A flask containing dyed green water is resting in a styrofoam bucket of icewater, as seen in the photograph at the left below. A plug seals the top of the flask so that there is no air on top of the water. Inserted into the water are a thermometer and a capillary tube, as seen in the close-up photograph at the right. Some of the water from the flask extends into the capillary tube.

i1 21  i1 21a 

If the water in the flask were to expand, the green water level would rise higher in the capillary tube. On the other hand, if the water in the flask were to contract, the green water level would fall lower in the capillary tube. The flask used is made of quartz glass, which has a very small coefficient of thermal expansion, so if the flask and the water change temperature the predominant effect will be due to the expansion or contraction of the water.

Suppose that the flask is now removed from its icewater bath and allowed to slowly warm up. What will the water level in the capillary tube do as the water in the flask warms up? Will it:

  • (a) rise.
  • (b) fall.
  • (c) rise for a bit and then fall.
  • (d) fall for a bit and then rise.
  • (e) remain the same.

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 The answer is (d): the water level in the flask will fall for a bit then rise, as seen in this sequence of close-up photographs of the thermometer and capillary tube taken at approximately 3 minute intervals over a time period of about 40 minutes.

 1. i1 21b  2. i1 21c  3. i1 21d  4. i1 21e  5. i1 21f
 6. i1 21g  7. i1 21h  8. i1 21i  9. i1 21j 10. i1 21k
11. i1 21l 12. i1 21m 13. i1 21n 14. i1 21o 15. i1 21p

The density of water is a maximum at about 4o C, so as the water warms up its volume decreases to about 4o C and then begins to increase.