Download Energy and Chemical Change Can changes be reversed

Energy and Chemical Change
Chemical changes involve a change in energy. Think
about a fireworks display. The release of thermal energy,
light, and sound are signs of chemical changes. All chemical
reactions involve energy changes.
Chemical reactions can also take in energy. Thermal
energy is often needed for a chemical change to take place.
To bake bread, for example, you have to put dough in a hot
oven.
expose
Some chemical reactions need energy in the form of
light. Plants and some unicellular organisms use the Sun’s
energy for photosynthesis. Photosynthesis is the chemical
reaction by which these organisms make sugar and oxygen.
This process occurs only if the organisms are exposed to
light.
(verb) to uncover; to make
visible
Can changes be reversed?
ACADEMIC VOCABULARY
Think again about a fireworks display. After the fireworks
explode, they cannot go back to the chemicals they were
before. Chemical changes cannot be reversed.
Reading Check
change that can be reversed
and one that cannot.
Key Concept Check
7. Explain how physical
and chemical changes affect
mass.
Conservation of Mass
Physical changes do not affect the masses of substances.
For example, when an ice cube melts, the mass of liquid
water will be the same as the mass of the ice cube. If you cut
a sheet of paper into pieces, the total mass of the pieces will
be the same as the mass of the paper you started with. Mass
is conserved, or unchanged, during a physical change.
Mass is also conserved during chemical changes. Antoine
Lavoisier, a French chemist, discovered this in the 1700s.
The masses of two substances that will chemically react can
be measured and added together. After the two substances
react to form new substances, the total mass after the
reaction can be measured. You will find that the total mass
before and the total mass after the reaction are the same.
The law of conservation of mass states that the total mass before
a chemical reaction is the same as the total mass after the chemical
reaction. This is always true because particles are only
rearranged. They cannot be created or destroyed, so the total
mass cannot increase or decrease.
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Matter: Properties and Changes
Reading Essentials
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6. Identify one physical
Some physical changes cannot be reversed, either. If you
cut an apple, it cannot be put back together. Other physical
changes can be reversed. For example, if you dissolve salt
in a pan of water and then boil the mixture, the water will
change to a gas and the salt will be left behind.