# Why is the specific heat of water high?

How is the specific heat of water so high?
I was reading the hyperphysics article on it but could not understand it properly.

• Higher specific heat of water is due to Presence of intermolecular hydrogen bonding in it The amout of heat supplied is used initially to break the bonding Due to this reason water has high specific heat capacity Jan 3 '16 at 15:38

Water has hydrogen bonding in it. Hydrogen bonding is some kind of intermolecular force (a tutorial and the wikipedia page) that is usually seen in molecules that have $$\ce{OH}$$, $$\ce{NH}$$ or $$\ce{FH}$$ somewhere in their structure.

## How does it happen?

Hydrogen atom is really small (atomic radius: About 37 pm) When it bonds with some very electronegative species, like O atom, (in the case of water), the electron is highly absorbed by the more electronegative. That results in a relatively high concentration of positive charge. The other O atom in water is attracted to H and this is the basis of hydrogen bonding.

## So...?

Hydrogen bonding in water is far too weak to be compared with intramolecular bonds like covalent or ionic bonds, but it's strong enough that to be broken, it will require lots of energy. That's why water boils at $$100 ° C$$.

Many know the reason of water's high specific heat to be hydrogen bonding. Partially, it's related to that: In giving heat to water, some heat is "spent" on loosening the hydrogen bonds rather than increasing water's kinetic energy. The argument is valid, and I haven't seen anyone bringing evidence as to its being wrong, but another reason, which was the one hyperphysics gave, relies on the motion of water.

Expanding the topic isn't necessary, and it will add up to confusion. In short, the water molecule is nonlinear. That will result in three rotational degrees of freedom that allows it to have an awful lots of rotational motions! (More types of motions are a cause of an additional bonus to the specific heat)

Liquid ammonia has a higher specific heat, just for the same reason.

Even a very easier reason to understand is the water's relatively low molar mass: $$\frac{18 g}{mol}$$. What should that mean? More moles of water per one kilogram.

As a summary, the reasons are:

1. The heated water will contribute much of the heat to loosening, bending or breaking the hydrogen bonds.
2. Water had three rotational degrees of freedom. In addition to vibration, rotations happen a lot to water molecules. This will result in a higher heat capacity.
3. Specific heat capacity is defined as the amount of heat required per unit mass to increase the temperature by a degrees celsius. The relatively low molar mass of water allows more moles of it to be there in a mass unit (either kg or g)

As a side note, the exceptional high specific heat is not the only weird property of water. See here for example.

Generally, an increase in the absorbtion of heat energy will result in an increase in the kinetic energy of the molecules and indeed, the individual atoms. The same can be said about water. However, since water has constantly forming H bonds, some of the heat energy will be used to break the water bonds. The more bonds that need to be broken and kept apart (not easy in water) the more heat the water can absorb. When the outside temperature cools down, then the bonds broken by the heat will reform and release energy into the surroundings.

• But liquid ammonia's specific heat is higher than water. Are you suggesting that is merely because H-bonding is stronger in ammonia? Aug 20 '16 at 16:51