3. What is an Acid?
An acid is a substance that produces
hydrogen ions, H+
in water.
An acid therefore can conduct
electricity.
pH < 7
It has a sour taste.
It has a stinging feeling.
It is corrosive.
5. What is an Alkali?
An alkali is a substance that produces
hydroxide ions, OH-
in water.
An alkali can conduct electricity.
pH > 7
It has a bitter taste.
It is caustic.
It is corrosive.
6. Examples of Acids
Names of acids Chemical formula Where can it be found?
Hydrochloric acid HCl In gastric juice in the stomach
Sulphuric acid H2SO4 In car battery
Nitric acid HNO3 In the preparation of fertilizers and
explosives
Carbonic acid H2CO3 In fizzy drinks
Citric acid C6H8O7 In oranges and lemons
Acetic acid CH3COOH In vinegar
7. Acidic or not?
Chemical name Name of
compound
Is it
Acidic?
H2SO4 (aq) Sulphuric acid Yes
HNO3 (aq) Nitric acid Yes
CH3COOH(aq) Ethanoic
acid/acetic acid
Yes
H2O (s) water No
8. Strength of an Acid
The strength of an acid depends on the
extent of ionisation.
The acid is stronger when more molecules
are ionised.
10. Strong Acids
A strong acid is one in which all the acid
molecules become ions in water.
Examples: Hydrochloric acid, HCl (aq)
Sulphuric acid, H2SO4 (aq)
Nitric acid, HNO3 (aq)
12. Weak Acids
A weak acid is one in which only a few acid
molecules become ions in water.
Examples: Acetic acid, CH3COOH (aq)
Citric acid, C6H8O7(aq)
Carbonic acid, H2CO3(aq)
13. Reaction of Acids with
Metals
General Equation:
Metal + Acid Salt + Hydrogen gas
Observation:
The metal dissolves
and effervescence occurs.
The gas extinguishes a burning
splint with a ‘pop’ sound.
15. Reaction of Acids with
Carbonates
General Equation:
Metal + Acid Salt + Water + Carbon
Carbonate dioxide
gas
16. Reaction of Acids with
Carbonates
Observation:
The metal dissolve and effervescence
occurs. The gas forms a white precipitate
with limewater.
17. Reaction of Acids with
Carbonates
MgCO3 (s) + 2HCl(aq) →
Magnesium +
carbonate
Hydrochloric
acid
→
→ CaSO4(aq) + H2O(l) + CO2(g)
Calcium +
sulphate
Water +→ Carbon
dioxide
gas
MgCl2 (aq) + H2O (l)+ CO2(g)
Magnesium chloride + water
+ carbon dioxide
CaCO3 (s) + H2SO4 (aq)
Calcium + sulphuric
Carbonate acid
18. Reaction of Acids with
Alkalis
General Equation:
Alkali + Acid Salt + Water
19. Reaction of Acids with
Bases
Base: Metal oxides, Metal hydroxides
NaOH + HCl
KOH + H2SO4
Sodium
hydroxide
Hydrochloric
acid
Potassium
hydroxide
Sulphuric
acid
+
+
+
+→
→
NaCl (aq) + H2O (l)
Sodium
chloride
water
K2SO4 (aq) + H2O (l)
Potassium
sulphate
water
20. Uses of Acids
Acids
Preserving food
-acetic acid in
vinegar
Making industrial
products
-nitric acid in
fertilisers
-sulphuric acid in
detergents
Rust removal
from iron and steel
-Sulphuric acid
-Hydrochloric acid
Car battery
-Sulphuric acid
21. Complete the Puzzle
Across
1. Acids react with _______ to give salt and water
only.
5. Only _______ solution of acids have acidic
properties.
6. In a strong acid, the acid molecules are _______
ionised.
8. Ethanoic acid is an example of a _______ acid.
9. Acids have a _______ taste.
Down
2. _______ acid is an example of a strong acid which
is found in car battery.
3. _______ react with acids to give off a gas that
forms a white precipitate with limewater.
4. When dissolved in water, acid molecules can ionise
to give _______ ions.
7. Hydrogen gas is evolved when acids react with
_______.
22. Answer
Across
1. Acids react with bases to give
salt and water only.
5. Only aqueous solution of acids
have acidic properties.
6. In a strong acid, the acid
molecules are completely
ionised.
8. Ethanoic acid is an example of a
weak acid.
9. Acids have a sour taste.
23. Answer
Down
2. Sulphuric acid is an example of a
strong acid which is found in car
battery.
3. Carbonates react with acids to
give off a gas that forms a white
precipitate with limewater.
4. When dissolved in water, acid
molecules can ionise to give
hydrogen ions.
7. Hydrogen gas is evolved when
acids react with metals.
24. Summary
An acid is a substance that produces
hydrogen ions in water.
Only an aqueous solution of acid has acidic
properties.
HCl (aq) H+
(aq) + Cl-
(aq)
The greater the extent of ionisation, the
stronger the acid.
25. Summary
An acid
- pH < 7
- turn blue litmus paper red.
- taste sour
- stinging feeling
- is corrosive (for strong acids)
- can conduct electricity.
26. Summary
Reactions of acids with
1. metals
Metal + Acid Salt + Hydrogen gas
2. carbonates
Metal + Acid Salt + Water + Carbon
Carbonate dioxide
gas
3. bases
Base + Acid Salt + Water
27. Summary
An alkali
- pH > 7
- turn red litmus paper blue
- taste bitter
- caustic
- is corrosive (for strong alkalis)
- can conduct electricity.
The difference between a strong acid and a weak acid is similar to the difference between a group of excellent netball players and that of terrible ones. A good netball player delivers the ball efficiently by passing it to another player or shooting into the net. She gets rid of the ball, just as a strong acid gives away or donates a proton. During a practice where everyone has a netball, nearly 100% of the excellent netball player will have passed the ball to another player or shot the it into the goal. Almost none will still be left holding the ball. This is similar to nearly 100% of the strong acid molecules dissolved in water giving away/ donating protons.
HCl(aq) H+ (aq) + Cl- (aq)
Almost none remain as protonated/un-ionized HCl(aq).
On the other hand, a bad netball player delivers the ball
Inefficiently. She is indecisive and tends to hold onto the ball, just like a weak acid does not readily donate its proton. At the end of practice, less than 100% of bad netball players will have pass the ball to others or shoot into the goal. For instance, only 10% of the netball player will pass the ball to others or shoot into the goal. The other 90% may be found holding the ball. Similarly, less than 100% of weak acid molecules dissolved in water may have delivered a proton. The rest of the molecules remain as protonated or un-ionized acid molecules. In conclusion, 100 aqueous strong acid molecules will successfully release 100 H+ ions into solution, whereas 100 aqueous weak acid molecules may only deliver 10 H+ ions.
Give examples of strong and weak acids.
