Obtaining Pure Salt from Rock Salt

Introduction

Rock salt is mined from underground deposits and spread on icy roads in winter. It looks grey or brown because it is not pure: it is a mixture of sodium chloride (common salt) with sand, clay and small pieces of rock. Chemists can turn this dull mixture into clean white salt crystals using three separation methods one after another. This page shows how dissolving, filtration and crystallisation work together, and why the order matters.

Core explanation

The key difference. Every separation relies on a difference in properties between the parts of a mixture. In rock salt the useful difference is solubility in water . Sodium chloride (NaCl) is soluble: about 36 g dissolves in 100 g of water at room temperature. Sand (mainly silicon dioxide, SiO₂) and clay are insoluble, so they stay as solid grains however long you stir.

Stage 1 — grind and dissolve. The rock salt is crushed into small pieces so that a larger surface area is in contact with the water. It is then stirred into warm water. The salt dissolves and forms salt solution; the sand and clay do not dissolve and remain suspended or settle at the bottom. Warm water speeds up dissolving, because the particles move faster and collide with the crystal surface more often.

Stage 2 — filter. The mixture is poured through filter paper in a funnel. The tiny water particles and dissolved sodium and chloride ions pass easily through the holes in the paper, so the filtrate is clear salt solution. The larger grains of sand and clay are trapped, forming the residue . At this point the insoluble impurities have been removed, but the salt is still dissolved.

Stage 3 — crystallise. The filtrate is warmed gently in an evaporating basin to drive off some of the water, until the solution is close to saturated. It is then left to cool and stand, so that crystals of sodium chloride grow as more water evaporates slowly. The crystals are separated from the remaining liquid and dried at a low temperature, for example on filter paper or in a warm place.

Why not simply heat to dryness? Boiling all the water away does recover salt, but any soluble impurities in the rock salt end up mixed with it, and strong heating can make the solid spit out of the basin. Slow crystallisation gives purer, well-formed crystals, because soluble impurities present in small amounts tend to stay dissolved in the remaining liquid.

Order matters. Filtration must come before crystallisation. If you evaporated the water first, the sand would still be mixed with the salt and there would be no easy way to separate two dry solids of similar grain size.

Step-by-step reasoning

To plan the purification of any mixture like rock salt:

1. List the substances present and a property of each (soluble or insoluble in water). 2. Dissolve so that the wanted substance goes into solution. 3. Filter to remove the insoluble substances as residue. 4. Crystallise the filtrate to recover the dissolved solid. 5. Dry the crystals gently.

Visual explanation

Imagine four pictures in a row. First, a beaker of cloudy brown liquid with grit swirling in it. Second, a funnel with a cone of filter paper: brown sand trapped in the paper, clear liquid dripping into a flask below. Third, an evaporating basin of clear solution steaming gently. Fourth, a watch glass holding small white cubic crystals.

Real-world analogy

Getting salt out of rock salt is like rescuing sugar cubes that have fallen into a sandpit. Dropping everything into a jug of water dissolves the sugar, pouring it through a fine sieve catches the sand, and letting the sweet water dry out on a plate leaves the sugar behind.

Real-world example

In "solution mining", water is pumped down into underground salt beds. The salt dissolves, and the resulting brine is pumped to the surface, leaving most insoluble rock underground. The brine is then purified and evaporated in large vessels to produce table salt and salt for the chemical industry.

Why?

Why does filtration separate salt from sand only after dissolving? Filter paper sorts particles by size. Solid salt crystals and sand grains are both too big to pass through, so both would be trapped together. Dissolving breaks the salt into individual ions that are far smaller than the holes, so only the salt passes through.

Common misconception

"The salt is caught in the filter paper because it is a solid." Once dissolved, salt is no longer a solid in the mixture — its particles are spread through the water and pass straight through the paper. The residue contains only the insoluble material.

Worked example

Question: A student dissolves 20.0 g of rock salt in water, filters it and dries the residue, which has a mass of 3.0 g. Estimate the percentage of salt in the rock salt, assuming all the soluble material is salt.

Reasoning: Mass of soluble material = 20.0 − 3.0 = 17.0 g. Percentage = (17.0 ÷ 20.0) × 100 = 85%.

Answer: About 85% salt.

Quick check

1. In the rock salt method, what is the filtrate? Answer: Clear salt solution — water with dissolved sodium chloride.

Exam focus

Examiners often ask you to describe this method in order. Name each step (crush, dissolve in warm water with stirring, filter, heat gently to concentrate, leave to crystallise, dry the crystals) and state the reason for each: solubility for dissolving, particle size for filtering, and evaporation of the solvent for crystallisation.

Advanced insight

Even after crystallisation, salt from rock salt contains small amounts of other soluble salts such as calcium and magnesium compounds. Industrial producers remove these by adding chemicals that turn them into insoluble solids, which can then be filtered off. Repeating crystallisation — recrystallisation — gives still higher purity, at the cost of losing some salt in the leftover liquid each time.

Summary

Rock salt is a mixture of soluble sodium chloride with insoluble sand and clay. Crushing and dissolving in warm water puts the salt into solution. Filtration removes the insoluble material as residue and gives salt solution as the filtrate. Gentle evaporation followed by slow crystallisation recovers pure salt crystals, which are then dried. The order of steps is essential.

Practice questions

1. Why is the rock salt crushed before dissolving? Answer: Crushing increases the surface area in contact with the water, so the salt dissolves faster. 2. Name the residue and the filtrate when a rock salt solution is filtered. Answer: The residue is sand and clay (insoluble impurities); the filtrate is salt solution. 3. Why is crystallisation preferred to heating the filtrate to dryness? Answer: Slow crystallisation gives purer crystals because soluble impurities tend to stay in the remaining solution, and it avoids spitting of the hot solid. 4. Explain why the steps must be done in the order dissolve, filter, crystallise. Answer: The salt must be dissolved so that it can pass through the filter while sand is trapped; only then can the salt be recovered from the filtrate. Evaporating first would leave salt and sand mixed.