Separating Mixtures

Refining Potash

(a simplified approach)

Background

• Potash is a major fertilizer for which there is a large world-wide demand especially in the growing economies of China and India.

• Saskatchewan has about 40% of the world’s potash reserves.

• Most of the potash ore is mined underground and brought to the surface as rock but there is one solution mine near Belle Plaine, between Regina and Moose Jaw, where the potash salts are dissolved underground and pumped to the surface to be refined.

• -a) Examine your Potash Ore. How many different substances can you see or find in your sample?

• -b) What properties helped you decide that the substances were different?

Materials

• 2 cups of water

• ½ cup of Potash Ore

• Pot

• Stove or Hot Plate

• Coffee filter and holder

• Coffee carafe or other clear container

• Stir spoon (slotted)

Procedure

• Put 2 cups of water and ½ cup of Potash Ore in the pot.

• Place the pot on the stove on high heat.

• Stir contents and bring to a boil.

• Shut off heat and continue to stir for several minutes (3 to 5 minutes) until the larger pieces have broken up.– c) What did you observe?

• Pour the mixture into the coffee filter keeping it stirred so that as little as possible is left in the pot.

• If you need to rinse the pot to filter more contents then use the clean filtrate (do not add extra solvent).– d) Describe the appearance of this first

residue.– e) Describe the appearance of this first

filtrate.

• Wash and dry the pot, place the filtrate in it, bring to a boil and simmer until the volume is reduced to less than half. (You can use a ruler to measure the height of the liquid inside the pot)

– f) What do you observe as the liquid evaporates?

• Filter the mixture.– g) Describe this second residue?– h) Describe the filtrate? What happens in the

clear filtrate as it drips into the carafe?

• Place the filtrate in the fridge to cool until next class and then filter it cold.– i) What happened in this second filtrate as it

cooled?

• Dry the three filters with their contents.– j) Describe the three dry residues?– k) What properties of the substances allowed you

to separate them into three fractions?– l) Are there any other substances still dissolved in

the last (third) filtrate? How could you find out?

Suggested Answers• a) You should be able to observe several

substances. One substance is red, another is gray and then there are large crystals that are shiny and clearer or whiter.

• b) The properties you used are colour, lustre (shinyness), and maybe hardness (did you try scratching the crystals). The ore tastes salty but you should not taste anything in the laboratory without the teacher’s permission.

• c) the mixture became very red and not clear.

• d) The residue is quite red and sometimes the gray material is visible.

• e) The filtrate is a clear, colourless liquid. Clarity and colour are two different properties.

• f) As the liquid evaporates a skin appears on the surface and where it splatters the spots dry to a white residue. Later there are a lot of white solid particles in the bottom of the pot.

• g) The residue is a white solid.• h) The filtrate is a clear, colourless liquid. As it

dripped into the carafe some white solid appeared in the filtrate.

• i) As the filtrate cooled more white solids appeared.

• j) The first residue is a mixture of a red solid (iron oxide) and a gray solid (clay). The second residue is white crystals (sodium chloride or table salt) and the third residue is also white crystals (potassium iodide or potash).

• k) The properties that allow us to separate this mixture are whether or not the substances are soluble and how soluble the dissolved solids are at different temperatures.

• l) There will still be some salt dissolved in the filtrate. We could find out by evaporating the water and crystallizing the remaining salt.