(Part 3) Mixtures + Separation Techniques

What are the two kinds of mixtures and explain the difference between them

• Homogenous → all the parts of the mixture are in the same state

• Heterogenous → the parts of the mixture are in different states

Name these parts of a mixture

• A solution

• Soluble

• Insoluble

• Solute

• Solvent

• Aqueous solution

• Suspension

• A solution: a mixture of a solid dissolved in a liquid

• Soluble: a solid that can dissolve in water

• Insoluble: a solid that cannot dissolve in water

• Solute: the solid that dissolves

• Solvent: the fluid in which the solute dissolves in

• Aqueous solution: a solution in which the solvent is water

• Suspension: a mixture of fine insoluble solid particles suspended in a liquid

Explain how the particle size of an insoluble solid affects how it settles in a mixture

• Large particles → settle fast → therefore don’t spend much time as a suspension

• Small particles → take longer to settle → spend a long time as a suspension

What happens once the insoluble particles in a suspension settle to the bottom

It is no longer a suspension

Explain the steps to obtain Salt from Seawater

1. The salt pans are filled with seawater

2. The sun heats the rock, causing evaporation - water turns into a vapour and leaves the pan

3. As the water evaporated, salt crystals form by a process called crystallization

4. Because this process takes days, the salt crystals grow large because they are allowed to form properly

What can filtration be used for

To separate an insoluble solid from a liquid or aqueous solution

LABEL THE FILTRATION DIAGRAM

What do the terms ‘residue’ and ‘filtrate’ refer to

residue: The insoluble solid that remains on the filter paper
filtrate: The liquid or aqueous solution that passes through the filter paper

Explain why these precautions were taken while conducting filtration

•    A few drops of distilled water are added to the funnel before inserting the filter paper

• The mixture is poured slowly during filtration

• The residue is washed with distilled water

• The filter paper was wet with distilled water before filtration

• A few drops of distilled water are added to the funnel before inserting the filter paper → helps filter paper stay in place

• The mixture is poured slowly during filtration → Prevents spilling, ensures good filtration

• The residue is washed with distilled water → removes any soluble impurities/filtrate stuck behind to ensure purity

• The filter paper was wet with distilled water before filtration → avoids paper absorbing product, therefore allowing a higher yield to be collected

What is evaporation to dryness used for

to obtain a soluble solid from a liquid solvent

Explain what happens in this separation technique

In this separation technique a solution (e.g. salt + water) is poured into an evaporating dish and placed over a Bunsen burner until all the solvent (water) from the solution has evaporated and only small* salt crystals are left

Why are the crystals that are left behind small

because the rate of evaporation in this case was very fast. The slower the rate of evaporation, the larger the crystals formed because they would have more time to form properly

LABEL THE DIAGRAM FOR HEATING TO DRYNESS

Since this technique involves direct heating from a Bunsen burner, what should this method only be used for

should only be used for salts (compounds) that are thermally stable and therefore do not decompose on heating

What would happen if heating to dryness was used on a thermally unstable salt

it might break apart or change chemically before all the water evaporates – you won’t obtain the same salt dissolved in the water

What is crystallisation used for

To obtain a soluble solid from a liquid solvent

Explain how crystallisation works

• First, a saturated solution (has as much solute as possible) is heated gently using a water bath until some of the water has evaporated.

• The remaining solution is then allowed to cool. Since the temperature of the water is colder, it becomes less soluble (can withstand less salt dissolved in it).

• Therefore, the extra solute (that the water cannot dissolve anymore) comes out of the solution and forms crystals within the solution

• The remaining solution (the water + salt that can be dissolved) is super saturated

What is the difference between crystallisation and evaporation to dryness

They both achieve the same effect - but in crystallisation the crystals obtained are larger since they have more time to form properly

LABEL THE CRYSTALLISATION DIAGRAM

Name the three cases where crystallisation needs to be used instead of evaporation to dryness

1. A hydrated salt is required

2. The salt required is thermally unstable

3. Large crystals are required

Explain why if a hydrated salt is required crystallisation needs to be used

If you used evaporation to dryness on a hydrated salt, it would drive away all the water of crystallisation leaving you with the anhydrous version of that salt

 

Explain why if the salt required is thermally unstable crystallisation needs to be used

  if a thermally unstable salt is exposed to the direct heating of the Bunsen burner in evaporation to dryness it would decompose (you wouldn’t get the same salt back)

Explain why if large crystals are required crystallisation needs to be used

Crystallisation involves evaporating water at a slow controlled rate therefore large crystals are formed, while in evaporation to dryness, the heating process is quick, so small crystals are formed