1What is a pure substance in chemistry?
A single element or compound not mixed with any other substance.
2How does the everyday meaning of pure differ from the scientific meaning?
Everyday: unadulterated and in its natural state, e.g. pure milk.
3How do pure and impure substances differ in melting and boiling points?
Pure substances have specific temperatures; impure substances melt and boil over a range.
4How do you calculate relative formula mass (Mr)?
Add the relative atomic masses of all atoms in the formula.
5What is the relationship between total Mr of reactants and products in a balanced equation?
The totals are equal when calculated using the quantities shown.
6What is relative atomic mass (Ar)?
The average mass of an atom compared with 1/12 of a carbon-12 atom's mass.
7What is an empirical formula?
The simplest whole-number ratio of atoms of each element.
8How do you calculate an empirical formula from atom numbers?
Divide each element's atom number by their highest common factor, e.g. Fe2O6 becomes FeO3.
9What is a formulation?
A mixture designed as a useful product, with carefully measured components.
10Why are metals often made into alloys?
Pure metals such as copper, gold, iron and aluminium are often too soft for everyday use.
11Which separation method is used for an insoluble solid?
Filtration; the solid remains as the residue.
12How do crystallisation and simple distillation differ?
Crystallisation obtains a dissolved solid by evaporating solvent and cooling; simple distillation collects the solvent by evaporation and condensation.
13What is fractional distillation used for?
Separating liquids with different boiling points, e.g. separating crude oil into fractions.
14What are the stationary and mobile phases in paper chromatography?
Stationary: paper; mobile: solvent.
15What is the Rf formula in chromatography?
Rf = distance moved by substance / distance moved by solvent
16How can chromatography distinguish pure from impure substances?
A pure compound produces one spot; a mixture separates into different spots.
17What are the stationary and mobile phases in thin-layer chromatography (TLC)?
Stationary: a thin layer of inert substance on a flat, unreactive surface; mobile: solvent.
18What are the phases in gas chromatography?
Mobile: inert carrier gas; stationary: liquid or solid on a solid support inside the column.