• Question: What is the difference between a chemical and a physical change ?

    Asked by lkelk12 to Dilwar, Lou, Rachel, Simon, Susan on 12 Nov 2013.
    • Photo: Rachel Dakin

      Rachel Dakin answered on 12 Nov 2013:

      A physical change is about energy or the state of matter. You can normally force a physical change temperature or pressure e.g. heating up water causes a physical change and it becomes steam. The molecules are still H2O though – no chemical change.
      Chemical changes happen on a molecular level so it’s much harder to see most of them. They involve forming a new substance. One you can actually see is iron going rusty. The iron starts out pure (Fe) over time it is oxidised becoming FeO. This is a new molecule, what’s good is that you can also see the new molecule as it is rust.

    • Photo: Simon Langley-Evans

      Simon Langley-Evans answered on 12 Nov 2013:

      Physical changes usually involve a substance changing from solid to liquid, liquid to gas, gas-liquid, liquid- solid. The substance is still the same chemical. Some molecules can do more extreme physical changes. Sulphur, for example, goes straight from solid to gas without becoming a liquid. This is called sublimation.

      A chemical change alters the substance into something completely different. When you eat food for example, the sugars in your food go through chemical changes that release energy for the body to use, leaving behind waste products that the body gets rid of (water and carbon dioxide are the end products of chemical changes to sugars).

    • Photo: Susan Skelton

      Susan Skelton answered on 13 Nov 2013:

      Hi Ikelk!

      Chemical changes are changes that happen to the molecules. A chemical change produces a new substance, for example cooking an egg, rusting an iron pan or mixing hydrochloric acid and sodium hydroxide to make salt and water.

      Physical changes do not produce new substances: physical changes are concerned with energy and states of matter. Changes in state or phase (for example, melting, freezing, vaporization, condensation, or sublimation) are physical changes. Physical changes are things like crushing a can, melting an ice cream, or smashing a glass.

      Basically, a chemical change makes a substance that wasn’t there before (rusty pan, cooked egg), whereas the starting and ending materials of a physical change are the same, even though they may look different (smashed glass, melted ice-cream).

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