Back to C4 Home
C4 E) Obtaining a Pure Salt
C4 E) Obtaining a Pure Salt
We can obtain a pure salt by reacting an acid and a suitable insoluble base. The insoluble base may be a metal oxide, a metal hydroxide or a metal carbonate.
I am going to look at the reaction of copper oxide (an insoluble base) with hydrochloric acid. The reaction here is a metal oxide and an acid, which will produce a salt and water. Hydrochloric acid produces chloride salts, and the metal used was copper. This means that the salt produced will be copper chloride. The word and chemical equations are shown below.
I am going to look at the reaction of copper oxide (an insoluble base) with hydrochloric acid. The reaction here is a metal oxide and an acid, which will produce a salt and water. Hydrochloric acid produces chloride salts, and the metal used was copper. This means that the salt produced will be copper chloride. The word and chemical equations are shown below.
We are now going to look at the process for obtaining a pure sample of copper oxide salt.
The first step is to gently warm the acid with a Bunsen burner. This is so that the particles in the acid have enough energy to react when we add the copper oxide. After the acid becomes warm, we turn the Bunsen burner off.
We then add powdered copper oxide a little bit at a time to the acid and stir the mixture. We keep adding powdered copper oxide until the copper oxide is in excess, which is when all of the acid has reacted – we will know when the copper oxide is in excess because the solid copper oxide that we add will remain unreacted and be clearly visible in the beaker.
The next step is to filter the mixture to remove the excess solid copper oxide. We filter the mixture by pouring the mixture through a funnel that is lined with filter paper. The solid copper oxide will be unable to pass through the filter paper and will therefore sit on the filter paper. The filtrate (the liquid that passes through the filter paper) will contain water and the copper chloride salt.
The final step is to use crystallisation to obtain crystals of the copper chloride salt. We undertake crystallisation by pouring our mixture/ filtrate into an evaporating dish and gently heating the solution using a Bunsen burner, which causes the water to evaporate. After some time, we will notice crystals starting to form in the solution; this is known as the point of crystallisation. When this happens, we remove the evaporating dish from the heat and leave the solution somewhere to cool. The salt will start to form larger crystals as the salt becomes insoluble in the cool more concentrated solution (the solution is now more concentrated because some of the water has evaporated). We then filter the crystals out of the solution and leave the crystals in a warm place to dry. We can speed up the drying process by placing the crystals in a drying oven or a desiccator.
We now have pure dry crystals of the copper chloride salt.
The process would be essentially the same if we were trying to obtain another salt from an insoluble base.