Silane is toxic and all reactions involving magnesium silicide to produce silane should be carried out in a fume cupboard.

In the August 2016 blog we looked at the reaction of magnesium with oxygen and chlorine.

Magnesium is so reactive it will take the oxygen away from silicon in sand, (silicon dioxide, SiO2) .

Magnesium reacts with sand to produce magnesium oxide and silicon.

2Mg + SiO2  -> 2MgO + Si

If an excess of magnesium is used, magnesium silicide (Mg2Si) is also formed.

2Mg + Si  ->  Mg2Si

Silane is produced when magnesium silicide is reacted with dilute acids, such as hydrochloric and sulfuric acids, to produce silane.

For example,  4HCl + Mg2Si  ->  SiH4 + 2MgCl2

Silane bursts into flames as soon as it is produced.

For example, SiH4 + 2O2  ->  SiO2  + 2H2O

Experiment 1 – heating magnesium and silicon dioxide using a Fresnel lens


The Fresnel lens here was taken from an old overhead projector


Quite a violent reaction, over in a few seconds


Boiling tube cracked by the ferocity of the reaction

Experiment 2 – heating magnesium and silicon dioxide using a Fresnel lens


Powdered magnesium and silicon dioxide


Heat using the focussed rays of the sun

Experiment 3 – heating magnesium and silicon dioxide in a crucible using a Fresnel lens


Adjust the lens to focus the suns rays onto the mixture


An exothermic reaction, over in a flash


The product mixture contains magnesium silicide which must be left to cool for several minutes

Experiment 4 – adding dilute sulfuric acid to the product in a boiling tube


Experiment 5 – adding dilute hydrochloric acid to the product mixture


The product mixture containing magnesium silicide


Small, sputtering explosions


Caught in the act

Experiment 6 – slow motion animation of adding dilute hydrochloric acid to the product mixture


Slow motion

Links to movies on You Tube:

Experiment 1
Magnesium and silicon (IV) oxide reaction 1 – boiling tube / Fresnel lens
16 seconds

Experiment 2
Magnesium and silicon (IV) oxide reaction 2 – boiling tube / Fresnel lens
39 seconds

Experiment 3
Magnesium and silicon (IV) oxide reaction 3 -crucible / Fresnel lens
24 seconds

Experiment 4
Magnesium silicide and sulfuric acid – boiling tube
14 seconds

Experiment 5
Magnesium silicide and sulfuric acid 3 – watch glass in the dark
40 seconds

Experiment 6
Magnesium silicide and hydrochloric acid at 120fps, no sound (watch glass)
56 seconds

Don’t look into the flame!

At the end of the last post which was about the reactions of the alkali metals with chlorine, I asked what you would see if burning magnesium were lowered into a gas jar containing chlorine gas. The answer is you would see a bright white flame.

The flame produced by magnesium in such reactions is so bright you must not look at it directly, because of the risk of damaging your eyes. Always wear eye protection and follow the recommended safety advice in your laboratory.

Sometimes its easier, although less exciting to look at a movie of such reactions. Movie here, (51 seconds).


Magnesium burns in oxygen with a bright, white flame

What is the product of the reaction between magnesium and oxygen? Can you write a balanced symbol equation for the reaction?

Magnesium also reacts violently with chlorine, producing the same bright, white flame. In the movie here (54 seconds), burning magnesium ribbon is lowered into a gas jar containing a mixture of chlorine gas and air. Listen for the crackles at about 18 seconds into the movie, indicative of the very energetic nature of the reaction.


Magnesium reacting with chlorine and oxygen from the air

What is the product of the reaction between magnesium and chlorine? Can you write a balanced symbol equation for the reaction?

Another movie of the reaction between magnesium and chlorine from the RSC Science Skool can be seen here,(1 minute 28 seconds).

Magnesium is so reactive, it will also take the oxygen away from silicon in silicon (IV) oxide. Carry out some research and find out the products of this reaction and an equation for it.

Answers to all of the questions in this post next time.


Making lithium chloride, sodium chloride and potassium chloride by direct combination

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


Experimental set-up

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).


