Understanding Chemical vs. Physical Changes

To identify which option is NOT a chemical change, we need to understand the difference between chemical and physical changes. A chemical change results in the formation of new substances with different properties. A physical change alters the form or appearance of a substance but does not change its chemical identity. These changes are often reversible.

Analyzing the Options

Option 1: Burning of coal

Burning is a combustion process. When coal (primarily carbon) burns, it reacts with oxygen (\(O_2\)) in the air to produce new substances like carbon dioxide (\(CO_2\)), ash, heat, and light. Since new substances are formed, this is a chemical change.

Option 2: Rusting of metal

Rusting is the process where iron reacts with oxygen and moisture to form iron(III) oxide (\(Fe_2O_3\)), commonly known as rust. Rust has different properties than iron. The formation of a new compound indicates a chemical change.

Option 3: Melting of ice

Melting is the process where solid ice changes into liquid water. The chemical formula for both ice and liquid water is \(H_2O\). The substance itself hasn't changed chemically; only its state (solid to liquid) has changed. This is a classic example of a physical change, and it is easily reversible by freezing the water back into ice.

Option 4: Reaction of acid with base

When an acid reacts with a base (neutralization reaction), they form a salt and water. For example, the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) produces sodium chloride (NaCl, a salt) and water (\(H_2O\)). Since new substances (salt and water) are formed, this is a chemical change.

Conclusion

Based on the analysis, melting of ice involves a change in the state of matter but not the chemical composition. Therefore, it is a physical change, not a chemical change.