Salt = an ionic compound that contains a cation (positive ion of a base) and an anion (negative ion of an acid)
→ produced by the reaction of an acid with a base
| Sodium Chloride | used to preserve food |
| Calcium Sulfate | blackboard chalk |
| Magnesium Carbonate | used to keep your hands dry in sports (so they won’t slip) |
| Aluminium Sulfate | added to dye to make them stick together |
| Ammonium Nitrate | fertiliser (help crops grow well) |
| Copper Sulfate | stop fungi growing on soya seeds |
Many methods for making salts start with acid (all acids contain hydrogen)
Hydrochloric acid (HCl) → chlorides
Sulfuric acid (H2SO4) → sulfates
Nitric acid (HNO3) → nitrates
Carbonic acid (H2CO3) → carbonates
Citric acid (C6H8O7) → citrates
Method 1:
Metal + dilute acid → Salt + Hydrogen
Zn + H2SO4 → ZnSO4 + H2
Method 2:
Metal oxide + dilute acid → Salt + Water
CuO + H2SO4 → CuSO4 + H2O
Method 3:
Metal carbonates + dilute acid → Salt + Water + Carbon dioxide
CuCO3 + HCl → CuCl2 + H2O + C2O
Method 4:
Acid + Base → Salt + Water
Hcl + NaOH → NaCl + H2O
Using metal to make salt (Method 1)
Metal + Acid → Salt + Hydrogen
Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen
ZnSO4 + H2SO4 → ZnSO4 + H2
Experiment
(1) Pour acid into the beaker.
(2) Add metal to the acid in the beaker.
(3) Once the mixture stops fuzzing (it means the reaction has stopped), pour it into an evaporating dish. Heat the dish very gently until crystals start forming at the edge of the solution.
(4) Remove from heat. Leave it for a few days to form crystals.
Safety tip: Be careful when heating the evaporation dish as the solution can spit and burn you.
Using metal oxide to make salt (Method 2)
- Some metals won’t react with acids to make salt (copper, silver → too unreactive to displace hydrogen)
- Use another method (using their metal oxide)
* metal oxides = base oxides *
Metal Oxide + Acid → Salt + Water
Copper Oxide + Sulfuric Acid → Copper Sulfate + Water
CuO + H2SO4 → CuSO4 + H2O
* copper sulfate → blue
* copper chloride → green
Experiment
(1) Pour acid into the beaker. Add metal powder into the acid in the breaker.
(2) Heat the mixture gently. (don’t boil as harmful fumes may be given off or the acid might split) Keep stirring all the time.
(3) Keep adding more copper oxide and stirring until there are a few powders left unreacted. When all the acids are reacted, turn off the heat. Allow the mixture to cool down. (the hot mixture might tear up the filter paper)
(4) Filter the mixture. Pour the filtrate into an evaporating basin.
(5) Heat the basin very gently until crystals start forming at the edge of the solution. Remove it from the heat. Leave it for a few days to form crystals.
Using metal carbonates to make salts (Method 3)
Metal Carbonate + Acid → Salt + Water + Carbon Dioxide
Calcium Carbonate + Sulfuric Acid → Calcium Sulfate + Water + Carbon Dioxide
CaCO3 + H2SO4 → CaSO4 + H2O + CO2
Calcium Carbonate + Nitric Acid → Calcium Nitrate + Water + Carbon Dioxide
CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2
Experiment
(1) Pour acid into the beaker.
(2) Add a spatula of metal carbonate (no need to heat or stir). Keep adding more metal carbonate until it stops reaction. There should be a small amount of unreacted metal carbonate left in the beaker. (to make sure that all acid are reacted)
(3) Filter the mixture (unreacted metal carbonates will be trapped in the filter paper).
(4) Pour the filtrate into an evaporating basin. Heat it gently.
(5) Stop heating when crystals start forming at the edge of the solution. Leave the solution for a few days to cool and evaporate slowly.
When we don’t know the concentration of something → use a titration method.
Titration = the process of determining the quantity of a substance A by adding measure increments of substance B, with which it reacts until chemical equivalence is achieved
Equipments needed for titration:
- Pipette (unknown concentration, known volume)
- Burette (known concentration, unknown volume)
- Conical flask
- Ring stand
Experiment
(1) Place some acid in a burette.
(2) Take an exact volume of alkali in a pipette and put it in a conical flask.
(3) Add a few drops of universal indicator into the flask. (the solution color would change to blue)
(4) Add the acid from the burette drop by drop slowly. Swirl the flask as you add the acid.
(5) Stop adding acid when you’ve produced a neutral solution. (the solution turns green)
(6) Add some activated charcoal powder to the green solution and mix it with a glass rod. (to decolorize the solution)
* activated charcoal powder → absorbs coloring matters from substances
(7) Filter the mixture.
(8) Pour the filtrate into an evaporating dish. Heat it gently.
(9) Stop heating when crystals start forming at the edges of the solution. Leave it for a few days to evaporate slowly and form crystals.
Indicators - show how acidic or alkaline a substance
(1) Universal Indicator
Strongly acidic → red (pH 1)
Weakly acidic → yellow (pH 4)
Neutral → green (pH 7)
Weakly alkaline → blue (pH 10)
Strongly alkaline → purple (pH 13)
(2) Litmus
Acid → red
Neutral → purple
Alkaline → blue
(3) Phenolphthalein
Acid → colorless
Neutral → colorless
Base → purple
(4) Methyl orange
Acid → red
Neutral → yellow
Alkali → yellow
