# When heat is increased, does water move upward in a straw due to increased capillary action or increased pressure?

I left a water bottle with an internal straw in my car throughout a long, hot day. When I opened the bottle at the end of the day, water immediately splashed out from the straw. This made me wonder if the increase in temperature caused the liquid to move with greater capillary action upwards in the straw due to higher movement of $\ce{H2O}$ atoms OR whether it was an increase in pressure that led the water molecules to automatically expel outwards from the straw once the bottle was opened.

I know this may seem like a silly question but what explanation would you give for the expulsion of the water from the straw? (I find this recurring event strange because the shape of the bottle never shrinks at all from the heat yet something forces the water outwards but I cannot discern what force causes this. By the way, this never happens to me with regular water bottles - only with ones containing straws/tubes on the inside.)

• The effect of heat would be negligible on the water itself (pressure, volume, capillary action, etc.). The cause just has to be air pressure. I'm not sure I have a very good image in my mind of your water bottle, but presumably there is air inside your bottle along with the water, it is under pressure from being warm, and when you remove the cap the pressure is allowed to escape. This happens to squirt bottles of water and other liquids in laboratories all the time when there is poor temperature control. On hot days the headspace air in the bottles heats, expands, and the liquid comes out. – airhuff Aug 9 '17 at 22:34
• I ran out of space above and I wanted to keep it in a comment rather than answer because I'm not sure I understand the design of your bottle yet. Does what I said make sense based on what you've seen happen? – airhuff Aug 9 '17 at 22:37
• Capillary action, if anything, decreases with temperature. – Ivan Neretin Aug 10 '17 at 5:04
• @airhuff Yes. I understand what you mean. A large volume of water was expelled so I assumed that the water had undergone more chemical change than the air. But, in reality, the air had experienced even greater thermal expansion than the water. – orangebull Aug 10 '17 at 17:43

It almost certainly was due to thermal expansion of water and the air above it. Consider that the thermal expansion coefficient of water near room temperature is ~$0.000214$ K$^{-1}$. [Though it is not linear over a broad range, assume for the small temperature rise in your car that it is.]

If the water was cold (e.g. ~$277$ K, maximum density) and it warmed to ~$300$ K ($80$o F), a temperature change of ~$23$ K, it would have expanded by a factor $(300 \ce{K} - 277 \ce{K}) \cdot 0.000214 \ce{K^{-1}} = 1.005$.

Assume you have a one-liter bottle filled completely with water when you start; by the time you return it's $1.005$ L. In the whole bottle, the change would hardly be noticeable, but the narrow straw greatly magnifies the rise of liquid. If the straw is $6$ mm in diameter, the volume of liquid is given as: $$V=\pi\cdot r^2\cdot h$$

$$5 \mathrm{mL} = 5 \ce{cm}^3$$

$$r = 0.3 \ce{cm}$$

$$h = \frac{5 \ce{cm}^3}{\pi \cdot (0.3 \ce{cm})^2} = 18 \ce{cm}$$

So the water would have risen about $18$ cm in the straw!

This exercise only took into account the water's expansion... for any air above the water in the bottle, and for other (approximately) ideal gases, $PV=nRT$ and thermal expansion would be far greater.

To some extent, the bottle itself expands, decreasing the rise of the liquid, so these calculations are not very accurate. Most likely, air expansion contributed more to the "mass ejection" than did that of water... it depends on whether you view your container as half-full or half-empty.

• Thank you. That was a very thorough answer. I have a much better understanding of this phenomenon now. :) – orangebull Aug 10 '17 at 17:32