C2.1 - Purity And Separating Mixtures Flashcards
What are the masses of subatomic particles described by?
- Their relative masses.
- Their masses compared to the mass of a proton.
What is relative atomic mass?
Relative atomic mass, Ar, is the mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom.
What does a chemical formula show?
A chemical formula tells you how many atoms of each element there are in a unit of a substance.
What is relative formula mass?
Relative formula mass, Mr, is the mean mass of a unit of a substance compared to 1/12 the mass of a Carbon-12 atom.
What does an empirical formula show?
An empirical formula shows the simplest whole-number ratio of the atoms of each element in a compound.
What does a balanced chemical equation show?
A balanced chemical equation shows the formulae (how the atoms are rearranged) and the relative amounts of each substance involved.
What does pure mean in everyday life and how does it differ from the scientific sense of purity?
- Pure in everyday life describes natural
substances that have not been processed or
changed. - In science, a pure substance consists of just
one element or compound.
What is a mixture and are they pure or impure?
A mixture contains two or more different substances that are not chemically joined together, (two or more different compounds), so are therefore impure.
Why is it difficult to obtain pure substances?
- Just one atom or molecule of something else
makes the substance impure. - Substances can easily become contaminated such
as with the carbon dioxide from the air which itself
is a mixture of nitrogen, oxygen, and other
substances.
What is an alloy?
An alloy is a mixture of a metal with at least one other element.
What is the melting point of a pure substance?
The melting point of a substance is a single temperature.
What is the melting point of an impure substance?
- Its melting point is less that that of the pure
substance. - It often melts over a range of temperatures.
- Greater the difference between the measured
melting point and the accepted melting point, the
lower its purity is likely to be.
How can you determine melting point?
Heating the substance and:
- Measure the temperature at which it melts.
OR
- Measure its temperature at regular time intervals
and plot a temperature against time graph.
What are the two important things to do when determining the melting point of a substance and why (what does this improve)?
- Heating the substance slowly - Allows the
temperature of the whole sample to
increase. - Stirring the substance - Ensures the entire
sample is at the same temperature.
- These two actions improve the accuracy of a
measurement of the melting point of a sample.
What is a solution?
A solution is a mixture formed when one substance dissolves in another.
What is a solute and solvent?
- A solute is a substance that dissolves in a solvent.
- A solvent is a substance that can dissolve a
solute to form a solution.
What happens when a substance dissolves?
When a substance dissolves, its particles separate and become completely mixed with the particles of the solvent.
What is a soluble substance and what is an insoluble substance?
- A soluble substance is one that can dissolve in
the given solvent. - An insoluble substance is one that can’t
dissolve in the given solvent.
What is the purpose of filtration and why does it work?
- Filtration separates an insoluble substance in the
solid state from substances in the liquid state. - It works because filter paper has tiny, microscopic
holes where small molecules can pass through
and larger insoluble substances can’t.
What are the components in filtration and explain why scientists flute filter paper.
- Filter paper, funnel, residue, conical flask, filtrate.
- Scientists flute filtrate paper to produce a larger
surface area for the filtrate to pass through.
What is crystallisation?
Crystallisation is the process by which crystals are formed during evaporation of a solvent from a solution.
What are the steps for crystallisation and explain each process?
- Use a water bath to heat up the solution,
(filtrate - depending on context), until the
solution becomes saturated (when no more
solute can be dissolved at that temperature). - Crystals will start forming at this point.
- Let the solution cool slowly - because the
solution then decreases the solubility of the
solute which results in more crystals
forming. - Separate the crystals from the remaining
solution by filtration, and dry them in a warm
oven or by patting them with filter paper.
What happens if you heat a solution too strongly in crystallisation, and what should you do instead?
- If you heat the solution too strongly, you get a
powder. - If you allow the solvent to evaporate slowly, you
get regularly shaped crystals.
What does simple distillation separate and what does it rely on?
