C2- Chemistry Analysing substances, making salts, acids and bases Additional Science C2- Chemistry Analysing substances, making salts, acids and bases

Analysing substances Instrumental methods of analysing substances such as elements and compounds are; accurate, rapid and sensitive Paper chromatography can be used to detect additives in food. Artificial colours can be identified.

Analysing substances Lid to prevent evaporation of the solvent. Unknown sample. Known samples. Pencil line; starting point, pencil does not dissolve in the solvent. Solvent; dissolves the colours and is absorbed through the paper. The colour dots from the unknown sample which match the known samples can be identified as to whether the additive is safe or not.

Analysing substances Gas chromatography linked to mass spectroscopy (GC-MS) is an example of an instrumental method: Gas chromatography separates mixtures of compounds, it uses a column packed with solid. The substance to be analysed is carried through the column using a gas The time taken for a substance to travel through the column can be used to help identify the substance

The number of peaks corresponds to the number of compounds present The position of the peak is the ‘retention time’, in other words how long they have spent in the column

Analysing substances The gas chromatography column can be linked to a mass spectrometer which can identify substances quickly and accurately and in very small quantities. The mass spectrometer can give the relative molecular mass of the substances (you look at the molecular ion peak on a graph)

Making salts H+(aq) + OH-(aq)  H2O (l) State symbols are shown after substances in reactions (s)= solid (l)= liquid (aq) = aqueous (dissolved in water) (g) = gas

Making salts Soluble salts can be made by reacting acids with: 1. metals – not all metals are suitable; some are too reactive and others are not reactive enough (MASH) 2. insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off (copper sulphate) 3. alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. (neutralisation)

Making salts Metal + Acid  Salt + Hydrogen Soluble salts can be made by reacting acids with: 1. metals – not all metals are suitable; some are too reactive and others are not reactive enough (MASH) Metal + Acid  Salt + Hydrogen

Making salts Hydrochloric acid produces metal chlorides e.g. Sodium chloride Sulphuric acid produces metal sulphates e.g. Sodium Sulphate Nitric acid produces metal nitrates e.g. sodium nitrate

Making salts Soluble salts can be made by reacting acids with: 2. insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off (copper sulphate)

Making salts Acid + Alkali  Salt + Water H+(aq) + OH-(aq)  H2O (l) Soluble salts can be made by reacting acids with: 3. alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. (neutralisation) Acid + Alkali  Salt + Water Hydrochloric + Sodium acid hydroxide Sodium + Chloride Water H+(aq) + OH-(aq)  H2O (l)

Salts solutions (aq) can be crystallised to produce solid salts (s) Making salts Salts solutions (aq) can be crystallised to produce solid salts (s)

Making salts Insoluble salts can be made by reacting two soluble salts together. This is called a precipitation reaction. Soluble except when with silver, mercury or lead. All soluble Soluble except when with silver, lead, mercury, barium, strontium and calcium.

Making salts Insoluble salts can be made by reacting two soluble salts together. This is called a precipitation reaction. All insoluble except for NH4+, barium and those of group 1 elements All insoluble except for calcium, barium and group 1 All insoluble except when with NH4+ and group 1 elements

An example precipitation reaction Making salts An example precipitation reaction Silver + Sodium nitrate chloride Sodium + nitrate Silver chloride AgNO3(aq) + NaCl(aq) → NaNO3(aq) +AgCl(s)

Making salts Precipitation reactions are used to remove unwanted ions from solutions, for example to treat drinking water or treating effluent

Acids and bases Metal oxides and metal hydroxides are bases e.g. sodium hydroxide, copper oxide. If the hydroxides are soluble they are called alkalis Hydrochloric + Sodium acid hydroxide Sodium + Chloride Water The name of the salt produced when an acid and alkali react depends on the acid used and the metal in the base

Acids and bases Ammonia (NH3) dissolves in water to form an alkali solution, NH3 (aq). It is used to produce ammonium salts These salts are used as fertilisers

Acids and bases The presence of H+ ions makes solutions acidic. The presence of OH- ions make solutions alkali. 1 2 14 13 12 11 10 9 8 7 6 5 4 3 Strong acid Neutral Strong alkali Weak acid Weak alkali More alkali More acid More OH- ions More H+ ions

Acids and bases Acid + Alkali  Salt + Water In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water. Acid + Alkali  Salt + Water nitric + potassium acid hydroxide potassium + nitrate Water H+(aq) + OH-(aq)  H2O (l)