Physical Science: Ch 7 Notes
A. Chemical Reactions
When a substance undergoes a chemical change, a chemical reaction
has taken place.
Reactions are known to produce: unexpected color changes, a
precipitate, gas, or a change in temperature
Common chemical reactions: rust forming, digestion of food, bleaching hair, milk
souring, apples turning brown, photosynthesis, and the combustion of
anything!
Chemical equation – a representation of a chemical reaction in which the
reactants and products are expressed as formulas
A chemical equation is written as
Reactants  Products
Reactants – the substances that undergo change
Products – the new substances formed as a result of that change
Ex: C + O2  CO2 “carbon reacts with oxygen gas to form carbon dioxide”
Ex: H2O  H2 + O2 “water decomposes into hydrogen gas and oxygen gas
Physical Science: Ch 7 Notes
B. Balancing Chemical Equations
This must be done to follow the Law of Conservation of Mass – states
that mass is neither created nor destroyed
Therefore, the products of a chemical reaction are not created
out of thin air and the atoms of the reactants are not
destroyed in the chemical reaction!
To balance a chemical equation:
1. Write the reaction in symbol form
2. Draw a vertical line down from the arrow in the equation and, on
each side, write the symbols of each of the atoms present
3. Count and record the numbers of each atom present on each side
of the arrow
4. Use coefficients to increase the number of atoms until they are
balanced (Do NOT change the subscripts already present in the
chemical formulas!)
Coefficients – the numbers that appear before the chemical
formulas in a balanced chemical equation
Physical Science: Ch 7 Notes
C. Classifying Reactions – chemical reactions are often classified by the type
of reactant or the number of reactants and products
Synthesis reaction – a reaction in which two or more substances react to
form a single substance
General form: A + B  AB
Example: 2H2 + O2  2H2O
Decomposition reaction – a reaction in which a compound breaks down
into two or more simpler substances
General form: AB  A + B
Example: 2H2O  2H2 + O2
Combustion reaction – a reaction in which a substance reacts rapidly with
oxygen often producing heat and light The products are ALWAYS carbon
dioxide and water
General form: X + O2  CO2 + H2O
Example: CH4 + 2O2  CO2 + 2H2O
Physical Science: Ch 7 Notes
Single Displacement – a reaction in which one element takes
the place of another element in a compound
General form: A + BC  B + AC
Example: Mg + HCl  MgCl2 + H2
Double Displacement – a reaction in which two different compounds
exchange positive ions and form two new compounds
General form: AB + CD  AD + CB
Example: HCl + PbO  H2O + PbCl2
Physical Science: Ch 7 Notes
D. Chemical Bonds and Energy
Chemical energy – the energy stored in the chemical bonds of a substance
Chemical reactions involve the breaking and formation of chemical bonds
The BREAKING of chemical bonds REQUIRES energy
The FORMATION of chemical bonds RELEASES energy
During a chemical reaction, energy is either a required reactant or
is released as a product
Endothermic reaction – reaction absorbs energy from its
surroundings as a reactant
Exothermic reaction – reaction releases energy to its
surroundings as a product