What it shows
Dissolving some solids makes the water colder (endothermic) and dissolving others makes it warmer (exothermic). The reason is the balance between the energy needed to break the solid apart and the energy released as water surrounds the particles.
The science
Dissolving takes in energy to pull the particles of the solid apart and gives out energy when water molecules attach to them. If the first is bigger, the solution cools. Urea, baking soda and citric acid all take in more than they give out. Calcium chloride gives out more, so it warms the water. Temperature change in a foam cup is a fair measure because foam is a poor conductor, so little heat enters from the room.
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.
- A thermometer reading to 0.5 or 1 degree
- 2 foam or paper cups nested together as a simple calorimeter
- A measuring cylinder or syringe for 50 ml of water
- Garden urea, baking soda, citric acid, table salt and sugar, 5 g of each
- Dehumidifier calcium chloride crystals, 5 g (adult only)
- A stopwatch and a spoon
- A kitchen scale
Safety
For this project: An adult must supervise and handle the calcium chloride and urea with gloves and wear goggles. Do not taste any chemical and wash hands afterwards. Calcium chloride is an irritant and can become hot.
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.
Put 50 ml of tap water into the foam cup and wait two minutes. Record the steady starting temperature.
Weigh 5 g of the first solid. Add it all at once, start the stopwatch and stir gently with the spoon, not with the thermometer.
Read and record the temperature every 15 seconds for two minutes.
Note the lowest or highest temperature reached and work out the change from the starting value.
Empty and rinse the cup, wait until it comes back to room temperature and repeat with the next solid. Use a fresh 50 ml of water every time.
Plot a graph of temperature against time for all the solids on the same axes in different colours.
Draw a bar chart of the temperature change for each solid, with a minus sign for cooling and a plus sign for warming.
Repeat the urea test using 10 g to see if double the mass gives about double the change.
What you should see
A bar chart showing that urea, baking soda and citric acid cool the water, salt and sugar change it very little and calcium chloride warms it. Doubling the mass roughly doubles the change.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does endothermic mean?
A process that takes in heat from its surroundings, so the surroundings get colder.
2Why use foam cups?
Foam is an insulator, so less heat enters or leaves and the temperature change is more accurate.
3Why is the same volume of water used every time?
To make it a fair test: the same mass of water needs the same heat for the same temperature change.
4Where is this idea used in daily life?
In instant cold packs, which use a cooling solid dissolving in water, and in hand warmers.
Take it further
Estimate the heat absorbed using q = mass of water x 4.18 x temperature change and compare the values per gram of each solid.
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
Science Projects for Class 9 and 10
Models and experiments for high-school physics, chemistry and biology