High Specific Heat of Water

Have you ever burned your tongue after drinking hot coffee that you thought had sufficiently cooled down? Have you ever tried cooking pasta in a rush and wondered why it takes so long for the water to boil? The reason it takes so long for water (or coffee, which is made of mostly water) to change temperature is something called the specific heat of water.

Here, we will discuss what specific heat of water means, why hydrogen bonding leads to a high specific heat, and what are examples in which we see this particular property.

What is the specific heat of water?

The quantity of heat that must be taken in or lost for one gram of material so that its temperature changes by one degree Celsius is referred to as specific heat.

The equation below shows the link between heat transferred (Q) and temperature change (T):

$Q=cm∆T$

In this equation, m represents the substance's mass (to which the heat is being transferred to or from) whereas the value c represents the specific heat of the substance.

Water has one of the highest specific heat among common material substances at approximately 1 calorie/gram °C = 4.2 joule/gram °C.

High specific heat of water and other examples

For reference, Figure 1 below compares the specific heat of water with other common substances.

Figure 1. This table compares water with several common substances in terms of their specific heat.

Because water has a high specific heat capacity, it takes a lot of energy to create temperature changes. It's why coffee takes a long time to cool down, or why "a watched pot never boils." It's also why it takes a long time for the environment to respond to external changes.

When a specific quantity of excess carbon dioxide (CO₂) is added to the atmosphere, for example, it takes time for warming impact on the air, land, and ocean to become fully apparent. Even if there were a means to directly add heat to the Earth (which is made up largely of water), it would take time for the temperatures to rise.

This means that the ocean can absorb a significant amount of heat before its temperature increases significantly. Similarly, when an external source of energy is removed, the ocean responds slowly and its temperature will not begin to fall immediately.

Put simply, the high specific heat capacity of water allows it to maintain a stable temperature, which is very crucial in sustaining life on Earth.

What is the relationship between the high specific heat of water and its chemical bond?

Water is made up of two hydrogen atoms connected by polar covalent bonds to one oxygen atom. When valence electrons are shared mutually by two atoms, it is referred to as a covalent bond.

Water is a polar molecule because its hydrogen and oxygen atoms share electrons unequally owing to electronegativity differences.

Each hydrogen atom has a nucleus composed of a single positively charged proton and one negatively charged electron orbiting the nucleus. Each oxygen atom, on the other hand, has a nucleus composed of eight positively charged protons and eight uncharged neutrons, with eight negatively charged electrons orbiting the nucleus.

Because the oxygen atom has a higher electronegativity than the hydrogen atom, electrons are drawn to oxygen and repelled by hydrogen. During the formation of a water molecule, the ten electrons link up and form five orbitals, leaving behind two lone pairs. The two lone pairs associate themselves with the oxygen atom.

As a result, oxygen atoms have a partial negative (δ-) charge, while hydrogen atoms have a partial positive (δ+) charge. While the water molecule has no net charge, the hydrogen and oxygen atoms all have partial charges.

Because hydrogen atoms in a water molecule are partially positively charged, they are attracted to partially negatively charged oxygen atoms in nearby water molecules, allowing a different type of chemical bond called hydrogen bond to form between nearby water molecules or other negatively charged molecules.

High specific heat of water molecule hydrogen bonding diagram

A hydrogen bond is a bond that forms between a partially positively charged hydrogen atom and an electronegative atom.

Hydrogen bonds are not 'real' bonds in the same way that covalent, ionic, and metallic bonds are. Covalent, ionic, and metallic bonds are intramolecular electrostatic attractions, meaning they hold atoms together within a molecule. On the other hand, hydrogen bonds are intermolecular forces meaning they occur between molecules (Fig. 2).

While individual hydrogen bonds are often weak, when they form in huge numbers--such as in water and organic polymers--they have a substantial impact.

How does hydrogen bonding lead to high specific heat of water?

Heat is basically the energy generated from the movement of molecules. Given that water molecules are linked to other water molecules via hydrogen bonding, there must be a huge amount of heat energy to first disrupt the hydrogen bonds and then to speed up movement of the molecules, thereby causing water temperature to rise.

As such, the investment of one calorie of heat results in relatively little change in water temperature because much of the energy is utilized to break hydrogen bonds rather than to quicken the movement of water molecules.

A method called calorimetry can be used to determine the specific heat of a substance or object.

Calorimetry can be summed up in four basic steps:

1. Bring the substance's temperature up to a predetermined level.

2. Put this substance in a thermally insulated container with water with a known mass and temperature.

3. Allow the water and the substance to reach equilibrium.

4. Take the temperature of both when they are in equilibrium.

Because the container is thermally insulated, heat energy is transferred only to the water and not to the surrounding environment. As a result, the heat transmitted from the item equals the heat absorbed by the water.

With this, we can use the formula $Q=cm∆T$ to write this heat transfer in terms of the following formula to solve for the specific heat of the substance or object.

$c_o=\frac{m_w{c}_w(T_{eq}-T_{cold})}{m_o(T_{hot}-T_{eq})}$

Where:

m_o is the mass of the object

m_w is the mass of the water

c_o is the specific heat of the object

c_w is the specific heat of the water

T_eq is the temperature at equilibrium

T_hot is the initial temperature of the object

T_cold is the initial temperature of the water

What is the importance of the high specific heat of water in sustaining life on Earth?

Temperature is an environmental factor that can limit or enhance the ability of organisms to survive and reproduce. Maintaining stable temperature is crucial to the survival of such many organisms. Water (whether in the environment or within the organism) can help regulate body temperature due to its high specific heat.

Large bodies of water can regulate their temperature due to water's high specific heat capacity. Oceans, for example, have a higher heat capacity than land because water has a higher specific heat than dry soil. As opposed to oceans, land tends to heat up faster and reach higher temperatures. They also tend to cool down faster and reach lower temperatures.

Similarly, water's high specific heat also explains why temperatures on land near bodies of water are more mild and stable. That is, because water's high heat capacity limits its temperature within a relatively small range, seas and coastal land areas have more stable temperatures than inland places. On the other hand, areas farther from the shore tend to have a significantly larger range of seasonal and daily temperatures.

We can also see how the role of the high specific heat of water in organisms' ability to regulate their internal temperature. Warm-blooded animals, for example, are able to take advantage of the high specific heat of water to attain a more uniform distribution of heat in their bodies. Like a car’s cooling system, water facilitates the movement of heat from hot to cold spots, helping the body to maintain a more consistent temperature.