The separation of sand and salt is a classic demonstration of using differences in physical properties to isolate individual components. This process relies on dissolution, exploiting the fact that one substance will dissolve in a liquid while the other will not. The method utilizes the distinct properties of solubility and particle size to recover both the solid sand and the crystalline salt.
The Role of Water
Introducing a solvent that interacts with the salt but not the sand is the first step. Water is the ideal choice because salt (sodium chloride) readily dissolves in the polar water molecules, forming a homogeneous solution. Sand (silicon dioxide) is insoluble and remains as a solid suspension.
To ensure high recovery, the mixture should be thoroughly stirred for several minutes. Applying gentle heat can accelerate the rate at which the salt dissolves. The liquid phase now contains the dissolved salt, while the solid sand particles are suspended, preparing the mixture for the next stage.
Removing the Sand
Since the sand remains solid and the salt is dissolved, the next step is filtration. This technique separates the insoluble solid from the liquid. A funnel lined with a porous medium, such as filter paper, is positioned over a clean receiving container.
The filter paper acts as a sieve, trapping particles larger than its pore size. As the suspension is poured into the funnel, the large grains of sand are held back on the filter medium. The liquid, called the filtrate, passes through the paper and collects below.
For best results, the mixture should be poured slowly down a stirring rod to prevent splashing. Once the liquid has passed through, the sand remains as a residue on the filter. Filtration achieves a much cleaner separation than decanting, which involves pouring the liquid off after the sand settles.
Retrieving the Salt
The final stage requires recovering the solid salt from the clear saltwater solution. This is achieved through evaporation, a phase change where the liquid solvent converts into a gas, leaving the solid solute behind. The saltwater is transferred to a shallow, open container, such as an evaporating dish.
The solution can be heated gently over a heat source, which increases the rate of evaporation by supplying energy to the water molecules. As the water gains thermal energy, it transitions into steam, escaping the container and leaving the non-volatile salt behind. Care must be taken during the final moments of heating, as the remaining salt can sometimes splatter.
Alternatively, the container can be left uncovered in a warm, dry area for passive evaporation. As the water slowly evaporates, the salt concentration increases until the solution becomes supersaturated. This causes the sodium chloride to crystallize and precipitate out, leaving the recovered white crystalline residue.