Name: _________________________________ Date: _______________________
8.1 Conservation of mass in chemical reactions
Year 9
Representing chemicals
When looking at a chemical reaction, scientists often represent substances using
chemical equations.
The substances formed at the beginning of a reaction are called
_________________________________.
The substances formed by the chemical reaction are called
_________________________________.
Scientists may choose to represent chemical reactions by writing a simple
_____________________________ ___________________________________.
The law of conservation of mass
The law of conservation of mass states that the total ____________________ of
chemicals at the beginning of a reaction will equal the total ___________________
of chemical at the ______________ of a reaction.
Example of chemical reaction
Methane gas is often used at home in cooking (on the stove). When methane burns,
it combines chemically with _____________________ (______) in the air to form
_____________________ ______________________ (CO2 ) and water
(__________).
In the figure below, count the different types of atoms present on the left hand
side of the arrow (reactants) and then count the different types of atoms present on
the right hand side of the arrow (products):
Reaction 1: Producing water
Hydrogen gas
Water
+
Water
Hydrogen gas
Oxygen gas
Atom type
Number of individual atoms
present in reactants
Number present in products
Hydrogen (H)
Oxygen (O)
Reaction 2: Burning methane (for cooking)
Methane
Oxygen
Carbon dioxide
Water
Atom type
Hydrogen (H)
Number of individual atoms
present in reactants
Number present in products
4
Oxygen (O)
Carbon (C)
By counting the atoms for yourself in the above exercises, what can you conclude
about mass in a chemical reaction?
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