C2.1- Purity And Separating Mixtures Flashcards
1
Q
What is relative atomic mass?
A
- the mean mass of an atom of an element compared to 1/12 the mass of a carbon atom
2
Q
What is relative formula mass?
A
- the mean mass of a unit of a substance compared to 1/12 the mass of a carbon atom
3
Q
What is a chemical formula?
A
- how many atoms of each element there are in a unit of substance
4
Q
What is an empirical formula?
A
- shows the simplest whole-number ratio of the atoms of each element in a compound
5
Q
What does pure mean?
A
- pure describes natural substances that haven’t been changed or processed
- a pure substance consists of just one element or compound
- mixtures are impure substances since they contain more than one element or compound
6
Q
Can mixtures be useful?
A
- difficult to obtain pure substances, just one atom or molecule of something else makes a substance impure
- many useful materials are mixtures of different substances
- an alloy is a mixture of a metal with one or more other elements
7
Q
How do you use melting points to determine purity?
A
- the melting point of a substance is the temperature at which it changes from the solid state to the liquid state
- the melting point of a pure substance is a single temperature
- If a substance is impure:
its melting point is less than that of a pure substance
it often melts over a range of temperatures, not just one
8
Q
How do you determine melting point?
A
- by heating it
- then either measure the temperature at which it melts, or measure its temperature at regular time intervals and plot a graph - it’s important to:
heat the substance slowly
stir the substance as it melts - heating a substance slowly allows the temperature of the whole sample to increase
- mixing ensures that the entire sample is at the same temperature
- these two actions improve the accuracy of a measurement of the melting point of a sample.
9
Q
What happens when a substance dissolves?
A
- a solution forms when one substance dissolves in another
- the solute is the substance that dissolves, and the solvent is the substance it dissolves in
- when a substance dissolves, it’s particles separate and become completely mixed with the particles of the solvent
- if a substance can dissolve in a particular solvent, it’s soluble in that solvent, if it can’t dissolve, it’s insoluble in that solvent
- substances can be soluble in one solvent but insoluble in another
10
Q
How does filtration work?
A
- filtration separates an insoluble substance in the solid state from substances in the liquid state
- it works because filter paper has tiny, microscopic holes
- when you filter a mixture of insoluble substance and a liquid substance, the liquid molecules are small enough to pass through the filter paper, but the after grains of insoluble substance cannot
- the insoluble substance stays behind in the filter paper as the residue while the water passes through as the filtrate
- particles of a dissolved solute are also small enough to pass through filter paper.
11
Q
How does crystallisation work?
A
- if you heat a solution, the solvent evaporates leaving the solute behind
- if you allow the solvent to evaporate slowly, you get regularly shaped crystals
- you need to heat the solution gently until it becomes a saturated solution
- a solution is saturated when no more solute can be dissolved at that temperature
- crystals will start forming at this point, so you 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, and dry them in a warm oven or by patting them with filter paper
12
Q
How does simple distillation work?
A
- separates a solvent from a solution
- 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’s then cooled and condensed back to its liquid state by a condenser, a piece of apparatus that is kept cold using a flow of cold water.
13
Q
What is fractional distillation?
A
- separates two or more substances from a mixture in the liquid state
- it relies on each substance having a different boiling point and uses a special piece of equipment called a fractionating column.
14
Q
How does chromatography work?
A
- relies on two different chemical phases:
a stationary phase that doesn’t move
a mobile phase that does move. - a phase is a substance in the solid, liquid or gas state
- paper chromatography is sometimes used to separate the coloured substances in ink or sweets
- in paper chromatography:
the stationary phase is absorbent paper
the mobile phase is a solvent in the liquid state, such as water or propane
15
Q
How does thin-layer chromatography work?
A
- works in the same way as paper chromatography but the stationary phase is a thin layer of silica or alumina powder spread over a plate or glass of plastic
- you can use TLC to separate a sample into its components for identification or analysis.
- 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 if it forms stronger bonds with the stationary phase than with the mobile phase
