A microscale mag sulfate experiment


The materials for making magnesium sulfate in a microscale reaction

Making salts on a micro-scale in a high school chemistry laboratory has the advantages of being quick, simple and easy to prepare. For example, here is an experiment we recently carried out making magnesium sulfate in an IGCSE Chemistry class.

A two decimal place electronic balance, tweezers and a 0.2ml (200 microlitre) automatic pipette were used in addition to the materials shown above.


1. Cut out a 10cm x 10cm plastic sheet (from an A4 plastic wallet) and record its mass on an electronic balance.

2. Place 0.2ml of 1M sulfuric acid on the centre of the sheet.

3. Take a 4 cm length of magnesium ribbon and bend it into an oval shape which can be placed around the globule of acid.

4. Place the magnesium around the acid.


Ring fence hubble bubble

5. When the bubbling has stopped, pick up the magnesium with a pair of tweezers and rinse off with a small volume of distilled water.


Rinse, rinse

6. Place the plastic sheet in a suitable place, such as a sunny windowsill or a low temperature drying cabinet, to allow the water to evaporate and the salt to crystallise.


Magnesium sulfate – has all the water gone or not?

7. Weigh the plastic sheet + the solid using an electronic balance and determine the mass of product formed.

8. If time allows, break up the crystals with a spatula and dry further using warm air from a hair dryer until a constant mass is obtained.

Calculations can be performed to determine the percentage yield of magnesium sulfate (presumably the heptahydrate) made.

The plastic sheets can be washed and re-used.

Thanks to Bob Worley for inspiring us to try out this and other high school chemistry lab experiments on a microscale. Here is a link to Bob’s Microchemuk website which is a goldmine of resources and ideas.

Two dark solids

Two dark solids, one giant ionic, the other covalent molecular.



Iodine is a molecular solid. It consists of I2 molecules. The molecules are non-polar and are held together by weak intermolecular attractions in the iodine crystals.


Potassium manganate (VII)

Potassium manganate (VII) is an ionic solid, made up of K+ ions and MnO4 ions in a giant crystal lattice structure.

The differences in bonding and structure between the two materials means they have quite different properties.


Water dissolves many polar molecules and ionic solids.

Here water is being added to iodine and potassium manganate (VII).


Which tube contains the iodine and which the potassium manganate (VII) ?

Hexane is a non-polar organic liquid which dissolves non-polar molecules, but not ionic solids.

Here hexane is being added to iodine and potassium manganate (VII).


Which tube contains iodine and which potassium manganate (VII) ?

Behaviour in an electric field

What would happen if solid iodine and solid potassium manganate (VII) were placed on some filter paper moistened with tap water and subjected to 10 volts?


In potassium manganate (VII) the K+ ions are colourless, whilst the MnO4 ions are deep purple in colour.

Which way do the purple ions move and why?

Iodine dissolves in potassium iodide solution

The I2 molecules in iodine do not dissolve very well in pure water, but they do dissolve in a solution of potassium iodide.  I3 ions are formed when iodine dissolves in potassium iodide.

Behaviour in an electric field

What would happen if solid iodine and solid potassium manganate (VII) were placed on some filter paper moistened with potassium iodide solution and subjected to 10 volts?


Why does the brown colour move to the left at the top of the slide?

This one maybe harder to explain fully. So let’s close with a little rhyme:

“Two dark solids, sometimes purple, sometimes brown. One ionic, one molecular, with behaviour that can make you frown.”




Sometimes things don’t go the way the way they’re planned. When this happens in chemistry the first thing is to see if you can repeat the observation. Some examples follow, maybe you can explain them.

Here’s an experiment we did recently using the juice extracted from red cabbage as an acid/base indicator. We were surprised by the colour produced in strong alkali on the far left of the picture.


Yellow at pH 12 to 14?

We checked with a second extract and got the same result


Red cabbage juice in 1M NaOH and 2M HCl

In another experiment we tried to grow one big crystal of sodium chloride by suspending a ‘seed’ crystal in a saturated solution of brine. The seed crystal fell off and then something magical happened around the string.


Boxes, little boxes..

Finally, when we tried to demonstrate diffusion by dissolving a small lump of potassium manganate (VII) in a beaker of water, things didn’t go as smoothly as planned:


Jittery time lapse animation (overnight)

Science, the fun part is in figuring out what happened.



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 Chemguide.co.uk 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?