2.37 describe simple tests for the cations:

Analysis of Ions and testing for gases

i           Li+, Na+, K+, Ca2+ using flame tests

To do a flame test, you need to use a piece of nichrome wire, dipped in concentrated hydrochloric acid.  Dip the wire in concentrated hydrochloric acid then place it in a roaring Bunsen flame to clean it.  Dip into the acid again, and then into the sample to be tested, and then hold in a blue Bunsen flame.  (Alternatively, use a wet wooden splint to put the sample into the flame.)  The metal ions in the compounds give different colours to the Bunsen flame:

Metal	Flame colour
lithium (Li+)	red
sodium (Na+)	yellow
potassium (K+)	lilac
calcium (Ca2+)	brick red (an orangey red)

Ii          NH4+ using sodium hydroxide solution and identifying the ammonia evolved

Add some dilute sodium hydroxide solution to a sample of the substance in a test tube and warm the mixture. Test any gas given off with moist red litmus paper.  If the substance contains ammonium ions, then ammonia gas will be given off.  The red litmus paper will turn blue, because ammonia is an alkaline gas.

NH4+(aq) + OH-(aq)  → NH3(g)   +   H2O(l)

iii         Cu2+, Fe2+ and Fe3+ using sodium hydroxide solution

Add a few drops of sodium hydroxide solution to a few cm3 of the sample to be tested.  Look for a coloured precipitate of the metal hydroxide to form.
copper(II) (Cu2+)	blue precipitate of copper (II) hydroxide Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)
iron(II) (Fe2+)	green precipitate iron (II) hydroxide Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)
iron(III) (Fe3+)	brown precipitate iron (III) hydroxide Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

2.36 understand the sacrificial protection of iron in terms of the reactivity series.

If iron is covered with a more reactive metal such as zinc, even if the coating is damaged to reveal the iron, the zinc will continue to react in preference to the iron because the zinc is more reactive, and the exposed iron is not affected. The more reactive metal is sacrificed to protect the iron.

A good example is bolting blocks of Mg or Zn to ships’ hulls. These react with the saltwater, but are easily replaced. The steel hulls are protected.

2.35 describe how the rusting of iron may be prevented by grease, oil, paint, plastic and galvanising

Grease, oil, paint and plastic are called “barrier methods”.  They stop air and water from accessing the iron.  Remember a good example of each method, e.g., grease for axles on cars, oil for a bicycle chain, paint for iron railings and plastic for the shelves in a fridge.

Galvanising involves covering the iron completely with zinc.  This is a form of “sacrificial protection” as well.  It is suitable for smaller objects, e.g., car bodies, buckets and nails.  See the next Section 2.36.