5.4: Physical and Chemical Changes
pg. 182
Key Concepts:
1. Understanding the physical and chemical
properties of a substance helps us identify its
uses.
2. Change in substances can be either physical
or chemical.
Physical Changes
Physical Change: a change in a substance that does not
involve the formation of a new substance.
When a physical property is changed, but the composition
is the same, this represents a physical change. Changes of
state are excellent examples of physical changes. When
water becomes ice as it freezes, the composition has not
change, but its physical property has gone from liquid to a
solid.
When you rip paper into smaller pieces, the composition of
the paper is the same, just smaller. This is still a physical
change.
What Happens in a Physical Change?
During physical change, physical properties of a substance
may change, but the composition of the substance stays the
same.
Changes of state: Solid, liquid, and gas. (freezing, melting,
boiling and condensing)
Dissolving is also a physical change. When water or sugar
dissolve in water. They disappear, but they are still there
when you taste the solution. If you evaporate the water the
salt and sugar will return.
Chemical Changes
Chemical Change: a change in which one or more new
substances is formed.
When substances are combined and a new substance is
formed, a chemical reaction has occurred.
Chemical Change Clues
Precipitate: is a solid that forms when two liquids are
mixed.
There are five indicators that can be used to determine if a
chemical has occurred;
1. a gas (bubbles) is formed,
2. heat or light is generated,
3. a solid precipitated is formed when two liquids are
mixed,
4. a new colour is formed,
5. the reaction cannot be reversed.
Figure 3: It’s a bird! It’s a plane! Yes, but it is still paper.
Check Your Learning:
Questions 1 – 6, pg. 185
Wrap Up:
- A physical change does not involve forming a new
substance, cutting wood, boiling water, and dissolving salt
water are examples of physical changes.
- A chemical change results in the formation of a new
substance. Burning a candle and cooking eggs are examples
of chemical changes.
- Evidence of a chemical change includes colour change,
odour change, gas production, precipitate production, and
production of light or thermal energy.