A glimpse into the analytical mind...

Caution - the following post contains insight into Jim's thought process. Steel yourself, or just decide now to not read it.

So my co-worker sends me an email. It's a joke, and the punch-line is at the very bottom, so far down that I can't see it when I first open the email.

Question: Which runs faster, hot or cold??

Joke Answer: Hot. Anyone can catch a cold!

Jim's Answer:

Temperature is a measure of the kinetic energy within a given system. As molecules vibrate and move, they collide, and release energy in the form of heat. The more molecular movement, the more collisions, the more heat, the higher the temperature.

But also consider that higher temperature (more movement) molecules are moving faster past one another when they do not collide. This faster movement is able to partly overcome the inter-molecular forces of attraction (van der Waals Forces), which in turn allows for more expansion*. This expansion leaves more "open space" between molecules in the solution. The end result is a lower density. For a given material in the liquid state, the lower density has a direct effect on viscosity, which is the material's resistance to flow.

So, assuming all other factors are equal**:

Cold water = lower temperature = less molecular movement = higher density = higher viscosity = less flow = slower "running"

Hot water = higher temperature = more molecular movement = lower density = lower viscosity = more flow = faster "running"

Therefore hot runs faster.

*This expansion is not due to weakened van der Waals Forces as may be experienced with molecular shearing of non-polar molecules or chemical decomposition resulting in lower molecular weights or loss of mass balance with the solution. The van der Waals Forces remain constant as long as the number of molecules and their molecular structures remain constant.

** Other possible factors which may impact flow rate include varying pressures of water through different fluid systems, vertical free-falling water under different gravitational fields, presence of solutes (for example, mineral content in "hard" water) in solution, ratio of isotope content in the molecular structures (such as Deuterium aka "heavy water") etc...