Video Transcript
A mixture of sand and sodium chloride is commonly known as rock salt. Which of the following shows the correct order needed to separate sodium chloride from rock salt? (A) Dissolution in water, filtration, crystallization. (B) Filtration, dissolution in water, distillation. (C) Filtration, crystallization, dissolution in water. (D) Crystallization, dissolution in water, filtration. Or (E) dissolution in water, crystallization, filtration.
To assist in determining the correct order of operations needed to separate sodium chloride from rock salt, let’s briefly define each of the four processes mentioned in the answer choices. Dissolution in water is when a solid solute dissolves into liquid water, forming a solution. Distillation is a separation technique that can be used to separate two miscible liquids that have different boiling points. Crystallization is a separation technique that can be used to separate a dissolved solid, also known as the solute, from a liquid, also known as the solvent. And finally, filtration is a separation technique that can be used to separate an insoluble solid, or insoluble solute, from a liquid, or from the solvent.
Switching to our rock salt sample, which according to the question is composed of sand and sodium chloride, it is important to note that the sample consists of two substances that are both in the solid phase. That being said, in general, both filtration and crystallization are separation techniques for mixtures containing a solid solute and liquid solvent, or for solid–liquid mixtures. The fact that neither of the components of rock salt are in the liquid phase means filtration and crystallization cannot be the first step in separating sodium chloride from rock salt. Therefore, answer choices (B), (C), and (D) which either list filtration or crystallization as the first step in the separation process can be eliminated at this point.
This means answer choices (A) and (E) are both options because the first step is to add water to the rock salt sample. This is because the Na+ and Cl− ions that make up sodium chloride dissociate in water, making sodium chloride soluble in water while sand is not. Therefore, adding water to rock salt creates a solid–liquid mixture. With the next part of the separation process of the solid–liquid mixture being either filtration in answer choice (A) or crystallization in answer choice (E), at a quick glance, it would seem that since both techniques can be used to separate solid–liquid mixtures, both techniques can be applicable second steps for this separation process.
However, one key difference is that the solid must be insoluble in the liquid for filtration to work and a dissolved solid, or soluble solid in a liquid, is when crystallization is used. While the sodium chloride is a dissolved solid in a liquid and might make one decide crystallization comes next in the process, the sand, or insoluble solid, would prevent crystallization from being a success, since all of the solids are not soluble. Therefore, we can eliminate answer choice (E). This means answer choice (A) must be the correct answer. To confirm this, let’s discuss how filtration followed by crystallization makes sense as the second and third step in the separation process.
Filtration would allow for the larger insoluble sand particles to remain in the filter paper, while the water and dissolved sodium chloride ions would pass through the filter paper and collect in the Erlenmeyer flask. This will result in the mixture containing only one soluble solid, the sodium chloride, in a liquid. This means by definition, one could use either evaporation or crystallization to separate sodium chloride from the water. Because both processes can involve heating this type of mixture until the water has evaporated off, leaving the solid sodium chloride behind and completely separated from the original rock salt sample.
So which answer choice shows the correct order needed to separate sodium chloride from rock salt? The answer is option (A) dissolution in water, filtration, crystallization.
