Problem 99 - Chemistry The Molecular Science

Question

You are given a mixture of sand, salt (sodium chloride), and naphthalene, a white solid that is not soluble in water. Write a detailed description of a procedure that would completely separate each component from this mixture.

Step-by-Step Solution

Answer

Dissolve salt in water, filter to remove sand, evaporate to get salt, and sublime to separate naphthalene.

Step 1: Dissolution of Salt in Water

Begin by adding the mixture to a beaker containing warm water. Stir the mixture well to ensure that the salt (sodium chloride) dissolves completely in the water. The sand and naphthalene will not dissolve because sand is insoluble, and naphthalene is not soluble in water.

Step 2: Filtration to Separate Sand

Pour the mixture through a filter paper that is placed in a funnel. The sand particles will be retained on the filter paper, while the salt solution (salt dissolved in water) and naphthalene will pass through. Collect the sand and allow it to dry.

Step 3: Evaporation of Water

Heat the salt solution to evaporate the water. This process will leave behind the dissolved salt. Continue heating until all the water has evaporated, ensuring that only salt remains.

Step 4: Sublimation of Naphthalene

Place the remaining mixture (naphthalene and salt, after water removal) in a sublimation apparatus. Heat the mixture gently — naphthalene, which sublimes at a relatively low temperature, will transform directly from solid to gas. Cool the vapor to condense it back into a solid, collecting the pure naphthalene.

Step 5: Collect and Verify Each Component

After sublimation, ensure that you have collected all three components separately: dry sand from the filtration, dry salt from the evaporation, and solid naphthalene from sublimation.

Key Concepts

Dissolution

Dissolution is the process of a solid, liquid, or gas becoming incorporated into a liquid to form a solution. In the context of our mixture, we are primarily focused on dissolving the salt, which is soluble in water. By adding warm water to the mixture containing sand, salt, and naphthalene, only the salt dissolves because it is the only component that is water-soluble. Stirring helps facilitate this process by increasing the interaction between the water molecules and salt, speeding up the dissolution. As a result, sand and naphthalene remain in their solid forms while salt forms a homogeneous saline solution.

Filtration

Filtration is a mechanical or physical process used to separate solid particles from a liquid. After dissolving the salt in water, the next task is to separate the insoluble sand. This is accomplished with the help of filter paper placed inside a funnel. When our mixture is poured onto the filter paper, only the liquid part, the salt solution, passes through, while the sand is left behind on the filter. This separation works because filter paper has tiny pores that allow liquid to pass but trap larger solid particles. As a result, we achieve the successful separation of sand from the salt solution and naphthalene.

Evaporation

Evaporation is the process of turning a liquid into vapor and is used to separate a dissolved solid from a solution. Once you have your filtered salt solution, heating it causes the water to evaporate, leaving the salt behind. During heating, the liquid phase (water) transitions into a gaseous form, gradually leaving behind the solid salt. Continue to apply heat until all the water has evaporated. What remains is dry salt, free from the liquid, effectively separating it from the initial solution.

Sublimation

Sublimation is a process where a solid changes directly into a gas without passing through the liquid phase. This property is unique to certain substances, such as naphthalene. After removing the water and leaving the salt behind from the previous step, you are left with naphthalene mixed with salt. Using sublimation, naphthalene is gently heated in a sublimation apparatus to transform it into vapor, skipping the liquid phase entirely. The vapor is allowed to cool and change back into a solid, free from salt. This enables the recovery of pure naphthalene, completing the separation of all mixture components.