In this post, let’s talk about salts as tested in O Level Chemistry. Check out our other posts on acids and bases which are very much related to this post on salts.
In this post, we will be covering these topics. Click on the link below to go straight to the topic.
- What are salts
- Methods of salt formation
- Method 1: Precipitation
- Method 2: Reaction between insoluble substance and acid
- Method 3: Titration
- More on acids, bases and salts
What are salts?
A salt is the ionic compound formed from the reaction between an acid and a base. The acid forms the anion portion of the salt, while the base forms the cation portion of the salt.
Recall the 3 reactions of acids which we talk about? They all result in the formation of salt as one of the products. Hence, in this topic on salts, some of the salt formation will be via this method.
Methods of salt formation
There are many ways to form salt. These very much depends on the solubility of the salt and their starting material. In O Level Chemistry, students learn 3 methods of salt formation:
- precipitation
- Reaction between insoluble substance and acid
- Titration
Let’s talk about when we use each method of salt formation.
Method 1 of salt preparation: Precipitation
This method is only suitable when the salt is insoluble e.g. lead(II) chloride, silver iodide, etc.
To form a salt from precipitation, start from 2 soluble substances containing the ions of the salt that you want. Since the salt formed is the only insoluble substance, it could be separated from the rest by filtration, followed by washing with distilled water to remove any impurity.
Method 2 of salt preparation: Adding excess insoluble substance to the acid
In method 2, the salt formed is a soluble one. This salt is also not a Group I salt, or ammonium salt, (else their carbonates, oxides and hydroxides are soluble). Hence, the bases and carbonates are insoluble and could be used to from this soluble salt.
To prepare the salt via this method, we will start of by adding excess carbonate, oxide, hydroxide (or even metal provided it reacts and also isn’t too reactive) to the acid. Excess carbonate, oxide or hydroxide is required to ensure all the acid react. The excess solid can be easily removed via filtration but not the excess acid. Hence, the solid must be in excess.
Next, filter off the excess solid reactant, and you will have a pure solution of the salt.
To obtain the salt in solid form, crystallisation is used. In crystallisation, the solution is heated to remove some of the water (solvent) to obtain a hot saturated solution. This hot saturated solution is then cooled (and solubility decreases). The salt is precipitated out. We’ll wash the salt with cold distilled water and dry them between filter paper.
Method 3 of salt preparation: Titration
The third method to prepare salt is via titration. Titration is used for soluble salts, and specifically for salts where both its starting materials and final products (i.e. the salt) are soluble in water. This method is used for Group I salts (e.g. sodium, potassium salts), and ammonium salts.
In this method, students will need to pipette (e.g. 25 cm3) of acid into a conical flask. Add an indicator (e.g. methyl orange) to the solution in the conical flask. Next, fill the burette with the alkali. Next, add the alkali dropwise until methyl orange turns orange, and note the volume of alkali added.
Repeat the entire experiment again, without the indicator, but adding the same amount of acid and alkali as the previous experiment. Now, you’ll get a pure solution of the salt. To obtain a pure salt (in solid form), crystallisation is used. However, if the salt is sodium chloride, you can heat to dryness (or evaporate the solution) to obtain the pure salt, since sodium chloride is stable to heat and will not decompose on heating.