Sodium Acetate

Introduction
Public summary: 

Find out about how handwarmers work, and see solid crystals form in seconds right in front of your eyes.

Rapid crystal growth from sodium acetate.
Useful information
Kit List: 

Sodium Acetate
Saucepan + cooker ring of some sort
Stirrer (not glass)
Metal cups for pouring
Plastic cups (ideally clear)
Cling film (not value cling film as it doesn't cling very well in this situation)
Cold water tray

You will probably need one person making the solution and the other demonstrating.

Packing Away: 

The sodium acetate solution can be cooled, crystallised and collected for re-use. When cooling, don't let it form one large solid layer in the pan! Try to break it in smaller chunks/crystals that are easy to put away. Talk to a committee member about whether it's clean enough to keep. Never pour hot sodium acetate solution down a sink: it will crash out on cooling and may block the drain!

Frequency of use: 
1
Explanation
Explanation: 

In a nutshell..

Sodium acetate forms a very stable supersaturated solution quite easily, so it is possible to grow crystals in front of the kids' eyes. First make a supersaturated solution and cool it, then add a crystal of sodium acetate to provide a nucleation centre for rapid crystallization.

How to set up the experiment

1) Dissolve the sodium acetate - it has already had water to it, so this should mean that you just have to warm it up. Try not to boil it too hard, as this will drive off water.
NOTE: If the sodium acetate does not seem to be going into solution, check carefully that you are heating it before adding more water. The solution in the pot usually has a thin crust on top and it still works fine.
2) Use the metal pots to pour the solution into plastic ups, and place cling film over the top, so you don't get small crystals forming on the surface and triggering the crystallisation. Turn the stirrer off before you pour.
3) Cool the cups down in cold water or ice. A mixture of cold water and ice works well. Expect at least a third of solutions to crash out before demonstrating!

Demonstration

Start by explaining that there is loads of stuff in the water and that it wants to from crystals but it can't, because water is holding it apart, so it's easy to grow crystals but hard to start them off. In explaining what sodium acetate is, possibly call it a cross between salt and vinegar or explain that it comes from a reaction between bicarb and vinegar.

Get a kid to feel the temperature of the cup.

Remove the cling film, get a kid to drop a crystal into the solution and it should grow crystals really rapidly, starting at the nucleation crystal. There ought to be different shapes of crystals growing at different speeds, too, so you can talk about that and rates at which they grow (maybe tie with liquid nitrogen ice cream).

Get the kids to feel the heat given off from the crystallisation process.

Heat is usually released when things crystallize in the same way it is when they condense or freeze; this release of heat is why steam is dangerous. Get kids to realise that they have to heat things to get them to melt/boil/often dissolve, so that energy is trapped in the liquid/gas/solution and is released again when the process is reversed (here crystallisation).

You can also use solutions to make towers in a plastic dish by pouring the solution out, this should be enough to trigger crystallization.

Where do we see this?

Nucleation:
Clouds won't form unless they have nucleation points so people have tried adding dust to clouds to stimulate rain - The Soviets used to seed clouds before they got to Moscow to stop it raining on days of the big parades.
The commercial hand warmers have a little clickey thing that you click to make it start crystallising. Apparently these work by trapping little crystals in the crack, but trapped deep so they can't act as nuclei, when you click the thing it releases them - starting the crystallisation.

Sodium acetate solution has a much lower freezing point than water (like brine). It is used to de-ice planes as it is less environmentally damaging than NaCl, KCl or ethylene glycol (the traditional alternatives). It is used as a commercial deicer in airports and similar places, because it is attracted to water so strongly it will decrease its melting point significantly.

Risk Assessment
Date risk assesment last checked: 
Wed, 17/01/2018
Risk assesment checked by: 
grh37
Date risk assesment double checked: 
Sun, 04/02/2018
Risk assesment double-checked by: 
Giedre
Risk Assessment: 
DESCRIPTION Dissolve sodium acetate in water, which requires heating to 80-90°C, then allow it to cool forming a supersaturated solution, which can be crystalised with a seed crystal.
RISKS # 1. Risk of burns from hot sodium acetate solution.
# 2. Risk of solution splashing into eyes - it is not poisonous, but has a high osmotic potential, so will sting like salt.
# 3. Slip hazard from spilled water/salt solution.
# 4. Risk of cuts from broken glass beakers.
# 5. Electrical parts risk - see Electrical Parts RA.
ACTION TO BE TAKEN TO MINIMISE RISKS # 1. The samples used must already have been cooled in order to crystallise, so the hotplate/hot liquid can and should be kept out of reach of the public at all times. In practice, for a long event, this may mean that two demonstrators are needed, one to heat the solution, the other to demonstrate. Demonstrator to wear eye protection (goggles) when heating up the solution.
# 2. Demonstrator to try to stop the kids putting their fingers in the liquid. Demonstrator must ensure eyewash is nearby, and know its location.
# 3. Use a tray to catch spills, and mop up any spills immediately.
# 4. Avoid using glass beakers to prepare sodium acetate solution if possible; otherwise, clear up broken glass immediately..
# 5. See Electrical Parts RA.
ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT # 1. Run burn under cold water for at least 10 minutes, and call first aider.
# 2. Call first aider. Use eyewash to wash out of eyes if trained and confident to do so.
# 3. Call first aider.
# 4. Call first aider.
# 5. See Electrical Parts RA.
This experiment contains mains electrical parts, see separate risk assessment.
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