Test for Cations

• The term 'cation' just means a positively charged ion. For example Mg2+, Na+, and Al3+ ions are all cations. 
• Nearly all cations are metals ions, as metals form positive ions. The only exception you're likely to come across is the ammonium ion, NH4+, which is a non-metal cation. 
• • Take a platinum (or nichrome) wire loop, and clean it by dipping it in some dilute hydrochloric acid, rinsing it in distilled water, and then heating it over a Bunsen burner flame.
  • Dip the wire loop into the compound you want to test.
  • Hold the wire loop in the clear blue part of the Bunsen burner flame (this is the hottest part).
  • See what colour the flame turns as the compound burns.
• Lithium ions (Li+) burn with a crimson flame.
• Sodium ions (Na+) burn with a yellow flame.
• Potassium ions (K+) burn with a lilac flame.
• Calcium ions (Ca2+) burn with an orange-red flame.
• Copper ions (Cu2+) burn with a green flame.
• A limitation of the flame test is that if you have 2 or more different metals in your sample, then the colours of the flames will mix together, and you probably won't be able to tell which metals you have.
• some metal ions form coloured precipitates when they react with hydroxide ions
•  when copper(II) reacts with hydroxide ions, it forms a blue precipitate of copper hydroxide. This means the entire solution turns a blue colour.
• A test we can do for metal ions is react them with a solution of sodium hydroxide (NaOH), and see what colour the solution turns.
• Calcium (formula: Ca2+) forms a white precipitate.
• Iron(II) (formula: Fe2+) forms a green precipitate.
• Iron(III) (formula: Fe3+) forms a brown precipitate.
• Magnesium (formula: Mg2+) forms a white precipitate.
• Aluminium (formula: Al3+) forms a precipitate that is white at first, but if there is excess NaOH, it then redissolves to form a colourless solution.