Natural and Processed Materials:
Subject List
Reaction and change

Changes

Whenever a change occurs in a substance or system of substances, such as a cake cooking, water boiling or mixing a cordial drink, scientists classify the change as either a physical change or a chemical change. Sometimes the boundary between physical change and chemical change is not easy to define, but the following describes typical physical changes and chemical changes.

Physical changes
Physical changes do not involve the formation of new substances and are usually easily reversible, that is, the substance can be changed back to its original form easily.

Examples of some common physical changes are:
Changes in state:

  • Boiling water to form steam.
  • Freezing water to form ice.
  • Condensing a steam to form liquid water.
  • Melting wax in a candle.
  • Subliming mothballs to form a gas.

In each case, a change in temperature has caused the change in state to occur. Simply reversing the temperature returns the substance to its original form.

Dissolving and mixing:

  • Mixing salt in water to form a solution.
  • Mixing oil and water.
  • Evaporating sea water to leave salt.

Simple forms of dissolving and mixing are considered physical changes, but mixing the ingredients of a cake is not a simple mixing process. A chemical change starts to occur when the ingredients are mixed, forming new substances.

Chemical changes
When a substance changes and cannot be easily returned to its original state, or when two or more substances combine to form a new set of substances, a chemical change is said to have happened.

Chemical changes create new substances and are usually not reversible.

Some examples of chemical changes are:

  • Cooking an egg - the yoke becomes solid and the protein turns white.
  • Burning wood - ash, water and carbon dioxide gas are formed.
  • TNT exploding - gas and bits everywhere!
  • Cement setting into concrete.
  • Baking bread - from the mixture rises a delicious loaf.
  • Photosynthesis - growing a plant from carbon dioxide, water, minerals and the Sun.

Chemical changes tend to be long lasting, if not permanent. You can let a cooked egg cool down and watch it for hours and it will not turn back into its original runny form! However, place a block of ice on a plate in a warm place and watch it melt.

Chemical change is not easily reversible, while physical change is easy to reverse.

Links:
Changes in state Changes in state - melting and freezing
Changes in state - boiling, evaporation and condensation Like dissolves like
Physical change or chemical change? Chemical reactions Classifying chemical reactions

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