C2.1- Purity And Seperating Mixtures Flashcards
What is the relative atomic mass (Ar)?
It is the mean mass of an atom compared to 1/12 the mass of a carbon-12 atom
What does a chemical formula tell you?
It tells you how many atoms of each element there are in a unit of a substance
What is the relative formula mass (Mr)
Relative formula mass is the mean mass of a unit of a substance compared to 1/12 the mass of a carbon-12 atom
What is the difference between relative formula mass and relative molecular mass
Relative formula mass refers to all substances that consist of 2 or more atoms. Relative molecular mass refers to substances that exist as molecules
What is an empirical formula?
An empirical formula shows the simplest whole number ratio for each element in a compound.
What does pure mean in every day life?
Natural Substances that have not been processed or changed
What is ‘pure’ in science
A substance that consists of one element or compound.
Are the atoms in alloys the same or different sizes
They are usually different sizes
What is an alloy?
A mixture of a metal with one or more other elements.
What is the melting point of a substance?
It is the temperature at which it changes from the solid state to the liquid state.
How can you tell if a substance is impure from its melting point.
- the melting point of a pure substance is a single temperature
- an impure substance melts over a range of temperatures and its melting point is less than that of a pure substance.
When you determine the melting point it is important to…
- Heat the substance slowly
- Stir the substance as is melts
Heating a substance allows…
The temperature of the whole sample to increase
What does mixing a sample ensure
That the entire sample is the same temperature.
What is a solution?
A mixture formed when one substance dissolves in another.
What is a solute
The substance that dissolves in a solvent
What is a solvent
A substance that a solute dissolved in
What happens to a substances particles when it dissolves?
It’s particles separate and become completely mixed with the particles of the solvent.
What does soluble mean
That a substance is able to dissolve in a solvent
What does insoluble mean?
A substance cannot dissolve
What is filtration?
The process by which insoluble substances are separated from soluble substances using a filter
What does filtration do?
It separates insoluble substances in the solid state from substances in the liquid state.
How does filtration work? (Example of sand and water)
It works because filter paper has microscopic holes.
When you filter a mixture of sand and water, water molecules are small enough to pass through the filter paper but the larger grains of insoluble sand cannot.
The sand stays behind in the filter paper as the reside while the water passes through as the filtrate.
What is a filtrate?
The liquid that passes through the filter during filtration.
What is residue?
An insoluble material left behind during filtration
Why do chemists flute their filter paper?
This produces a larger surface area for the filtrate to pass through.
What is evaporation?
When a substance turns from the liquid to the gas state at a temperature below the boiling point.
What happens if you heat a substance too strongly?
You get a powder
What happens if you allow the solvent to evaporate slowly?
You get regularly shaped crystals
What is crystallisation?
The process by which crystals are formed during the evaporation of a solvent from a solution
How can you tell if a solution is saturated?
No more solute can be dissolved at that temperature.
What is solubility?
A measure of how much solute can dissolve in a given solvent at a certain temperature
How does crystallisation work?
- You need to heat the solution gently until it is a saturated solution
- crystals will start to form at this point so you then let the solution cool slowly.
- as the solution cools the solubility of the solute decreases so more crystals form.
- you can separate them from the remaining solution by filtration.
What equipment is needed for filtration?
Filter funnel with filter paper
Conical flask
What equipment is needed for crystallisation
An evaporating basin with the solution
A flask with boiling water
A tripod
Bunsen burner
What does simple distillation do?
It separates a solvent from a solution
How does simple distillation work?
- it relies on the solvent having a much lower boiling point than the solute
- when the solution is heated the solvent boils but the solute does not.
- the solvent escapes from the solution in its gas state
- it is then cooled and condensed back to its liquid state by a condenser
What is condensation?
Changing from the gas to liquid state
What is a condenser
A piece of apparatus that is is kept cold using a flow of cold water
What is vapour?
A substance in its gas state and below its boiling point.
What does fractional distillation do?
It separates 2 or more substances from a mixture in the liquid state.
How does fractional distillation work?
- it relies on each substance having a different boiling point.
- then both substances leave the liquid mixture as vapours
- in the fractioning volume the vapours condense on the inside surface, hearing it up. When the temperature reaches the boiling point of one of the vapours (the one with the lower boiling point) it cannot condense any more so it passes into the condenser. While this is happening the one with the higher boiling point can condense further and water droplets fall back into the flask.
What are advantages of the fractioning column?
It has a large SA on which the vapours can continually condense.
During fractional distillation it is hottest at the bottom and coolest at the top.
What is chromatography
A separation method that relies on the distribution of a substance between a mobile phase and a stationary phase.
What is the stationary phase
A substance in the solid or liquid state that doesn’t move during chromatography
What is the mobile phase?
A substance in the liquid or gas state that moves during chromatography
What are the mobile and stationary phase in paper chromatography?
Stationary phase- absorbent paper
Mobile phase - solvent in the liquid state
What is an Rf value ?
It is the relative distance travelled by a substance during chromatography
Why do we use Rf values
To compare the spots on a chromatogram.
How do you calculate the Rf value?
Distance travelled by substance ➗ distance travelled by solvent
What are the stationary and mobile phase in gas chromatography
Stationary phase- silica or alumina powder packed into a metal column
Mobile phase- unreactive carrier gas which doesn’t react with the sample
What does gas chromatography do?
It separated the components of a mixture and also measures their amounts
How does gas chromatography work?
- The sample is turned into the gas state when it is injected into the column.
- the carrier gas pushes the sample through the column.
- a detector sends a signal to a computer as each component leaves the column.
- the computer produces a chromatogram
What are the advantages of thin layer 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.
How would you separate a mixture of soluble and insoluble substances?
Dissolving followed by filtration
How would you separate a solute dissolved in a solvent (a solution)
Crystallisation to obtain the solute, simple distillation to obtain the solvent
How would you separate a mixture of 2 or more substances in The liquid state
Fractional distillation
How would you separate a mixture of coloured soluble substances?
Paper chromatography or thin layer chromatography
