elements, compounds and mixtures

what is the most common element in the air 78%

nitrogen

what is a compound

substance that is made of two or more different elements which are chemically bonded together

what is a mixture

made of two or more substances that haven’t been chemically bonded

what is a pure substance

a substance is pure if its completely made up of a single element or compound

what do pure substances have vs what happens if its not pure

has a specific, sharp melting and boiling points —- if its not pure the melting or boiling point will vary (increass boiling) over a range of temperatures

what is fractional distillation used for

to separate a mixture of liquids

how does fractional distillation work

different liquids have different boiling points, liquids with lowest boiling points evaporate first, but the column is cooler towards the top, so they condense and run into a flask

what is simple distillation used for

to separate out solutions - A liquid from the solution.

how does simple distillation work

the part of the solution with the lowest boiling points evaporates, the vapour is then cooled, condensed and collected.

what is a problem with simple distillation

only used to separate things with a very different boiling points evaporate, otherwise, have to use fractional distillation

example of simple distillation

to get pure water from sea water

why is a thermometer used in simple distillation

to make sure the highest boiling point is not exceeded, both would boil and they wouldn’t separate

what is filtration used for

used to separate an Insoluble Solid from a liquid

how does filtration work

put filter paper into a funnel and pour mixture into it

what is crystallisation used for

to separate a Soluble solid from a solution

how does crystallisation work

• pour the solution into a evaporating disk and heat the solution

• some of the water will evaporate so the solution will get more concentrated

• when crystals start to form remove dish

• filter out crystsls from solution

how would you use filtration and crystallisation to separate rock salt (salt + sand]

1. grinding - w pestle and mortar

2. dissolving - in a beaker and stir

3. filtering - filter paper + funnel

4. crystallisation - evaporate in dish

method for chromatography

1. use a pencil to draw a horizontal line near the bottom of the filter paper

2. add spots of ink evenly spaced next to the unknown substance

3. place the paper in the beaker

4. make sure level of solvent is below baseline (dont want ink to dissolve)

5. place a lid to stop the solvent from evaporating

6. each different dyes in the ink will move up the paper at a different rate and form a spot at a different place

why do we use pencil to mark the baseline

pencil marks are insoluble so they wont dissolve in the solvent

how does chromatography separate mixtures

3 reasons

1. different dyes move up the paper at different rates

2. some will stick to the paper while others dissolve better and travel quicker

3. the distance depends on the solvent

what is the end result of chromatography

chromatogram - used for Rf values

what do we call a pure substance thats run next to a tested substance to see its a component in the mixture

reference substance SRMS (standard)

why is it incorrect to say that a reference substance

two different substances can have the same Rf value