4.8.3.2 - metal hydroxides

what type of ions does adding sodium hydroxide solution identify

cations

describe how to carry out a sodium hydroxide solution test

• add just a few drops of NaOH at first and very slowly

• If it is added too quickly and the precipitate is soluble in excess, you might miss the formation of the precipitate dissolves as quickly as it forms if excess solution is added

• a small amount is thus added, very gradually and any colour changes or precipitates formed are noted

• then the NaOH is added in excess and the reaction is observed again

positive result of the sodium hydroxide solution test with aluminium

• white precipitate (aluminium hydroxide)

• this dissolves in excess sodium hydroxide solution to form a colourless solution

positive result of the sodium hydroxide solution test with calcium

• white precipitate (calcium hydroxide)

• insoluble so remains in excess NaOH

positive result of the sodium hydroxide solution test with magnesium

• white precipitate (magnesium hydroxide)

• insoluble so remains in excess NaOH

positive result of the sodium hydroxide solution test with copper(II)

• blue precipitate (copper hydroxide)

• insoluble so remains in excess NaOH

positive result of the sodium hydroxide solution test with iron(II)

• green precipitate (iron hydroxide)

• insoluble so remains in excess NaOH

positive result of the sodium hydroxide solution test with iron(III)

• brown precipitate (iron hydroxide)

• insoluble so remains in excess NaOH

how to form a balanced equation

Cu²⁺ + 2OH^- → Cu(OH)₂

you need as many OH- ions as the charge on the metal ions

the Na from the NaOH and whatever the metal ion was bonded with will react to form a compound together

Cu(OH)2 = precipitate

CuCl₂ + NaOH → Cu(OH)₂ + 2NaCl