Flaming metals and chlorine

Here are three pictures taken from a movie of the reactions which can be viewed on You Tube here (3 minutes).


Lithium reacting with chlorine to make lithium chloride


Sodium reacting with chlorine to make sodium chloride


Potassium reacting with chlorine to make potassium chloride

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.


Combustion spoon: lithium and chlorine


Combustion spoon: sodium and chlorine


Combustion spoon: potassium and chlorine


Products from experiments carried out in porcelain crucibles were all white crystalline solids

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.


Close-up: potassium reacting with chlorine

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?


Li sizzles, Na balls & K-pot explodes

Comparing the reactions of lithium, sodium and potassium with water.


A lump of potassium explodes

The alkali metals all react speedily with water and with oxygen.

For this reason they are stored in oil to limit the oxygen and water in air getting to them in the chemical store.


Lumps of lithium, sodium and potassium metals in hexane prior to dropping them into water

Students at GCSE are often asked to compare the trend in reactivity of the metals on going down the group, by comparing the observations made as each one reacts with water.

To this end experiments are sometimes carried out, demonstrating the reactions of lithium, sodium and potassium with water.

All three metals float on water and react quickly with it producing hydrogen gas.


Potassium always catches fire producing a lilac coloured flame and explodes if too big a chunk is used.


“Li sizzles, Na balls and K-pot explodes lilac”

Equations for the reactions are:

2 Li   +   2H2O  –>  2LiOH   +   H2
2 Na   +   2H2O  –>  2NaOH   +   H2
2 K   +   2H2O  –>   2KOH   +   H2

Students are also often asked to predict how rubidium and caesium metals would react with water.

The answer is that they would both react producing similar products according to the equations above, but even more vigorously than potassium, with caesium being the most reactive.

Links to movies on You Tube from which the above gif animation was made are here:

1. Short version 39 seconds

2. Longer version 2 minutes and 3 seconds


Why has no one has ever dropped a lump of Francium into water and made a movie of it?



Fresnel Thermite

We carried out some experiments recently lighting a length of magnesium ribbon with a Fresnel lens. The Fresnel lens was used to focus light rays from the sun on the magnesium. The magnesium ribbon acted as a fuse to ignite a thermite reaction mixture in which it was embedded. The experiments went rather well providing a relatively safe and unhurried way of starting this energetic reaction.


Using a Fresnel lens to start a thermite reaction


The power of the sun


We used 20g of thermite mixture made from aluminium powder (5g) and iron (III) oxide (15g) which reacts according to the following equation:

Fe2O3 + 2 Al → 2 Fe + Al2O3


Stand well back


and focus


a sample of iron can be obtained once the mixture has cooled down

The experiments were carried out in the tropics and it would be interesting to see if the same method works in a more temperate clime.

Links to movies of the experiments on You Tube can be found here:

  1. Fresnel / Thermite 1
  2. Fresnel / Thermite 2



Blue ruby

This post contains a collection of animated gifs and other pictures from experiments we have been doing recently.

First a heart made by putting copper foil into silver nitrate solution (copper is more reactive than silver and displaces silver ions from solution).


Silver heart

Next, heating a beaker of ice to water and steam.


Ice water steam

Those are anti-bumping granules you can see at the bottom of the beaker in the boiling water.

Now for a rather pretty B/Z reaction we carried out recently.


A Belousov-Zhabotinsky reaction

Captured using time-lapse mode


99 pictures at 15 seconds interval

Finally, the curious case of the colour of chromium (III) ions in water. And hence the title of this post ‘Blue ruby’.

When asked “How do you make a blue heart red?”
“With a torch and chromium (III) ions”, I said

Jim Clark, writing on notes that “The simplest ion that chromium forms in solution is the hexaaquachromium(III) ion – [Cr(H2O)6]3+.” He also goes onto say that “The hexaaquachromium(III) ion is a “difficult to describe” violet-blue-grey colour.”

Our AQA A’ level Chemistry describes [Cr(H2O)6]3+ as being ruby in colour, but our solution of chromium (III) chloride definitely looks blue to me.


chromium (III) chloride solution

That is, until one looks at it under torchlight (tungsten filament bulb), then
indeed it does appear a ruby colour.


blue or ruby?


Use a torch


shine a light


then you’ll see ruby delight


off on off


has that boiling tube got a red eye?


Sweet TLC hydrolysis

Aspartame sweetener is a modified dipeptide. When aspartame is reacted with 6M hydrochloric acid the peptide bond should be hydrolysed (broken), producing aspartic acid and phenylalanine methyl ester. The methyl ester may also be hydrolysed, depending on the severity of the conditions.


Whiteboard of the aspartame hydrolysis by my friend and colleague Tony Pluck

We carried out a hydrolysis reaction by boiling aspartame sweetener (one individual sachet) with 10ml of 6M HCl in a boiling water bath for 30 minutes. The results were analysed using thin layer silica chromatography, (silica TLC).


Running a chromatogram (silica TLC plate)

Samples of aspartame sweetener were spotted before and after the hydrolysis, together with phenylalanine and aspartic acid standards.

The plate was run in a solvent system of butan-1-ol (12ml), glacial acetic acid (3ml) and distilled water (6ml) taken from the old Nuffield A’level Chemistry textbook .

After allowing the solvent to run up to within a few cm of the top of the plate it was removed from the beaker and dried using a hot hair dryer. The TLC plate was then sprayed with ninhydrin and heated in an oven at 110 C for approximately 10 minutes.


Samples left to right, phenylalanine, hydrolysis mixture, aspartic acid and aspartame sweetener.


Same as the above with labels on the plate

Another experiment was carried out, identical to the above except with a much shorter hydrolysis time (of about 3 minutes). This was to see if the whole experiment could be done in a 55 minute period of chemistry at school.


The same again with a much shorter hydrolysis time of a few minutes

We want to run further chromatograms spotting smaller samples, as both of the above plates suffered from streaking due to sample overloading.



Tropical freeze

Cool a bottle of water in the freezer compartment of a refrigerator and it is possible to supercool the water below its freezing point for a short period of time. If one removes the bottle and gives it a sharp bang on a hard surface the rapid formation of ice crystals ensues in what is popularly referred to as the ‘instant ice’ experiment. I’ve been trying to capture the instant ice freezing experiment on camera for several weeks now without much success.

One of the biggest problems of doing this experiment in the tropics is that as soon as the bottle is removed from the fridge condensation forms on the outside of the bottle. This prevents one seeing whats going on inside.

If you try this at home I can only repeat the advice given on several You Tube movies; use as many bottles as you can and keep monitoring them to arrive at just the right moment to take them out by trial and error. My 600ml bottle took somewhere between one and a half to two hours.

Tropical freeze…


Whoosh brothers

The whoosh bottle demonstration involves setting light to methanol vapour in a thick walled 20L water bottle. By adding a few drops of water into the methanol, salts such as LiCl, NaCl and KCl can be incorporated into the ignition mixture and add colour to the resulting flame.

Here are some images of the same.

First three pictures of the experiments:


Lithium whoosh – the bottle on the right did not ignite.


Sodium whoosh – only the middle one


Potassium whoosh

Next are three animated gifs of the experiments:


What colour do you see in the flame in the bottle? Is it the characteristic flame colour from Li, Na or K?


Tried for a triple whoosh, but only the middle one worked. Was it Li, Na or K?


Better colours with the lights out? It’s usually difficult to see the colour of the flame with this shy violet. Li, Na or K?

The whoosh brothers! A composite gif animation of all the above.


Whoosh brothers!

Try as I might I could not get more than one bottle to ignite at a time. Movies uploaded to You Tube soon.



Follow appropriate safety guidelines when igniting the bottles. Methanol vapour is toxic. Only use thick walled plastic bottles. Do not try more than one ignition per bottle. Wash each bottle by filling completely with cold water after each ‘burn’. Our laboratory was ventilated with the all the windows open after the each experiment for more than an hour.