Purifying crude salt

01

Level the balance first. Line both pans with matching paper. Put crude salt on the left and weights on the right to prepare about 5 g.

This 5 g is the prepared sample, not an amount that must all be added to the water.

02

Stand the measuring cylinder upright on a level surface. Measure about 10 mL of water, then transfer it to the beaker.

Read at eye level with the lowest point of the concave meniscus.

03

Add a small portion of crude salt, then stir gently with the glass rod. Watch the white salt grains shrink while the sand remains.

Avoid striking the sides or bottom of the beaker with the rod.

04

Stop adding when no more salt dissolves. Keep and weigh the unused sample. Subtract it from the starting mass to find the amount added.

Undissolved sand alone does not mean the brine is saturated. Check whether salt grains continue to dissolve.

05

Fold the circular paper into a semicircle, then fold again. Open one side to form a cone with one layer on one side and three on the other.

Identify the thicker, triple-layer side. The glass rod will rest gently against it.

06

Place the paper in the funnel and wet it with a little water. Fit it against the inner wall and release trapped air bubbles.

A good fit helps filtration. Air bubbles mainly slow the flow; they do not necessarily make the filtrate cloudy.

07

Support the funnel securely in the iron ring. Keep the paper edge below the funnel rim and the liquid below the paper edge.

Position the receiving beaker so the funnel stem touches its inner wall.

08

Keep the spout against the rod, the rod against the triple paper layer, and the funnel stem against the receiving beaker’s inner wall. Pour slowly, retaining sand on the paper.

The filtrate should be clear. If it is cloudy, check for torn paper, an overflowing liquid level or a dirty receiving beaker.

09

Slowly pour the clear filtrate into the evaporating dish. Keep the liquid below two-thirds of the dish’s capacity.

Two-thirds refers to volume, not two-thirds of the dish’s height.

10

Secure the dish in the iron ring. Heat it with the spirit lamp while stirring continuously. Water decreases and salt begins to crystallize.

Stirring distributes heat and reduces splashing. Water vapour is invisible; watch the falling liquid level and crystals.

11

When plenty of solid appears but the dish is still damp, cover the lamp with its cap. Leave the dish in place and let residual heat finish drying.

Do not keep heating until completely dry, and never blow out a spirit lamp.

12

Let the dish cool and lift it with crucible tongs. Use the glass rod to transfer the salt onto weighing paper, then weigh the recovered solid.

Record the measured mass. The salt is only partly purified; soluble impurities may remain. Recovery yield is not the same as purity.

Experiment steps

Weigh the crude salt

Follow the materials through the separation.

Dissolved salt is invisible to the eye.

Paper retains solid particles; brine passes through.

As water evaporates, salt crystallizes.

Each action is available independently. Select a step heading to inspect it on the laboratory stage.