Water expands at temperatures over $5^\circ\mathrm C$ strictly monotonic. If you now drink $1~\mathrm{L}$ water ($5^\circ\mathrm C$) which will expand in your body on $37^\circ\mathrm C$, did you actually drink $1 ~\mathrm{L}$ water or $37^\circ\mathrm C$-volume?


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  • $\begingroup$ If you put a red ball into a box with no lights inside, does the red ball turn black? It's a matter of perspective. $\endgroup$ – Neil Jul 25 '16 at 14:25

$1$ litre of water will remain almost $1$ litre as long as it is in the liquid state, no matter what the temperature is.

The following formula gives you an order of magnitude estimate of the expansion:

$$\Delta V=V_0\ \Delta T \ \beta$$

where $\beta$ is the coefficient of thermal expansion and $V_0$ is the initial volume.

For water, $\beta \approx 10^{-4}\; \mathrm K^{-1}$.

Therefore, if $1$ litre of liquid water is heated by $40^\circ\mathrm C$, the volume will only change by $\approx 1 $ millilitre.

However, you might have notices that when you heat up a closed plastic bottle filled with water, it seems to expand a lot more and may even make the bottle explode if heated for long enough. This is due to part of the water turning into vapour, which occupies a much greater volume $\approx 1000$ times more. The higher the temperature, the more evaporation you have.


Volume is not a meaningful measure of quantity, for the reason you hint at in your question.

You can say how many moles (or grams) of water you drank - more useful if you want to know about the impact on your body chemistry.

This is related to my answer about scales measuring in grams rather than Newtons. Can you see how?


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