Chemistry C3 Chemical Detection

Qualitative Analysis - any method used to identify which chemical substances are present, eg, using an indicator to find out if acids are present.

Quantitative Analysis - any method used to determine the amount of a particular chemical substance present, for example, carrying out an acid-base titration to find out how much acid is present.

The presence of a Hydrogen ion means that the substance is an acid. The more hydrogen ions that there are the stronger the acid, and therefore the lower the pH on the pH scale.

The pH of a substance can be measured using an indicator such as litmus solution (universal indicator) or litmus paper.

Purple and blue litmus paper will turn red if the substance is an acid.

Common acidic substances include...

Hydrochloric Acid (HCl)

Sulphuric Acid (H SO )

Nitroc Acid (HNO )

When an acidic substance dissolves in water (aq) it produces H ions.

Acid + Metal --> Salt + Hydrogen

Acid + Metal Carbonate --> Salt + Water + Carbon Dioxide

Acid + Alkali --> Salt + Water

Acid + Metal Oxide --> Salt + Water

Remember:

Hydrochloric Acid makes chloride salts,

Sulpuric Acid makes sulphate salts,

Nitric Acid makes nitrate salts.

The presence of OH ions in a substance means it is an alkali. The more OH ions there are the stronger the alkali, and therefore the higher it is on the pH scale.

If an unknown substance is alkali it will turn purple or red litmus paper blue.

When an alkali dissolves in water it produces OH ions:

Sodium Hydroxide --> Sodium Ions + Hydroxide Ions

When an alkali is heated with ammonium salt in the lab, ammonia is produced:

Calcium hydroxide + ammonium chloride --> Calcium Chloride + Steam + Ammonia

When held in a Bunsen flame, different metals burn with different distinctive flames:

· Sodium (Na ) = Yellow Flame

· Potassium (K ) = Lilac Flame

· Calcium (Ca ) = Brick-Red Flame

· Copper (Cu ) = Blue-Green Flame

Therefore, metal ions can easily be identified by burning a paste of an unknown ionic substance in a hot Busen flame.

If the Flame Glows a particular colour the ions of that particular metal must be present in the substance.

Identifying Ions from Precipitates

Precipitation reactions form the basis of some tests for ions. Metal compounds in solution contain either...

• Metal ions and non-metal ions, or
• Metal ions and compound ions.

Sodium hydroxide solution is added to a solution of an ionic substance. If the ionic substance contains metal ions then the following coloured precipitates will be made...

Silver nitrate solution + Copper (II) ions (Cu ) ---> Blue Precipitate

Soduim hydroxide solution + Iron (II) ions (Fe ) ---> Green Precipitate

Sodium hydroxide solution + Iron (III) ions (Fe ) ---> Brown Precipitate

Sodium hydroxide solution + Aluminium ions (Al ) ---> White Precipitate

Calcium ions (Ca ) and zinc ions (Zn ) also form white precipitates when added to sodium hydroxide solution.

Calcium Ions form calcium hydroxide:

Ca + 2OH ---> Ca(OH)

Amuminium ions form Aluminium hydroxide which only dissolves in excess sodium hydroxide solution:

Al + 3OH --> Al(OH)

Zinc ions form zinc hydroxide which dissolves in both sodium hydroxide and ammonium hydroxide:

Zn + 2OH ---> Zn(OH)

Dilute nitric acid and silver nitrate solution are added to a solution of an ionic substance. If the ionic substance contains Halide Ions (i.e. Chloride, Bromide or Iodide) then the following coloured precipitates will be made:

Silver nitrate solution + Chloride ions (Cl ) ---> White Precipitate

Silver nitrate solution + Bromide ions (Br ) ---> Pale Yellow Precipitate

Silver nitrate solution + Iodide Ions (I ) ---> Yellow Precipitate

Eg...

Potassium Chloride + silver nitrate ---> Silver Chloride + Potassium Nitrate

KCl + AgNO ---> AgCl + KNO