Is boiling water considered a chemical change or a physical change, and why?

Introduction / Context:
Understanding the difference between physical and chemical changes is a fundamental part of school level chemistry. Physical changes involve alterations in state or appearance without changing the chemical identity of the substance, whereas chemical changes create new substances with different properties. Boiling water is a common everyday phenomenon and an excellent example to test whether you can correctly apply this distinction. This question asks whether boiling water is a chemical or physical change and why that classification is correct.

Given Data / Assumptions:

• The substance involved is pure water, H2O.
• Boiling water means heating liquid water until it changes to water vapour (steam).
• No additional chemicals are added, and there is no electrical decomposition.
• The options describe various possible explanations about forming new substances or changing state.

Concept / Approach:
In a physical change, the molecules themselves remain the same but may move further apart or closer together, change arrangement or change energy. In boiling, liquid water changes to gaseous water vapour, but each molecule remains H2O. There is no breaking of water into hydrogen and oxygen in this process. In a chemical change, the original substance is converted into one or more new substances with a different composition, such as when water is electrolysed to hydrogen and oxygen gases. Since boiling only changes the physical state from liquid to gas and does not change the chemical identity, boiling water is a physical change, not a chemical change.

Step-by-Step Solution:
Step 1: Identify the initial and final states: initially, water is in the liquid state; after boiling, it is in the gaseous state as steam. Step 2: Note that in both states, the chemical formula of the substance is still H2O, so the molecules have not changed composition. Step 3: Physical changes typically involve changes in state, shape or size, but no new chemical substances are formed. Step 4: Chemical changes require breaking and making of chemical bonds to create new substances with different chemical formulas. Step 5: During boiling, the heat energy supplied overcomes intermolecular forces between water molecules, allowing them to move apart into the gas phase. Step 6: Since no hydrogen gas or oxygen gas is produced and the water molecules remain chemically unchanged, boiling is a physical change. Step 7: Therefore, the correct statement is that boiling water is not a chemical change but a physical change because only the state changes.

Verification / Alternative check:
Evidence for boiling being a physical change includes the fact that if you collect steam and cool it, it condenses back into liquid water with the same taste and properties. There is no change in composition before and after. The boiling point is a characteristic physical property of water and is used to help identify it. In contrast, if you electrolyse water, you produce hydrogen and oxygen gases that cannot simply be condensed back into water without a chemical reaction. This clearly shows that boiling is reversible and does not create new substances, reinforcing its classification as a physical change.

Why Other Options Are Wrong:
Option B claims that hydrogen and oxygen are formed during boiling, which is incorrect; those gases form only in electrolysis or certain chemical reactions, not in simple boiling. Option C suggests that water molecules are destroyed, which does not happen; they remain intact as H2O. Option D mixes up the idea of energy supply with chemical change; although heat is supplied, this alone does not guarantee that a chemical change occurs. Option E states that boiling is both physical and chemical at the same time, which would require new substances to form, but here they do not. Only option A correctly states that boiling water is a physical change because only the state changes from liquid to gas.

Common Pitfalls:
Students sometimes mistakenly think that any process involving heat must be a chemical change, but many physical changes, such as melting ice or boiling water, also require heat. Another common error is to confuse boiling with electrolysis and assume that hydrogen and oxygen must always be produced when water is heated. To avoid these mistakes, ask whether the chemical composition has changed. If the formula before and after is the same, it is a physical change; if it is different, it is a chemical change. Applying this simple test helps classify many processes correctly.

Final Answer:
Boiling water is not a chemical change; it is a physical change because only the state of water changes from liquid to gas while the chemical composition remains H2O.