What it shows
When citric acid and baking soda dissolve and react in water, the mixture gets colder, which shows an endothermic change that takes heat from the surroundings. It is the opposite of burning, which gives heat out.
The science
Chemical changes may give out energy (exothermic) or take it in (endothermic). When baking soda (sodium hydrogen carbonate) reacts with citric acid in water, carbon dioxide is released and the energy needed to break the bonds and dissolve the solids is taken from the water as heat. The water therefore cools, and the temperature fall is larger when more reactants are used, up to the point where one of them runs out.
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.
- Citric acid powder (nimbu ka sat), about 20 g
- Baking soda, about 20 g
- Three plastic cups
- A thermometer reading 0 to 100 °C
- Water at room temperature, 100 ml
- A spoon and a teaspoon
- A kitchen scale or measuring spoons
- A notebook
Safety
For this project: Citric acid powder irritates eyes, so an adult must supervise and the child must wear glasses. Do not taste, and wash hands after. Never use a closed bottle for the reaction.
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 100 ml of water in a cup and measure its temperature. Wait until it is steady, and note it.
Weigh 5 g of citric acid and 5 g of baking soda in separate cups.
Add both powders into the water together, stir gently with the spoon, and watch the thermometer, which needs to be placed carefully without touching the bottom.
Write the lowest temperature reached and the time it took.
Repeat with 10 g of each powder in a fresh 100 ml of water at the same starting temperature.
Repeat again with 15 g of each, and plot a graph of mass used against temperature fall.
Try citric acid alone in water, and baking soda alone in water, and note whether each one cools the water.
Write whether the change is exothermic or endothermic and explain with your graph. Do not use a closed container: the gas needs a way out.
What you should see
Water cools by about 3 to 8 °C depending on the mass used, and each powder alone gives a smaller drop than the pair together.
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 change that absorbs heat from its surroundings.
2What gas is formed?
Carbon dioxide.
3Why do we start from the same water temperature?
To make the experiment a fair test so only the mass changes.
4Name an exothermic change.
Burning of fuel or adding quicklime to water.
Take it further
Find the mass ratio of acid to soda that gives the largest temperature fall.
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.
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