The Rainbow Snow Globe demonstration

Abstract:
With a simple winter-themed demonstration, I show how you can use a little diet salt to make a homemade snow globe that forms needle-like crystals of potassium chloride with a simple trick. Although, it is also time to party for the new year, so these little guys also love to refract light into a mesmerising array of colours. The demonstration pulls this off by manipulating temperature solubilities, so you can enjoy repeating it again and again with the aid of only a heat source.

Introduction:
Well winter is here and in full force, and Christmas has only just recently passed, so I think some snow would be lovely. Here in the south of the UK it never snows, it just stays uncomfortably cold, but with a little chemical magic we can deny the weather and predict hurricanes of snow! Except our hurricanes of snow are contained in a jam jar and the snow is just potassium chloride.

The reality of this demonstration is a little more than just that though. The basic idea is we are manipulating the temperature solubility of potassium chloride in water. A common chemist fact is that most chemicals are more soluble, and more will dissolve, in hot water, and vice versa. This allows us to saturate a high temperature solution of water, meaning no more potassium chloride will dissolve, and let the temperature fall by cooling in air, causing the solubility of the potassium chloride to decrease and crystals of the compound to form.

Sure this is pretty and all, crystals raining down from above that look like snow, but why use potassium chloride and not something else, such as ammonium chloride which crystallises to form what looks more like actual snowflakes? Good question! I have a few answers for that.

Firstly, potassium chloride will form cube-like crystals that are similar to its brother sodium chloride. This all changes though once the solution is agitated from side to side. I stumbled upon this accidentally when recrystallising some potassium chloride. The compound starts to slowly switch crystal shapes over a minute or so to form needle-like crystals (as well as the odd rhombus it seems).

Secondly, these new crystalline shapes seem to possess the ability to refract light, maybe due to their size, in such a way that shining a bright white light on them as they fall reveals a rainbow of colours. Some crystals will blink blue, some red, purple, green, yellow, pink – hence the ‘rainbow’ in the demonstration title. Every second you get a multitude of different colours, although each crystal blinks a colour only for a fraction of a second so unfortunately photos are not great at showing this effect and you may only catch one or two blinks in a photo. You will see this later on.

Safety:

Substance Hazard information
Potassium chloride Low Hazard
  • This is an extremely safe demonstration and I can only mutter pernickety safety precautions such as boiling water. Potassium chloride is about as dangerous as sodium chloride, common table salt. This is one of the few experiments on my blog where gloves and goggles are an inconvenience and I recommend not to use them.

Experimental:
This demonstration can be carried out with potassium chloride found as a low sodium salt alternative, but if it is not a pure product, such as the brand I will show here which is 51% potassium chloride and 48% sodium chloride, this can significantly impact the quality of your finished snow globe and a purer source of potassium chloride is recommended. With the brand I used it was not nearly as successful a snow globe as with pure potassium chloride. It still works and I will show the method here.

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A thermometer is very useful to make sure you reach 80oC and not go over or under, but if you do not have a thermometer handy do not worry, it does not need to be precise – just heat the water until the sides of the container start to mist up and form droplets of water.

A 250ml beaker was placed on a hotplate and 100ml of distilled water was added along with a stir bar. The magnetic stirrer was turned on low in anticipation of adding the potassium chloride source to dissolve. Heating was also turned on to try and reach a water temperature of 80oC.

I recommend 80oC because as this demonstration is repeatable, the solution only needs to be heated again to redissolve all of the crystals and then left to cool like before. A microwave oven is much more convenient though to heat up the solution than a hotplate though – it is faster, in every household, but struggles to get water above 80oC. Therefore if you do not own a hotplate, a microwave oven can be used in its place for this initial step too. If the solution cools off too much while dissolving the potassium chloride just give the solution another blast in the microwave oven.

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It does not matter if you over-shoot and add too much potassium chloride that will not dissolve – it will be filtered out in the next step.

Once the water reached the preferred temperature, I kept pouring in the potassium chloride in the form of a brand of low sodium diet salt as mentioned until no more of the salt dissolved.

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I covered the funnel with plastic wrap while filtering to help keep in the heat of the solution.

I then opted for a hot gravity filtration. I stuffed a cotton ball into a funnel to act as the filter and ran the hot solution slowly through. This step is to remove any insoluble materials added to the salt which, depending on brand, can be various anti-caking agents that we do not want in our final snow globe. If you have a pure source of potassium chloride this filtration step can be skipped.

I had to reheat the filtrate to redissolve some crystals that inevitably formed as the solution cooled while waiting for everything to filter through. Otherwise I am done and I can pour the solution into my chosen container. I recommend a thick-walled, glass container with a large open top – something like a jam jar works very well.

Results:

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The top of the solution needs to be exposed to air so that it cools down on top quicker, encouraging crystals to form on top and then rain down like snow, dragged down under their own weight.

I left the solution to cool in its chosen container and after a couple of minutes crystals started to form on the bottom of the container and on the top of the solution.

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It does remarkably look like snow. It is not the desired effect I was looking for though.

With gentle shaking of the container from side to side a few times and then waiting for a couple of minutes, tiny specks of crystals will start forming and falling by themselves. Now that is looking like a beautiful white winter! The needle-shaped crystals do form a little, but their presence is quite limited. I was not able to get a good photo of them unfortunately. I think we need to step it up.

 

I repeated the method except using a pure source of potassium chloride I had purchased online. With the same treatment, long needle-like crystals formed much quicker and in larger quantities. Soon I had achieved a real snow blizzard – chemistry style.

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If you look closely in the centre you can spot a crystal producing a lovely red hidden amongst the masses of white crystals.

It is quite obvious to see that the purer source of potassium chloride was the champion, I highly recommend using a purer source than the brand I used of diet salt but even that still worked. You could make impure diet salt products work better for this demonstration by collecting the crystals that form in the demonstration (which are now purer potassium chloride) and then producing a new solution with these crystals. This is how I stumbled onto this effect in the first place.

I wish I could add a video here as filming captures the tens of coloured sparkles every second very well whereas the still photos for some reason only capture a measly one or two. Pictures just do not do the demonstration justice, I have many friends who would completely agree after I showed it personally to them!

I would link to a YouTube video of someone covering this demonstration but nobody seems to have uploaded a video on it surprisingly! Then again, I did stumble upon it in my own time and made up a demonstration name. The only results I get are to do with a band and the videos are titled ‘KCL – The Snow Globes’ which I think is hilarious for this post as KCl is the chemical symbol for potassium chloride! I should have got them to sponsor this post… missed opportunity!

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Although this photograph is not too spectacular, I wanted to include it as I wanted to show more of the colours the crystals could make. I hope you can spot the two golden-yellow crystals near the top centre if you look closely enough.

The demonstration is just a simple manipulation of temperature solubilities that many chemists are used to, but I hope you can appreciate the potential for beauty of this Rainbow Snow Globe demonstration! Happy new year everyone!

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