C2.1 Flashcards
Explain what is meant by the purity of a substance, distinguishing between the scientific and everyday use of the term ‘pure’
- A pure substance is a single element or compound, not mixed with any other substance.
- In everyday language, a pure substance is one that has nothing added to it and is in a natural state.
Use melting point data to distinguish pure from impure substances.
- Pure substances melt and boil at specific temperatures.
- ## impure substances melt and boil over a range of temperatures.
Calculate relative formula massess of species separately and in a balanced chemical equation.
- Relative Formula Mass:
Separately - Sum of relative atomic masses of the atoms in the numbers shown in the formula.
Balanced Chemical Equation - Sum of relative formula masses of the reactants + Sum of relative formula masses of the products.
Deduce the empirical formula of a compound from the relative numbers of atoms present or from a model or diagram and vice versa.
- Empirical Formula: Simplest whole number ratio of atoms of each element in a molecule.
- From relative number of atoms present / diagram, you can form the molecular formula first, eg. Fe2O6. To then find the empirical formula you divide the number of each element by the HCF.
Explain the meaning of a formulation.
- A Formulation = mixture that has been designed as a useful product.
Describe and explain the process of filtration
- 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, the liquid molecules are small enough to pass through the filter paper, but the larger insoluble substances cannot.
- This substance stays behind in the filter paper as the residue while the water passes through as the filtrate.
Describe and explain the process of crystallisation
- If you have a mixture of a solute and solvent, you could separate them using Crystallisation.
- You would first heat up the solution so that the solvent evaporates leaving the solute behind. If you allow the solvent to evaporate slowly, you get crystals starting to form from the solute.
Describe and explain the process of simple distillation
- Simple distillation is used to separate a solvent from a solution.
- Simple distillation works because the solvent has a much lower boiling point than the solute.
- When the solution is heated, the solvent boils and evaporates. It escapes from the solution in its gas state and is then cooled and condensed back to its liquid state by a condenser.
Describe and explain the process of fractional distillation
- Fraction distillation is used to separate a mixture of two or more substances in the liquid state.
- It relies on each substance having a different boiling point.
- The substance with the lower boiling point would boil and evaporate first, where it will go through the fraction in column. The vapour would condense when it reacher a part of the column that is below the temperature of its boiling point.
- The same process occurs with the next substance.
What is chromatography?
- It is used to separate mixtures and give information to help identify substances.
- Chromatography relies on two different phases: a stationary phase that does not move, and a mobile phase that does move.
Describe the techniques of paper chromatography
Paper chromatography:
- stationary phase = absorbent paper
- mobile phase = solvent in the liquid state
- The more soluble the substance is, the further up the paper it travels.
- If one dot is produced, the substance is pure. If more than one dot is produced, the substance is impure.
Describe the techniques of thin-layer chromatography.
Works in the same way as paper chromatography however:
- stationary phase is a thin layer of silica or alumina powder spread over a plate of glass or plastic.
- mobile phase is a solvent in the liquid state.
How do you work out Rf values and what do they mean?
- Rf value = distance moved by substance / distance moved by solvent
- Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.
Describe and explain the techniques of gas chromatography.
- The stationary phase is silica or alumina powder packed into a metal column.
- The mobile phase is an unreactive carrier gas, such as nitrogen, which does not reach with the sample.
- Gas chromatography separates the components of a mixture but also measures their amounts.
- The sample is turned into the gas state when it is injected into the column. The carrier gas pushes the sample through the column. Different components take different times to travel through the column. A detector sends a signal to a computer as each component leaves the column, and the computer then produces a chromatogram.
