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Matter is constantly undergoing changes around us. From water boiling in a pot to food cooking on a stove, these transformations can fundamentally alter how a substance appears or behaves. To understand these changes, chemists classify them into two main categories: physical changes and chemical changes.

Physical Changes: Altering Appearance, Not Identity

A physical change is a transformation that alters the physical properties of a substance but does not change its chemical composition or chemical nature. The original substance remains chemically the same, even if it looks different.

Examples of physical properties that can be observed and specified include:

• Color
• Hardness
• Rigidity
• Fluidity
• Density
• Melting point
• Boiling point

A prime example of a physical change is the interconversion of states of matter (e.g., solid to liquid, liquid to gas, or vice versa). When ice melts into water or water boils into water vapor, their physical properties (like state, density, and appearance) change. However, chemically, they are all still H?O. No new substance is formed, and the underlying chemical identity remains unchanged.

Other common examples of physical changes include:

• Cutting of trees (wood is still wood, just smaller pieces).
• Melting of butter in a pan (butter melts but is still butter).
• Boiling of water to form steam (water changes state but remains water).
• Dissolving common salt in water (salt and water are still salt and water, just mixed).
• Making a fruit salad with raw fruits (fruits are cut and mixed, but chemically they remain the same fruits).

Chemical Changes: Creating New Substances

In contrast, a chemical change, also known as a chemical reaction, involves a fundamental alteration in the chemical composition of a substance, leading to the formation of new substances with entirely different chemical properties. During a chemical change, one substance reacts with another to undergo a change in chemical composition.

Consider the example of water and cooking oil. Both are liquids, but their chemical characteristics are distinct. Oil burns in air, a property known as inflammability, whereas water extinguishes fire. This ability to burn is a chemical property of oil. When oil burns, it reacts with oxygen in the air to form new substances (like carbon dioxide and water vapor), fundamentally changing its chemical nature. Therefore, burning is a chemical change.

Other examples of chemical changes include:

• Rusting of an almirah (iron reacts with oxygen and moisture to form iron oxide, a new substance).
• Passing electric current through water and the water breaking down into hydrogen and oxygen gases (water, H?O, is chemically broken into its constituent elements).
• Burning of paper and wood (these materials react with oxygen to form ash, smoke, and gases, which are new substances).

The Candle Conundrum: Both Changes at Once!

Interestingly, some everyday phenomena involve both physical and chemical changes simultaneously. The burning of a candle is a perfect example.

• Physical change: The wax near the flame melts (solid to liquid), and then the liquid wax vaporizes (liquid to gas) – these are changes of state.
• Chemical change: The wax vapor burns, reacting with oxygen in the air to produce carbon dioxide, water, soot, and light/heat – this is a chemical reaction that creates new substances.

Distinguishing between physical and chemical changes is vital for understanding chemical processes and how matter interacts and transforms in the world around us.

Question for You: If you bake a cake, is that primarily a physical or a chemical change? Explain why.