The alkali metals lithium, sodium and potassium all react with chlorine gas to produce their respective chlorides.
2Li + Cl2 –> 2LiCl
2Na + Cl2 –> 2NaCl
2K + Cl2 –> 2KCl
In the animation below samples of lithium, sodium and potassium metals are shown being heated in a blue Bunsen burner flame until they catch fire. The flaming metals are then dropped into gas jars containing chlorine gas. Each metal reacts showing its characteristic flame colour and a product seen as a white smoke (solid particles of the chloride).
Here are three pictures taken from a movie of the reactions which can be viewed on You Tube here (3 minutes).
We carried out the experiments in broken porcelain crucibles because when we used combustion spoons the iron metal, in the steel from which the combustion spoons were made, also reacted with the chlorine gas (movie here). Dense red smoke, presumably of iron (III) chloride, was produced in addition to the alkali metal chlorides.
We had to use broken crucibles because whole crucibles would not fit inside our gas jars.
As for comparing the reactivity of the three metals, potassium reacted the fastest, with the reaction over in a couple of seconds. Both lithium and sodium reacted for several seconds, with longer lasting flames.
However, the reaction between potassium and chlorine shown in the animation above did not go to completion because there was unreacted chlorine gas at the end of the experiment, (visible in the picture). There was also unreacted potassium and this reacted violently with water when the gas jar was was washed out, after allowing it to cool.
An animation of the reaction between potassium and chlorine in another experiment is shown below (movie here). This one did not react so violently, but was also over in a couple of seconds.
Thus, burning potassium reacts quicker than lithium and sodium with chlorine gas, but its almost as if the potassium chloride produced smothers the reacting metal.
What colour flame would you expect to see if burning magnesium ribbon was dropped into a gas jar full of chlorine gas?