Separation Techniques: Crystallization

This method is usually used to separate dissolved solids (solute) from a solution, or in other words, to separate the heat-liable solutes from their solutions. Crystallization not only prevents the solute from decomposing but also, most soluble impurities would be left behind.

Crystallization is commonly used to obtain pure sugar in industries and to obtain very pure silicon which is used in computer chips. To begin, you will need a saturated solution, meaning a solution that contains the maximum amount of solute dissolved in a solvent at a particular temperature.

Real-life scenario...

Let's say you and your business partner are going to start a new crystal factory. Your partner insists that rapid cooling would be better for the company since production would be faster. However, you think that larger and more nicely shaped crystals are better for selling. Hence, you need to prove to your partner that the crystals would indeed be larger and more nicely shaped. So you decided to conduct an experiment with hydrated copper (II) sulphate.

Hypothesis: Slow-cooling would form better, larger and nicer-looking crystals

Process...

Step 1: Heat about 20cm^3 of water in a beaker. Stop heating and remove from the tripod stand once bubbles are observed in the ater or when the water boils

Step 2: Add one spatula of copper (II) sulphate to the hot water

Step 3: Stir the mixture until all the copper (II) sulphate can be dissolved

Step 4: Repeat step 3 until no more copper (II) sulphate can be dissolved

Step 5: Filter the solution if there are any solid impurities

Step 6: Heat the copper (II) sulphate solution in an evaporating dish

Step 7: Stop heating when about half the solvent has evaporated from the solution. DO NOT heat to dryness. If a crust is formed on the surface of the solution, stop heating and add a some distilled water to the solution, stir to redissolve the crystals to form the crust

For those doing slow cooling...

Step 8: Pour the solution into a clean small boiling tube and allow it to cool and measure the time taken for crystals to appear

Step 9: Collect the crystals and dry on filter paper

Step 10: Observe the crystals formed and compare with rapidly cooled crystals

For those doing rapid cooling...

Step 8: Pour the solution into a clean small boiling tube and cool it in ice-water and measure the time taken for crystals to appear

Step 9: Collect the crystals and dry on filter paper

Step 10: Observe the crystals formed and compare with slowly cooled crystals


Observations...

Conclusion...

Crystals formed by rapid cooling are of smaller size and are more irregular in shape. To obtain more crystals we can actually a) Add more water to dissolve more copper sulphate or b) apply slow cooling instead or rapid cooling.

And that is how the story of your very successful crystal company started...

Separation Techniques: Distillation

Distillation is used to separate the solvent from a solution of solutes. It can also be used to purify water. In fact, many industries use distillation to manafacture clean water, hence the name distilled water.

How to carry out simple distillation:

Step 1 - Set up the bunsen burner

Step 2 - Clamp the distillation flask in position

Step 3 - Place the three way connector into the distillation flask

Step 4 - Place the thermometer into the top of the distillation flask

Step 5 - Place the receiving flask

Step 6 - Attach the tubing to the water inlet and water outlet condenser

Step 7 - Ensure all joints are not under stress

Step 8 - Turn on cold water supply to the condenser (and check for water leaks)

Step 9 - Add the liquid to your distillation pot and then the boiling stones to ensure smooth boiling

Step 10 - Start heating the liquid and collect product in the flask

How does it work?

As the temperature rises, substances with a lower boiling point rises first, allowing us to isolate different components of the mixture. Since different substances have different boiling points, and by using the thermometer to take accurate temperature readings, we can determine which substance will be received by the flask. Once the vapor comes into contact with the cooler surface of the condenser, the substance then condenses into its liquid state and drips into the receiving flask.

Hence, distillation is based on the principle of evaporation and condensation.

What is it used for?

Distillation is used in Desalination plants such as NeWater, to obtain pure drinking water. It is also used in oil refineries to separate crude oil from petroleum.

e collecting at any given stage of the distillation as we do not want to be collecting any harmful substances

Separation Techniques: Paper Chromatography

There are several methods to separate substances from each other, such as: filtration, chromatography, distillation and fractional distillation, evaporation, crystallization, magnetization and the separation of immisicible liquids.

But today, we shall only focus on chromatography.

Paper chromatography is used to separate mixtures of solutes with different solubility and degree of absorption such as ink dyes or sugar mixtures.

How to carry out paper chromatography:

Step 1 - Draw a pencil line 2cm from the edge of the chromatography paper

Step 2 - Place a drop of the mixture on the line. Wait for awhile before dripping about 2 more times, this is to ensure there is a concentration of the mixture

Step 3 - Dip the chromatography paper in a suitable solvent, and make sure the pencil line does NOT touch the solvent

Step 4 - Observe the solvent "run" up the chromatography paper

Food coloring "running" up chromatography paper

Step 5 - Remove the chromatography paper from the solvent once it has reached the solvent front

Step 6 - Calculate the Rf value

What is Rf value?

Rf value stands for retention factor. It is the distance moved by substance over the distance moved by solvent. This means substances with a higher Rf value are less easily absorbed, and smaller Rf values means the substance is more stongly absorbed.

How does it work?

As the solvent travels up the paper, the dyes are dissolved. Some dyes are more soluble, hence they travel faster up the paper, whilst others are less soluble and are absorbed strongly on the paper. This is all based on the principle of solubility.

The difference of solubility of the substances allows separation. The more strongly absorbed substances  travel slower.

What is it used for?

Paper chromatography is used to separate and identify compounds in a mixture. It can also be used for testing the purity of substances.