- Simple distillation separates a solvent from a
solution. - It relies on the solvent having a much lower
boiling point than the solute.
What happens during simple distillation?
- When the solution is heated, the solvent boils
but the solute does not. - The solvent turns into vapour and travels from the
solution to the condenser so it can be condensed
back to its liquid state, leaving the previously
dissolved solute behind.
Why is simple distillation useful in the laboratory?
Simple distillation is useful in the laboratory if you want to purify a solvent.
What is fractional distillation and what does it rely on?
- A separation technique which separates two or
more liquids in a mixture. - It relies on each liquid having a different boiling
point.
What does the fractionating column improve?
The fractionating column improves the separation of the mixture due to its large surface area on which the vapours can continually condense.
What happens in the fractionating column in the separation of bioethanol from water for use as a fuel?
- The vapours condense on the inside surface,
heating the fractionating column up. - When the temperature inside reaches the boiling
point of ethanol, ethanol vapour cannot condense
any more, but water vapour can. - Water droplets fall into the flask, and ethanol
vapour passes into the condenser…
What are the two different chemical phases of chromatography?
- A stationary phase that does not move.
- A mobile phase that does move.
What is the stationary and mobile phase in paper chromatography?
- The stationary phase is absorbent paper.
- The mobile phase is a solvent in the liquid state,
such as water or propanone
What is the stationary and mobile phase of thin layer chromatography?
The stationary phase is a thin layer of silica or
alumina powder which is spread over a plate of
glass or plastic.
- The mobile phase is a solvent.
What is a chromatogram and what does it show?
- A chromatogram is a pattern produced when
separating a mixture using chromatography. - It shows the separate components of a sample.
What does the pattern produced on a chromatogram depend on?
- The pattern produced depends on how each
component is distributed between the two
phases. - A component travels further up the plate if it
forms stronger bonds with the mobile phase
than with the stationary phase. - A component will not travel very far is if it forms
stronger bonds with the stationary phase than
with the mobile phase.
What does gas chromatography do in addition to separating the components?
Gas chromatography separates the components of a mixture and also measures their relative amounts.
What is the stationary phase and mobile phase in gas chromatography?
- The stationary phase is silica or alumina powder
packed into a metal column. - The mobile phase is an unreactive carrier gas
such as nitrogen, which does not react with the
sample
What are the steps in gas chromatography?
- The sample is turned into the gas state when it is
injected into the column. - The carrier gas pushes the sample through the
column. - The different components take different times to
travel through the column, depending on how
strongly they bond to the stationary phase. - A detector then sends a signal to a computer in
which each component is a peak plotted against
the retention time.
- What is the Rf value?
- How can you tell if two components are the same
using the Rf value? - How to you calculate Rf value?
- The Rf value is the relative distance travelled by
a substance during chromatography. - If two spots have the same Rf value and are the
same colour, they are likely to be identical. - Rf = Distance travelled by substance / Distance
travelled by solvent (distance from base line to
solvent front).
How do you interpret a gas chromatogram?
- Each peak represents a component /substance
present in the mixture. - The area under each peak shows how the relative
amount of each substance in the mixture. - The retention time is the time taken for a
substance to travel to the chromatography column
and is different for different substances.
What are the advantages of TLC than paper chromatography?
- It is quicker.
- It is more sensitive, so a smaller sample can be
used. - There is a larger range of stationary phases and
solvents to choose from. - You can also scrape an individual spot from a thin-
layer chromatogram for further analysis, for
example by gas chromatography.
How do you separate a mixture which contains insoluble and soluble substances?
Dissolving followed by filtration.
How do you separate a mixture which contains a solute dissolved in a solvent (a solution)?
Crystallisation to obtain the solute or simple distillation to obtain the solvent.
How do you separate a mixture which contains two or more substances in the liquid state?
Fractional Distillation.
How do you separate a mixture which contains coloured soluble substances?
Paper chromatography or thin layer chromatography.
