What it shows
A fixed amount of gas at constant pressure expands steadily as its temperature rises. A graph of volume against temperature is a straight line that points to about minus 273 degrees C.
The science
Charles’s law says that the volume of a fixed mass of gas at constant pressure is directly proportional to its absolute temperature in kelvin, so V divided by T stays constant. A graph of V against temperature in degrees C is a straight line. Extending it back to zero volume gives about minus 273 degrees C, the absolute zero, which is why the kelvin scale begins there. In the syringe the pressure stays about equal to the air pressure because the plunger slides freely.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 50 or 60 ml plastic syringe with a smooth plunger (from a medical store)
- A cap or a piece of clay to seal its tip
- Ice, warm water and a kettle of hot water (adult use)
- A large steel bowl or pan
- A thermometer reading to 100 degrees C
- A rubber band to hold the syringe on a ruler
- A drop of cooking oil for the plunger
- Graph paper
Safety
For this project: An adult must handle the hot water. Do not use water above 70 degrees C. Do not push the syringe hard against a closed tip, since the plunger can pop out.
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.
Pull the plunger to the 40 ml mark, cap the tip tightly and put one drop of oil on the plunger edge so that it slides freely.
Fill the bowl with a mix of ice and water and wait for the thermometer to read about 5 degrees C.
Put the syringe in the bath with the whole barrel under water for three minutes, tapping the plunger lightly so that it moves freely.
Read the volume from the scale at the top of the plunger seal and write the temperature of the bath next to it.
Replace the bath with water at room temperature, then 40 degrees C and 60 degrees C, mixing hot and cold water, giving three minutes each time. A common mistake is to read before the gas has reached the temperature of the bath.
Convert each temperature to kelvin by adding 273 and find V divided by T for every pair of readings.
Plot volume against temperature in degrees C and draw the best straight line through the points.
Extend the line back until it meets the temperature axis and read the temperature at zero volume.
What you should see
A straight line graph that, when extended backwards, meets the axis near minus 270 degrees C, and V divided by T that stays almost constant for the readings.
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 Charles’s law say?
At constant pressure the volume of a fixed mass of gas is proportional to its absolute temperature.
2Why is the plunger left free to move?
So the pressure of the gas remains equal to the outside air pressure.
3What is absolute zero?
The lowest possible temperature, minus 273.15 degrees C or zero kelvin.
4Why does the line not hit exactly minus 273?
Friction, small leaks and the volume of the tip cause small errors.
Take it further
Keep the temperature fixed and press the plunger by adding known weights to test Boyle’s law, that pressure times volume is constant.
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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