O(g): This is the sublimation of ice (a phase change from solid to gas), which is also a physical change because the molecular structure of H₂O remains the same.
- d) H₂O(g) → H₂O(l): This is condensation (gas to liquid), another physical change, as the molecules remain unchanged, only the phase of water changes.
Conclusion:
In option c, water undergoes a chemical reaction where it is decomposed into hydrogen and oxygen gases. This is the only option where new substances are formed, marking it as a chemical change. The other options involve phase changes (solid to liquid, liquid to gas) that do not alter the chemical composition of water.