flame tests and metal hydroxide

what can flame tests be used for

Flame tests can be used to identify some metal ions (cations)

describe the two methods of flame test

1. place a small amount of chemical onto wire mounted in a handle and place the end of this into a blue Bunsen burner flame and observe the colour of the flame to work out the metal ion present

2. Dip sample onto a damp splint/nichrome wire making sure splint/wire is clean by dipping in acid first and place in blue flame of bunsen Bruner

what are the distinct colours produced in flame tests

Lithium,

sodium,

potassium,

calcium

and copper

-crimson flame

-yellow flame

-lilac flame

-orange-red flame

-green flame

problems with flame tests

-colour of a flame test can be difficult to distinguish. This is especially true if there is only a low concentration of the metal compound

-sometimes sample contains a mixture of metal ions which can mask the colour of the flame

what happens if you add sodium hydroxide to solutions of calcium ions, magnesium ions and aluminium ions and how can problem be overcome

they will all produce white precipitates meaning cannot distinguish between these three tests

problem can be overcome by adding excess sodium hydroxide solution, where the aluminium precipitate redissolves

and use flame tests to find out which one is calcium ions (orange-red) and magnesium (no colour)

Solutions of copper(II), iron(II) and iron(III) ions form coloured precipitates when sodium hydroxide solution is added. what are they?

Copper(II) forms a blue precipitate of copper(II) hydroxide

, iron(II) a green precipitate of iron (II) hydroxide

and iron(III) a brown precipitate of iron (III) hydroxide

1. calcium nitrate + sodium hydroxide

2. magnesium nitrate +sodium hydroxide

3. aluminium nitrate +sodium hydroxide

1. sodium nitrate + calcium hydroxide

2. sodium nitrate + magnesium hydroxide

3. sodium nitrate +aluminium hydroxide

sometimes a damp splint can be used instead of nichrome wire. Why should splint be damp before using it in a flame test

so splint doesn’t burn, which will mean that u will not be able to see the colours

why is nichrome wire dipped in acid before each salt solution

to clean it

why is a different splint used for each solution

to prevent contamination

why was a roaring flame used instead of a safety flame

to see the colours properly