Hand warmers - click the button for instant warmth. Boil to re-use.
A nice rainy-day project that has a bit of science behind it but will also be popular with kids in the winter is making re-usable hand-warmers.
There are a group of girls from my daughter's class who come over a few times a year to do a day's science or technology project with me. We try to illustrate some scientific or technical feature and use that as the basis of making something to take home and use/play with. We aim to use tools and methods that might not normally be encountered by little girls (they were 7-8 years old) and always make sure there is something to decorate!
One autumn we made re-usable hand warmers. They are not hard to make and illustrate many scientific concepts. At a basic level, there is the idea of states of matter; solids, liquids and gasses, and phase changes due to heating/cooling. At a deeper level, there is crystals and crystallisation, seeding crystalisation and super-saturation and at a relatively high level there is latent heat of fusion (enthalpy of fusion) and conservation of energy. You could pitch this project anywhere from age around 6 up to maybe 13-14 depending upon how deeply you analyse what's happening. Most importantly, they are fun to make!
This instructable covers all the steps of making the warmers. One or two of these I did in advance of our project because you need strong adult hands. Cutting the metal discs, for example and pre-cutting the PVC sheets to a standard size. The handout that I gave to my kids is attached in the last step as a .pdf file.
CREDIT: This project was based upon the excellent work of kevin_mic in this instructable: https://www.instructables.com/id/Sodium-Acetate-BODY-Warmer/ I have tweaked the project a little to make it more suitable for little hands but the fun-fun-fundamental ideas are all his.
Step 1: What You Need
These are the materials that I used for a pair of small-ish hand warmers. They get to 50'C so large ones could be a danger to little hands so we kept them fairly small and put them in a little cloth bag to give a slightly lower heat for longer.
The following are the tools and materials that we used.
Materials (for each pair of hand-warmers):
4 x 11cm by 13 cm clear PVC*
2 x small disc of metal cut from a slap-bracelet
1 x 150 g Sodium acetate (tri-hydrate)
1 x 30 ml water
2 x Small cotton draw-string bag (optional)
Scissors and/or roller-cutter
Fabric Pens (if decorating the bags)
Step 2: Preparation
Before letting the kids loose on the materials, I did the following preparation:
Slit the slap-bracelet with a knife and remove the outer cover.
With strong scissors, cut the resulting metal strip approx into squares. You should get around 8-10 from a bracelet.
Cut each square into a circle with scissors.
Roughly sand the edges to avoid cutting the kids
Cut the PVC sheet into 11 x 13cm squares with a cutting mat and roller-cutter.
Step 3: Make the Starter
To make the starter, we are going to smooth off the metal discs and punch a hole through each with a nail.
Firstly, the kids use some fine (300 grit +) sand paper and smooth off any rough edges on the discs.
Now put the nail through something that won't hurt if it gets hit - I used the insulation from a bit of old mains wire. A little bit of scrap wood would be fine too. Remember: these kids have probably never held a real hammer before!
On a bit of scrap wood, get the kids to bang the nail through the middle of the disk. If it goes through a long way then you will have to help them remove the disc from the nail with pliers.
The kids then put the disk on the flat side of a brick and bash it flat with the hammer.
Step 4: Decorate the Plastic
On each PVC sheet, when you start decorating, put a dot near the top-right corner - this allows you to tell which side has been decorated.
We will decorate the inside of the hand-warmers so that the ink does not come off. However, some colours of even permanent markers will dissolve in your solution! We found the turquoise sharpies just made the solution blue - you might like to test your marker pens!
Deocrate with any pattern you like. i printed some pictures at 8 cm x 10 cm and let the kids trace and colour them. Any writing needs to be mirror-writing so it reads correctly. You can always write on a scrap piece of PVC and turn it over to trace it.
Keep all decoration away from the edges - leave half an inch or so all around. Some marker pens seemed to interfere with the heat-sealing, meaning that if they overlapped with the sealed edge, the seal would be weak or not hold at all.
Step 5: Make a Pouch
Line the pairs of plastic sheets up with the decorated sides together.
Seal all around, then seal around again to give a double-seal. This helps with a robust finish.
Trim all around using the cutting wheel or scissors, then clip off one corner. You need to leave an opening that will admit the starter you made earlier, and also will take a funnel for pouring in the acetate solution.
Step 6: Fill 'er Up!
Weigh out your sodium acetate - we used 150g for two pouches, based on 11 x 13 cm pieces of vinyl.
Place the acetate in a small sauce-pan and heat with two table-spoons of water (2 x 15 ml).
Meanwhile, place the starter into the pouch and thread in a straw to help keep it open to the bottom when you fill it up (& to support the funnel). Place a funnel in the opening and support in s suitable vessel (we used a mug).
Once the acetate is liquid, pour half into each pouch, remove the straw and clean off any excess on the opening using a damp tissue. Dry the opening, squeeze out as much air as possible and double-seal with the heat sealer.
Step 7: Heat, Cool & Use
You will probably find your hand warmers will crystallise initially, so before use you will need to heat them in a simmering pan of water. Remover with tongs and leave to cool.
When you are ready for warm hands, simply "snap" the metal disk and you should see the acetate start to crystallise. Sometimes they take a few goes to get started first time but after a few uses they do start easier. If snapping does not work to start, try rubbing the hole in the starter between thumb and finger (through the plastic obviously). That will often start a stubborn hand-warmer.
To re-use, just repeat the boiling and cooling.
Enjoy your project - and enjoy warm hands, whatever the weather!