OCR GCSE Chemistry Notes | C2
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- Pure: a single element or compound, not mixed with any substance.
- Pure substances melt and boil at specific temperatures so compare data.
- Relative formula mass: add up the mass numbers that make a compound.
- Empirical formula steps:
- Relative atomic mass: ratio of the average mass of an atom compared to 1/12th of a carbon-12 atom.
- Relative molecular mass: ratio of the average mass of a molecule of an element or compound to 1/12th of a carbon-12 atom.
- Relative formula mass: weighted average of the masses of the formula units relative to 1/12th of a carbon-12 atom.
- Mixtures: two or more elements not chemically combined
- Formulation: a mixture for a specific purpose e.g. paint
- Alloys: a mixture of two or more metals. They are harder than pure metals as the layers cannot slide over each other easily.
- Filtration: use a funnel and filter paper to separate precipitates from a solution.
- Crystallisation: to separate a soluble salt from a solution; heat, evaporate, allow solution to cool, dry and form crystals.
- Simple distillation: separate a solvent from a solution; one of them evaporates and escapes and condenses separately.
- Fractional distillaton:
- Chromatography: used to separate mixtures and identify substances.
- Stationary phase e.g. paper
- Mobile phase e.g. solvent
- Rf Value: distance of substance/ distance of solvent (always 0-1).
- TLC (thin layer chromatography): stationary phase is a thin, inert substance.
- We can also use chromatography to check if something is pure as it will only produce one spot.
- Gas chromatography:
- Stationary phase: liquid/ solid on a solid support
- Mobile phase: inert carrier gas
- We want to see how soluble a substance is in the gas.