Elements, compounds and mixtures + separation techniques

Crystallisation (what is it used for? Method?)

Used to obtain ONLY the solute from a solution.

Heat the solution on a Bunsen burner until it bubbles and crystals start to form, then turn it off. Leave the solution to cool slowly until all crystals have formed. The filter to remove excess liquid and leave crystals to dry completely (or use kitchen towel or blotting paper to dry crystals)

Filtration (what is it used for? Method?)

Used to separate insoluble solid and liquid from a mixture.

Use filter paper on a funnel on a beaker and pour mixture into it. The residue stays on the filtration paper, while the liquid (filtrate) passes through the paper as they are small enough to pass.

Element

A pure substance made of only one kind of atom and cannot be split into anything simpler by chemical means

Compound

A substance made up of atoms of two or more different elements chemically bonded

Mixture

A combination of two or more different elements and/or compounds that are not chemically combined

How do we know when a substance is pure?

Pure substances have a fixed boiling and melting points, whereas mixtures boil or melt over a range of temperatures.

Method to obtain pure salt from rock salt

- Crush rock salt with pestle and mortar

- Mix it in hot water so salt dissolves but impurities don't

- Filtrate it so the residue/impurities stays on filter paper while the filtrate passes.

- Salt solution (filtrate) is crystallised (see instructions for how to perform crystallisation)

Simple distillation (what is it used for? Method?)

Used to obtain a solvent from a solution.

A solution is heated so the solvent evaporates and the vapor is immediately cooled and condensed by the cold water in the condenser to obtain the liquid (distillate) that is collected in the beaker.

After all the water is evaporated from the solution, only the solid solute will be left behind

Fractional distillation (what is it used for? Method? Example)

Used to separate mixtures of liquids with different boiling points

Mixture is heated up to the temperature of the substance with the lowest melting point so that it evaporates toward the fractional column and then immediately cooled and condensed by the condenser, turning into a liquid that is collected in a beaker

Ethanol + water.

What are the there posible sources of heat for fractional distillation? Why can't we use a Bunsen burner when dealing with a mixture of ethanol and water?

Bunsen burner, water bath and electrical heater.

Because ethanol is flammable.

What is the job of the fractionating column? What are the glass beads for?

When the mixture is heated up, some liquids with a higher melting point may also start to evaporate. But the column is cooler at the top so they won't be able to go all the way up before they condense and run back to the flask.

They increase surface area for better separation of two vapours.

Separation of ethanol and water: ethanol produced is about 96% pure, it is impossible to remove last 4% of water

What are the 3 options as sources of heat used in fractional distillation (only ONE can be correct at a time)? Why can't we use a Bunsen burner when separating ethanol from water?

Bunsen burner, electric heater, water bath.

Because ethanol is flammable.

Chromatography (When do we use it? How does it work + conclusions?)

Used to separate a mixture of usually inks or food colourings to see its composition.

The mobile phase (water) flows through he stationary phase (chromatography paper) and carries the components of the mixture with it. If there are different components, these will travel at different rates as their solubility is different. Insoluble substances will stay on the pencil line.

Describe how you would carry out chromatography (4]

- Pour solvent into beaker

- Draw start/base line in pencil above the solvent and place dyes/ink on top of the pencil line

- Leave the paper until the solvent is almost at the top of the paper

- Leave paper to dry

State 2 things that should be the same in two experiments so that results can be compared fairly

- Same solvent

- Same type of chromatography paper

ALLOW reference to use of pencil for start line/spots must start on horizontal line/solvent must be below line of spots

ALLOW same distance travelled by solvent

Why do we draw a pencil line instead of ink? Why must the pencil line sit ABOVE the level of the solvent?

Because the ink would also dissolve and travel up the paper, affecting the results.

So the samples don't wash into the solvent container/dissolve into the water.

Why is a lid used in chromatography?

Stops solvent evaporating

MAYBE toxic, or risk of fire (esto si te especifica que es flammable)

Difference between chromatogram and chromatography paper

Paper chromatography is the name given to the overall separation technique while a chromatogram is the name given to the piece of chromatography paper with the visibly separated components after the run has finished (results)

Rf value equation

Rf = distance travelled by substance / distance travelled by solvent