Why do some liquids take longer to heat up than others?

Question

Sorry the format sucks, I’m on mobile.

So, I have a 4 month old. On occasion I warm formula that has been in the fridge in the microwave. 20 seconds, All it needs. Yes, I shake it and test it on my arm everytime to make sure it is just right and i barely do this as i have a baby brezza. Don’t come at me. Lol.

Now today my 9 year old who is dealing with sensitive teeth asked me is I could warm her pear juice for her. Sure, no prob. The pear juice took 40 seconds just to reach room temperature. Why is this? Is it the sugars? I don’t know why this just occured to me and is bothering me but here we are.

In: Chemistry

Answer 1

If you look at [This Gif](https://lh3.googleusercontent.com/proxy/dX6cV5Tlq8gQS98Xk6CLmkUgVEXG62OY3zSbtGfW8omcECLehyV8L-NllN16thf_ljpCBRHDbMP7I69NUILBTaEAFlfjzFL3BjzR8nTX4qg_ZA) you can see how a single molecule of water can distribute extra kinetic energy through different modes of vibration.

If one unit of energy causes one of those modes of vibrations, then it requires one unit of energy for *each* type of vibration for a one unit increase in the temperature.

Water is unique in that it has a very high number of modes of vibration compared to many other substances. Because each mode of vibration requires its own unit of energy, then the more modes of vibration available, the more energy will be required to increase the temperature.

This is known as the *specific heat* and is a quantitative measure of the relationship between energy and temperature.

The second thing to keep in mind is how a microwave works. Microwaves are explicitly designed to cause high energy vibrations in water, and lesser energy vibrations to anything that is not water.

As formula will contain more water content than a pear for equal mass, it should take longer for the pear to heat up, because the pear has many components that are not sensitive to microwaves.

However if you were to heat them up in an oven for example, the pear has a much better chance of heating up quicker, as now the sensitivity to microwaves is no longer a limiting factor. The pear takes less total energy to heat up, but the formula gets an extra kick from the microwave specifically.