C2.1 Purity and separating mixtures Flashcards
What is relative atomic mass?
the mean mass of an atom of an element compared to 1/12 of the mass of a carbon-12 atom
What does a chemical formula tell you?
how many atoms of each element there are in a unit of a substance
What is a relative formula mass?
the mean mass of a unit of substance compared to 1/12 of the mass of a carbon-12 atom
define empirical formula
the simplest whole number ratio of the atoms of each element in a compound
Calculate the relative formula mass of Mg(OH)2
Mg = 24.3, O = 16.0, H = 1.0
M(r) of Mg(OH)2 = 58.3
How do you find the empirical formula of a chemical formula?
Divide the chemical formula by the highest common factor
What does pure mean in scientific terminology?
a substance consisting of just one element or compound
is water pure?
no, because it contains multiple minerals, distilled water is pure, however
What is an example of a helpful mixture?
Alloys (a mixture of a metal with one of more other elements)
What is the melting point of a substance?
The temperature at which it changes from the solid state to the liquid state
How many melting points does a pure substance have?
Only one
How many melting points does an impure substance have?
Multiple, however many elements there are in the compound
How can you tell a substance is impure looking at its melting point?
- it’s melting point is less than of the pure substance
- it often melts over a range of temperatures, not just one temperature
Does the temperature of a pure substance change while it melts?
No, it stays constant
How must you heat a substance to find out its melting point and why?
- slowly
- stirring it
So that the temperature of the whole sample can increase, not just the surface
Why should you stir a substance while it melts?
To ensure that the entire sample is at the same temperature
What does chromatography separate?
mixtures of soluble substances
what happens when a substance dissolves?
- a solution forms
- the solute’s particles separate and become completely mixed with the solvent’s particles
what does filtration do?
separates an insoluble substance in the solid state from substances in the liquid state
what feature of the filter paper allows filtration to occur?
it has tiny microscopic holes which prevent insoluble substances from passing
what is the substance called which is left behind during filtration?
residue
what is the substance which passes through the filterpaper called
filtrate
how does crystallisation work?
- a solution is heated slowly until it becomes a saturated solution (when no more solute can be dissolved at that temperature)
- crystals start forming at thus point so saturated solution is left to cool
- as solution cools, solubility of the solute decreases, so more crystals form
- the crystals can be separated from the remaining solution by filtration
Bite size:
To obtain large, regularly shaped crystals:
Put the solution in an evaporating basin
Warm the solution by placing the evaporating basin over a boiling water bath
Stop heating before all the solvent has evaporated
After the remaining solution has cooled down, pour the excess liquid away, or filter it. Dry the crystals using a warm oven or air-dry them.
why must you heat a solution slowly (crystallisation)?
if you heat a solution too fast, a powder is formed (no crystals)
how do you dry crystals?
- place them in a warm oven
- pat them with filter paper
What is simple distillation used to separate?
to separate a solvent from a solution
eg. (oil and water)
what does simple distillation rely on?
solvent having a much lower boiling point than solute
describe simple distillation
1) solution is heated
2) solvent has low boiling point and so heats quicker than solute
3) solvent rises and condenses in vapour condenser
4) travels down the test tube
what equipment is used during simple distillation?
condenser, test tube, bunsen burner, thermometer
what is simple distillation useful for (in a lab)?
purifying water for experiments
what does fractional distillation seperate?
Separates one or more liquids from a mixture of liquids
how does fractional distillation separate substances?
depending on their boiling points
how is fractional distaillation used industrially?
to separate crude oil into separate hydrocarbons
describe the process of fractional distillation (in terms of an ethanol-water mixture)
1) vapours condenser on inside surface, heating it up
2) when the temperature reaches the boiling point of ethanol, ethanol vapour cannot condense any more, but water vapour can
3) water droplets fall back into the flask, and ethanol vapour passes into the condenser
4) in the condenser, the ethanol vapour is cooled and condensed back to its liquid state. It drips into the collecting container
how does the fractioning column help improve the separation of the mixture?
- large surface area where vapours condense
- column becomes hottest at bottom and coolest at top
what are the advantages of TLC (thin-layer chromatography) over paper chromatography?
- it is quicker
- more accurate results (as TLC is more sensitive)
- a smaller sample can be used
- large range of stationary phases and solvents to choose from
How does TLC’s stationary phase work?
a thin layer of silica or alumina powder is spread over a plate of glass or plastic
how is gas chromatography different to paper or TLC?
- the stationary phase is silica or aluminium powder packed into a metal column
- the mobile phase is an unreactive carrier gas
what does gas chromatography do?
separates the components and measures their amounts on a chromatogram
how is the the sample separated during gas chromatography?
depending on how strongly they bond to the stationary phase
how do you calculate the rf value?
distance travelled by solute (or spot) / distance travelled by solvent
how do you separate a soluble and insoluble substance?
filtration
how do you separate two or more substances in the liquid state?
fractional distillation
how do you separate coloured soluble substances?
chromatography
if a component travels very high during chromatography, what property does it have?
highly soluble (dissolves easily in solvent)
when calculating rf value, where do you measure the spot from, and where the solvent?
spot - from the PENCIL line to MIDPOINT on spot
solvent - from PENCIL line to where it stops
How does gas chromatography work?
The sample is turned into the gas state when it is injected into the column, depending on how strongly they bond to the stationary phase. A detector sends a signal to a computer as each component leaves the column. The computer produces a chromatogram in which each component is a peak plotted against the travel time.
