What it shows
A supersaturated solution holds more solute than it normally should. Touching it with one seed crystal sets off fast crystallisation that gives out heat, so the liquid turns into a warm solid.
The science
Baking soda and vinegar react: NaHCO3 + CH3COOH gives CH3COONa + H2O + CO2. The product is sodium acetate in water. Boiling off water makes a concentrated solution. When it is cooled very gently with no seed, it stays liquid although it holds more sodium acetate than it can normally keep at that temperature. A single seed crystal triggers crystallisation of sodium acetate trihydrate, and the crystallisation gives out latent heat, so the solid warms to near 50 degrees C.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- White vinegar, 1 litre
- Baking soda, 4 tablespoons
- A wide steel pan
- A stove (adult only)
- A clean glass jar with a lid and a clean saucer
- A spoon and a small piece of paper
- A fridge for cooling
- A thermometer if possible
Safety
For this project: An adult must supervise and do all boiling. Vinegar fumes irritate the eyes and the pan can overflow with fizzing, so add soda a little at a time in a ventilated room. Do not taste the product. The hot solution can scald, so cool it before pouring.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Pour 500 ml of vinegar into the pan and add baking soda one teaspoon at a time. Wait for the foaming to stop between each addition.
Keep adding until a new spoon of soda no longer fizzes. The solution is now sodium acetate.
Boil the liquid gently, with an adult, to reduce it until a crust of crystals forms on the surface and the volume is a quarter of the start. Do not boil it dry.
Pour the hot liquid into a clean jar leaving behind any solids, cover it and let it cool without moving it. Cooling in a fridge for 30 minutes helps.
Put a few drops of the original solution on a saucer and let them dry to make seed crystals.
Test a small amount in a cup: touch it with a seed crystal and see it crystallise upwards.
Pour a thin stream of the cooled liquid onto a seed crystal on a saucer so a tower of solid builds as you pour. Touch it and feel the warmth.
Re-melt the solid in hot water and cool again, to show the process can be repeated. Make a note of the temperature reached.
What you should see
The clear liquid freezes into a white solid tower at once, and the solid feels warm. It can be melted and made to freeze again many times.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is a supersaturated solution?
A solution holding more dissolved solute than it normally could at that temperature.
2Why does the solid feel warm?
Crystallisation is exothermic and releases latent heat.
3What is the seed crystal for?
It gives particles a surface to start growing on.
4Where is this idea used?
In reusable hand warmers and heat packs.
Take it further
Measure the temperature rise with a thermometer and compare it for different amounts of solution.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Safe Chemistry Projects for School
Experiments and models with kitchen materials and proper safety
Science projects
Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology