Thermodynamics covers the physics of heat and its relationship with matter and other forms of energy, and is involved with virtually every process in chemistry and physics many of which we wouldn’t ever think of.
For the purpose of this article, I will try to concentrate on areas useful to heating engineers and may gloss over certain terminology such as entropy and enthalpy as they often only serve to add confusion when trying to explain just the basics that we need to know of the subject.
So to begin with, ask yourself what is heat? What is heat measured in?
It’s important to get off on the right foot here as terms often get mixed up. Heat is the stored kinetic energy of a substance. As it is a measure of energy it is measured in Joules, or more commonly in the heating engineers’ line of work Watts and kilo Watts.
Kinetic means movement doesn’t it? How can a glass of water sitting still on a table top have kinetic energy?
Well, while in physics kinetic energy often refers to the movement of an object, it also refers to the energy held within the object. You probably already know that everything you see around you and many things you don’t see, are made up of atoms, too small to see and too many to count but they are the building blocks of every solid, liquid and gas and they never sit still.
The atoms in any substance around you are always in motion, the energy of this motion is what gives you a temperatures to measure in a substance.
In a 250ml glass of water there is in the region of 8,356,989,178,075,501,872,000,000 molecules of H2O (eight septillion three hundred fifty-six sextillion nine hundred eighty-nine quintillion one hundred seventy-eight quadrillion seventy-five trillion five hundred one billion eight hundred seventy-two million, 😂, molecules) Every one of which is vibrating and moving around bouncing off each other constantly, this is the stored energy we know of as heat.
When we add more heat into the system the molecules vibrate and move faster and with more energy than before, as the average energy of the molecules that you measure in the water rises so would any measurement of temperature.
In that case how much heat energy is stored in a block of ice at 0°c?
Well, you first need to realise that 0°Celsius, is not the same as zero heat. Celsius just like Fahrenheit, is simply a scale made up in the past for convenience. True temperature is measured in the Kelvin scale, with 0° Kelvin being “absolute zero” which would sit on the Celsius scale around minus 273.15°c.
So a block of ice at 0°c would be approximately 273° above absolute zero. Notice I wrote 273° there without saying Celsius or Kelvin? That’s because a change in 1°C is the same as a change in 1°K, the scales have the same integers, just shifted by 273 degrees, unlike Celsius and Fahrenheit where a change in 1°C is the same as a change of 1.8°F.
Any substance with temperature above absolute zero (the minimum theoretical temperature possible) has stored thermal energy in the form of kinetic energy within the atoms or molecules it is comprised of. Like pressure, heat always wants to achieve an equilibrium where everything would be at the same temperature, there’s a consistent theory in physics called the “heat death of the universe” where everything finally reaches one temperature and therefore there can be no further transfer of heat.
Again like pressure heat can only move from higher to lower, hotter to less hot, hold an ice cube in your hand and it feels “cold” to us, but it has heat, your hand is at a higher temperature so heat will move from your hand to the ice cube. This is the basis for heat pumps, as even cold outside air or ground has stored thermal energy, the heat pump system takes advantage of the refrigeration cycle to extract the thermal energy contained in a substance that would otherwise not be hot enough to directly heat our homes.
The diagrams below give a rough visualisation of the movement of water molecules in different states.
Gas: Gasses are individual molecules that move around freely, at atmospheric pressure they have a lot of space in between the different particles and move at very high speed (in excess of 1000mph) however they rarely will travel far before colliding with other particles and changing direction. When thinking about the air around us, even in a space only 1cm3 there would be about 25,000,000,000,000,000,000 gas molecules so you can maybe imagine there will be a lot of bumping into each other.
The higher the temperature the more energy in the molecules, this means more movement in them. That will take the form of most of the molecules moving at higher speeds, but in the case of gasses also faster rotation of the molecules. With the higher speeds and stronger collisions they will push against each other harder which propels them further apart, this is important as that is why a gas will expand when hot reducing the density of the gas (hot air rises anyone?) well it does, because the cooler denser gas is heavier per unit of volume and will displace any sparser lighter gas upwards.
Liquids: Like gasses, the molecules in a liquid can still move around reasonably freely, with much less space in between the particles as they don’t have the momentum and energy to break free from the bond strength that pulls the molecules together. In the case of water, the liquid phase is approximately 1600 times denser than when it’s a gas. For example, in the case of a 250-litre unvented cylinder, should someone bypass the safety devices and the cylinder were to overheat past 100 degrees, the water could flash-boil to steam that would then very quickly take up a volume of 400,000 litres.
Adding more heat to a liquid, like a gas, will put more energy into the molecules similarly causing them to move faster and push away from each other again decreasing the density of the hotter liquid, leading to convection currents in the fluid where the hotter, less dense fluid is being pushed upwards to the top of the fluid by the cooler denser fluid underneath.
Solids: Different to gas and liquid phases of a substance, the molecules in a solid cannot move freely, they do not have enough energy to overcome the bonds holding the molecules in place but that doesn’t mean they don’t still have kinetic energy. In a solid the atoms are always vibrating, as they are heated they vibrate more and will oscillate towards and away from each other faster and further apart which on our visible scale of the material is seen as the substance expanding as it gets hotter. The case of water is unusual in that from 4°c and below it actually expands as it cools, most people will have seen or at least heard of water pipes bursting as they freeze when the ice takes up more volume than water. However this is not the normal for most substances, a particular lattice arrangement of the molecules caused by the hydrogen bonds in water is what causes this phenomenon.
Why does a boiling pot of water never go above 100°C?
I expect most people will never have tried this, and until more recently I myself had never really thought about it, but if you put a pot of water on a cooker, turn it on and start heating the water what happens?
Assuming it’s a gas cooker, the hot gasses from the cooker flame (over 1900°C) makes contact with the pot, this transfers heat energy to the solid metal molecules of the pot which will vibrate more, and in turn, transfers more energy to the liquid water molecules in the pot.
As the water heats, the molecules gain more and more energy (temperature approaching 100°C) to the point where they have enough energy and speed to break free of the hydrogen bonds which keeps the molecules near each other. Once free, those molecules are considered to be a gas and will escape into the air in the room.
But at this point, where you see the water bubbling away, if you measured the temperature of the water, regardless of whether it’s near the top of the pot, or the sides or the bottom, will be almost uniformly 100°C.
So where is all the heat going from the gas cooker if the temperature is no longer rising in the water?
This is what we call latent heat. It is the additional heat that’s required to be added to a system to break the hydrogen bonds holding the molecules as a liquid. So up until 100°C the molecules move and vibrate faster and faster. Once that temperature has been reached all the additional heat you put into the pot of water is taken up by separating the bonds, the molecules continue to vibrate and move fast but this kinetic energy does not increase further.
It takes a considerable amount of heat energy to break the bonds, in fact at 100°C we would have to add an amount of heat the equivalent of raising the water temperature to 540°C.
This process also works in reverse, when water vapour changes phase back to a liquid the additional latent heat that initially broke the hydrogen bonds is released. This is the main concept of condensing boilers, where by water vapour produced as a product of combustion is condensed inside the boiler releasing the latent heat into the heating system as opposed to condensing later and releasing heat outside in the atmosphere.
The diagram below illustrates the concept of latent heat.
As you heat a solid close to liquid temperature you will reach point x on the diagram, where you would have to continue to add heat energy with no increase in temperature to break the bonds holding the molecules in a solid lattice, once the phase change to liquid is complete then continuing to add heat to water will increase its temperature up to 100°c where the temperature will stop increasing and the additional heat energy goes in to breaking the bonds holding the molecules together as a liquid (Y), once the liquid changes phase to a gas the temperature of the gas then continues to increase if we assume a constant source of heat is being put into the system.
For every litre of water that is vaporised 0.63kw of heat energy is consumed as latent heat, equally for every litre of condensed water 0.63kw of heat will be given off.
This incidentally is why we sweat. The water on our skin absorbs additional heat as it evaporates which draws heat from our skin to help cool us down even if the air around us is warmer.
Does everything hold heat equally?
Although all materials are made out of atoms, they don’t all have the same ability to hold heat. The amount of heat energy a material can store is referred to as the “Specific Heat Capacity” of a substance.
Materials with a high specific heat capacity can hold more heat energy without changing temperature significantly compared to those with a low heat capacity. (see thermal mass article)
A material with a low heat capacity will take much less heat energy to reach the same temperature. This is particularly useful for insulation. If we choose a material with a very low heat capacity, then it will only absorb a small amount of heat energy before reaching the same temperature as its surroundings and therefore reaching close to a thermal equilibrium and significantly slowing further heat transfer through it.
As insulation, a material with a high heat capacity would be less useful as it would need to absorb much more thermal energy in order to increase its temperature, reducing the heat from the area we are trying to keep warm.
Specific heat capacity is measured as an amount of energy taken to increase a given mass of material by a temperature. Commonly “kJ/Kg.K” Which would be kilojoules per kilogram of mass per degree Kelvin (or Celsius) rise in temperature.
For example, it takes approximately:
0.8kJ to heat 1kg of Aluminium by 1°c
0.24kJ to heat 1kg of Silver by 1°c
0.88kJ to heat 1kg of Concrete by 1°c
4.19kJ to heat 1kg of Water by 1°c
What stands out here to me, is that counter intuitively some metals don’t hold a great deal of heat energy, but water on the other hand holds a massive amount of energy for every degree rise in temperature compared to most other materials.
This is what makes water very useful as a heat storage and transfer fluid. We can dissipate a large amount of heat into water for the purpose of cooling without raising the temperature beyond the point where it would change to gas. We can also add a lot of heat energy to water before pumping it round our heating systems to warm up radiators or storage cylinders, but also means we must be mindful about how much water we heat up, as heating more than is necessary leads to high energy wastage.