Natural and
Processed Materials:
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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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