1. EVIDENCES OF A CHEMICAL REACTION:
1) Color change.
2) Formation of a solid (precipitate).
3) Evolution of a gas (bubbles).
4) Temperature changes
(exothermic or endothermic).

2. Law of Conservation of Mass
The mass is neither created nor destroyed.
The total mass of all substances before a chemical change (reactants) equals the total mass of all substances remaining after the change (products).

3. Law of Conservation of Mass
Example 1:
2 Mg (s) O2 (g) → 2 MgO (s)
24 g = ?

4. Law of Conservation of Mass
Example 1 :
2 Mg (s) O2 (g) → 2 MgO (s)
24 g = g

5. Law of Conservation of Mass
Example 2:
2 Mg (s) O2 (g) → 2 MgO (s)
12 g ? = g

6. Law of Conservation of Mass
Example 2:
2 Mg (s) O2 (g) → 2 MgO (s)
12 g g = g

7. Law of Conservation of Mass
Example 2:
2 Mg (s) O2 (g) → 2 MgO (s)
12 g g = g
x = 20 g – 12 g
x = 8 g

8. Law of Conservation of Mass
Example 3:
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O
40 g g = g ?

9. Law of Conservation of Mass
Example 3:
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O
40 g g = g g

10. Law of Conservation of Mass
Example 3:
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O
40 g g = g g
76.5 g = g x
x = 76.5 g – 58.5 g
x = 18 g

11. Balancing Chemical Equations:
Determine the number of atoms of each element in
the reactants and products. Count a polyatomic ion
as a single unit if it appears unchanged on both
sides of the equation.
2. Balance each atom one by one (use coefficients).
(*) Never change the subscripts in a chemical formula.
(Each substance has only one correct formula).
3. Check that the quantity of each atom is equal on both
4. Make sure all the coefficients are in the lowest
possible ratio.

12. Balancing Chemical Equations(Sequence):
1st -- Metals (such as Fe, Na, K, Mg…).
2nd -- Polyatomic ions (such as SO42-, PO43- that cross from reactants to products unchanged).
3rd -- Non-metals (such as Cl, S, Br)
4th-- Hydrogen.
5th-- Oxygen.

13. Practice: 1) Na(s) + HCl(aq) → NaCl(aq) + H2(g)

14. 2) Al(s) O2(g) → Al2O3(s) 4 x 2 3 2
Al = 4
Al = 4
O = 6
O = 6

15. 3) C4H8 (g) + O2(g) → CO2(g) + H2O(g) Reactants Products6 4 4
C = 4
C = 4
H = 8
H = 8
O = 12
O = 12