Which one of the following is an example of a chemical change?
Burning of paper
Understanding Chemical and Physical Changes
Changes in matter can be broadly classified into two main categories: physical changes and chemical changes. It's important to distinguish between these types to understand how substances behave.
What is a Chemical Change?
A chemical change, also known as a chemical reaction, involves the formation of new substances with properties different from the original substances. In a chemical change, the chemical composition of the substance is altered. These changes are often irreversible, and energy (like heat or light) is usually absorbed or released during the process.
What is a Physical Change?
A physical change involves a change in the form or appearance of a substance, but not its chemical composition. The molecules themselves do not change. Examples include changes in state (solid, liquid, gas), shape, size, or texture. Physical changes are often reversible.
Analyzing the Options for Chemical Change
Let's examine each given option to determine whether it represents a chemical change or a physical change.
- Option 1: Burning of paper
When paper burns, it reacts with oxygen in the air. The paper is transformed into ash, smoke (containing carbon dioxide and other gases), and water vapor. These are completely new substances with different chemical properties than the original paper. This process is irreversible. Therefore, the burning of paper is a chemical change. - Option 2: The magnetization of soft iron
When soft iron is placed in a magnetic field, it becomes magnetized. This involves aligning the magnetic domains within the iron. The chemical composition of the iron itself does not change; it remains iron. This change is often temporary in soft iron and can be reversed by removing the magnetic field or heating the iron. This is a physical change. - Option 3: Dissolution of cane sugar in water
When cane sugar (sucrose) dissolves in water, the sugar molecules disperse throughout the water molecules, forming a sugar solution. The sugar molecules themselves do not break down or change chemically. The chemical properties of sugar and water remain the same in the solution. The water can be evaporated to recover the solid sugar. This is a physical change. - Option 4: Preparation of ice cubes from water
When liquid water freezes to form ice cubes, it changes from the liquid state to the solid state. The chemical composition of water is \(\text{H}_2\text{O}\), and it remains \(\text{H}_2\text{O}\) whether it is liquid water, solid ice, or gaseous steam. This is a change of state, which is a physical change. The ice can be melted back into liquid water. This is a physical change.
Based on the analysis, the only option that involves the formation of new substances is the burning of paper.
Summary of Changes
| Change | Type of Change | Explanation |
|---|---|---|
| Burning of paper | Chemical Change | New substances (ash, gas, etc.) are formed. Irreversible. |
| Magnetization of soft iron | Physical Change | No change in chemical composition; arrangement changes. Often reversible. |
| Dissolution of cane sugar in water | Physical Change | Substances are mixed but retain original properties. Reversible. |
| Preparation of ice cubes from water | Physical Change | Change of state (\(\text{H}_2\text{O}\) remains \(\text{H}_2\text{O}\)). Reversible. |
Revision Table: Key Differences
| Feature | Chemical Change | Physical Change |
|---|---|---|
| Nature of Substance | New substances formed | No new substances formed (only appearance/form changes) |
| Chemical Composition | Altered | Remains the same |
| Reversibility | Often difficult or impossible to reverse | Often easily reversible |
| Energy Change | Usually significant energy absorbed or released | Often minimal energy change (except state changes) |
| Examples | Burning, rusting, cooking, digestion | Melting, freezing, boiling, dissolving, cutting, breaking |
Additional Information: Signs of Chemical Changes
Several observable signs can indicate that a chemical change has occurred. These are not always present, but their appearance strongly suggests a chemical reaction is taking place:
- Change in Color: For example, rust forming on iron changes its color from metallic gray to reddish-brown.
- Production of Gas: Bubbles forming, like when vinegar is mixed with baking soda.
- Formation of a Precipitate: A solid substance forming in a liquid mixture.
- Change in Temperature: The mixture becoming hotter (exothermic) or colder (endothermic).
- Production of Light or Sound: For instance, during combustion (burning).
- Change in Odor: A new smell being produced.
Recognizing these signs helps in identifying chemical changes in various contexts.
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