2. A.Signs of a Chemical
Reaction
s Evolution of heat and light
s Formation of a gas
s Formation of a precipitate
s Color change
3. B.Law of Conservation of
Mass
s mass is neither created nor destroyed
in a chemical reaction
s total mass stays the same
s atoms can only rearrange
4H 4H
36 g
2O 2O
4g 32 g
6. D. Writing Equations
2H2(g) + O2(g) → 2H2O(g)
s Identify the substances involved.
s Use symbols to show:
• How many? - coefficient
• Of what? - chemical formula
• In what state? - physical state
s Remember the diatomic elements.
7. D. Writing Equations
Two atoms of aluminum react with
three units of aqueous copper(II)
chloride to produce three atoms of
copper and two units of aqueous
aluminum chloride.
• How many?
• Of what?
• In what state?
2Al(s) + 3CuCl2 (aq) → 3Cu(s) + 2AlCl3 (aq)
8. E. Describing Equations
s Describing Coefficients:
• individual atom = “atom”
• covalent substance = “molecule”
• ionic substance = “unit”
3CO2 ⇒ 3 molecules of carbon dioxide
2Mg ⇒ 2 atoms of magnesium
4MgO ⇒ 4 units of magnesium oxide
9. E. Describing Equations
Zn(s) + 2HCl(aq) → ZnCl2(aq) +
• How many?
H2(g)
• Of what?
• In what state?
One atom of solid zinc reacts with
two molecules of aqueous
hydrochloric acid to produce one unit
of aqueous zinc chloride and one
molecule of hydrogen gas.
