C2.1 - Purity And Separating Mixtures Flashcards
What are the masses of subatomic particles called?
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 is something that contains only one type of particles, it has a fixed melting and boiling point
What is a pure substance ?
A pure substance is something that contains only one type of element or compound, it has a fixed melting and boiling point
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 elements or 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 different metals ( more than one element )
What is the melting point of a pure substance?
The melting point of a substance is a single temperature, a fixed melting point
What is the melting point of an impure substance?
- Its melting point is less than that of the pure
substance. - It often melts over a range of temperatures, does not have a fixed melting point/ a variable melting point
- Greater the difference between the measured
melting point and the accepted melting point, the
lower its purity is likely to be.
Do you know how to draw cooling/heating curves for pure and impure substances ?
yes I hope.
This is for a pure substance since the lines when there is a change of state, showing it has a fixed melting boiling point
For a mixture, the lines representing a change of state would be slanted since a mixture has a variable melting/ boiling point - KNOW HOW TO DRAW THAT TOO
Why are lines on a cooling, heating curve horizontal during a change of state ?
At a change if state the temperature does not change as the energy is involved in making/breaking bonds
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.
Why does filtration work ?
It works because filter paper has tiny, microscopic holes where small molecules can pass through and larger insoluble substances can’t.
What is filtration used to separate ?
Filtration separates an insoluble substance in the
solid state from substances in the liquid state.
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.
explain how sand and water are separated using filtration ?
- The mixture of sand and water is poured into the filter funnel, which is lined with filter paper.
- The filter paper has tiny microscopic holes
- The water molecules are small enough to pass through the paper to collect in the beaker.
- The large insoluble sand particles cannot pass through the filter paper and collect in the filter funnel.
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/ evaporating basin to heat up the solution (filtrate - depending on context) until it becomes a saturated solution (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 rely on?
It relies on the solvent having a much lower boiling point than the solute, the substances being separated having a large difference in boiling points
What does simple distillation separate ?
- There is quite a difference in boiling points
- The two substances being separated are miscible
What happens during simple distillation?
- When the bunsen is lit, the substance with the lowest boiling point evaporates first
- The substance rises up towards the thermometer. The vapour travels through the condenser, were the cold water “jacket” cools and condenses the ethanol into a liquid
- When all the substance with the lowest boiling is collected the temperature rises, and eventually reaches the temperature if the boiling point of the other substance
- It is now the other substance’s vapour that rises, travels through the condenser, cools and condenses to liquid water
Label the equipment in simple distillation ?
Why is simple distillation useful in the laboratory?
Simple distillation is useful in the laboratory if you want to purify a solvent.
What does fractional distillation rely on?
It relies on each liquid having a different boiling
point.
What is fractional distillation used to separate ?
- Mixtures with substances that have similar boiling points
- When there are many substances, a mixture with many different substances
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 substance with the lowest boiling point evaporates first and travels up the column
- It condenses, as the beads inside the column are cold and falls back down
- This continues until the beads are hot enough for the vapour not to condense
- A temperature then increases as the second substance evaporates, rises up the column and the beads at the top of the column get hotter
- Each substance that evaporates and condenses and is collected at a different temperature is called a fraction
eg. Crude oil ( a black mixture containing lots of liquids with different boiling points )
What are the two different chemical phases of chromatography?
- A stationary phase that does not move.
- A mobile phase that does move.
Why is chromatography used ?
It is used to separate substances contained in a mixture when they are in small quantities and they are coloured.
What is the stationary and mobile phase in paper chromatography?
- The stationary phase is paper.
- The mobile phase is a solvent in the liquid state,
such as water or propanone
What is a mobile and stationary phase ?
Stationary : This is the substance that the mobile phase moves on
Mobile : This is the substance that moves up/along the stationary phase, usually a solvent
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 longer retention time
- A component will not travel very far is if it forms
stronger bonds with the stationary phase than
with the mobile phase, a shorter retention time
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 inert carrier gas ( hydrogen or nitrogen ) pushes the sample through the column ( the column is a very long coil of thin layer of silica coated on it - the stationary phase )
- The different components take different times to
travel through the column, depending on how
strongly they bond to the stationary phase and eventually travel through it and reach the detector - 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?
The Rf value is the relative distance travelled by
a substance during chromatography.
What is Rf value used for ?
It is used to compare different spots on a chromatogram.
eg. if two spots have identical Rf values and are the same colour - most probably the same substance
How can you tell if two components are the same using the Rf value?
If two spots have the same Rf value and are the same colour, they are likely to be identical.
How to you calculate Rf value?
Rf = Distance travelled by solute ( spot ) / Distance
travelled by 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 highest peak therefore means the highest percentage of that component - The further the peak the longer the retention time
What is retention time ?
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 ( thin layer chromatography ) 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 - sample can be retreived
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.
