# Is heat just a change in temperature?

According to my chemistry textbook, heat is defined as "Energy that is exchanged because of a difference in temperature or a change in phase." My textbook also says, "Heat is a form of energy. Temperature, on the other hand, is a measure of how quickly the molecules (or atoms) in a substance are moving," implying that temperature is not a form of energy, just a measurement of a physical condition.

This seems all wrong to me. If heat is just a change in temperature, but temperature isn't energy, doesn't that imply that energy is created when two objects of different temperature interact, or is there potential heat energy in the temperature of an object?

Also, the textbook says that the reason we feel hot is because of heat from temperature change, and not temperature itself, but this doesn't explain why chemical reactions occur faster at different temperatures. Nor does it explain why there are different states of matter at different temperatures, or even why our bodies need to stay at a certain temperature at all. Where is the energy from heat and other chemical processes coming from, exactly?

Other sources online also say that heat is a change in temperature and temperature is not energy, only a measurement, so I don't think it's just my textbook. I feel like I'm missing something.

## 4 Answers

[...] is there potential heat energy in the temperature of an object?

The energy associated with temperature is called thermal energy. If you know the heat capacity of an object, you can figure out how much energy is required to change the temperature by a certain amount.

If heat is just a change in temperature, but temperature isn't energy, doesn't that imply that energy is created when two objects of different temperature interact [...]?

Heat is energy transferred from one object to another. When heat is transferred from a hotter to a colder object, the hotter object loses thermal energy and the colder object gains it. No energy is created.

Where is the energy from heat and other chemical processes coming from, exactly?

A chemical reaction can give off heat because electrons can be in higher states (weak bonds) and in lower states (strong bonds). Potential energy can turn into thermal energy, and vice versa. It takes energy to melt ice, so melting ice cools down the surroundings. An explosive reaction releases energy, so it increases the temperature of the surroundings by transferring heat to it.

• I really like your answer. I think it addresses the major puzzlements of the OP without dumbing down the concepts so that erroneous ideas are created. Temperature and heat are surprisingly hard to define. – MaxW Apr 3 '19 at 6:11

There are two concepts here, temperature and heat. The temperature of a 'system' is only that property that determines whether or not a system is in thermal equilibrium with another system.

This accords with our experience that we can distinguish by touching between something that is hot and that which is cold but have little feeling if it is at the same temperature as we are.

We can measure temperature by many different means, such as pressure of a gas, electrical resistance, expansion of a liquid, magnetic susceptibility of a paramagnetic salt and by radiant emission, which is how we can determine the temperature of a furnace or of the surface of a star. The reference of zero is then some property such as the triple point of water for the celsius scale.

A molecular interpretation of temperature indicates that in a molecule with an infinite number of energy levels, as the temperature is raised more and more energy levels become populated but any given level always has less population than the one immediately below it. This is in accord with the Boltzmann distribution. Thermodynamically this is expressed as the rate of change of internal energy with entropy, i.e the slope of a graph of internal energy vs entropy (at constant volume).

( In systems where there are a finite number of energy levels, such as an arrangement of spins, (for example in some types of nmr experiments) then negative temperatures are possible, i.e. larger population in upper than lower levels. This does not mean a temperature below absolute zero.)

Heat is internal energy in transit, it flows from one part of a system to another, or between two systems both by virtue of a temperature difference, and can only be quantified when the transfer has finished. It is incorrect to refer to 'the heat in a body' just as it is incorrect to refer to 'the work in a body'. The heat and work are ways in which the internal energy of a body is changed. Put another way, it is impossible to divide the internal energy into some amount of heat and another amount of work. We have no direct knowledge of heat from our senses (or instruments) and heat is quite distinct to 'hotness'.

The first law defines the change in internal energy $$\Delta U$$ as the sum of heat $$Q$$ and work $$W$$, $$\Delta U = Q+W$$.

This means that we can define heat as that energy transfer brought about by non-mechanical means, and is equal to the internal energy change less the work done when the system is at a different temperature to it surroundings.

The first law has three features, (a) it is based on conservation of energy, (b) to satisfy (a) it introduces the idea of internal energy and (c) it defines heat as energy in transit by virtue of the temperature difference.

That heat is energy was first determined quantitatively by J. Joule in the mid 1800's in elegant experiments where the increase in temperature of water agitated by a paddle wheel, rotated by the lowering of a weight under gravity, was measured.

This seems all wrong to me. If heat is just a change in temperature, but temperature isn't energy, doesn't that imply that energy is created when two objects of different temperature interact, or is there potential heat energy in the temperature of an object?

One example of how heat and temperature are related is when describing mixing cold water and hot water. Take for example a cup of water at room temperature. The water molecules will have an average kinetic energy that leads to the "room temp" feeling. If you add "heat" to this system, the temperature will rise, adding kinetic energy to the molecules of water in the system. When talking about heat, you're talking about the energy of the water molecules, as well as the amount of energy you could add to the water molecules to raise them to another temperature, so it's important to be specific when saying "room temperature water has been heated from absolute zero using xxxxxx joules of heat". This would allow you to keep a mental pointer of the amount of heat that any specific sample of water has.

I recommend looking at PhET's States of Matter Simulator, click states, then water, then heat it from 0 K and watch what happens with the water molecules. (remembering that the heat in that container is constant so there is no loss in temperature when you let go of "heating".

Also, the textbook says that the reason we feel hot is because of heat from temperature change, and not temperature itself, but this doesn't explain why chemical reactions occur faster at different temperatures. Nor does it explain why there are different states of matter at different temperatures, or even why our bodies need to stay at a certain temperature at all. Where is the energy from heat and other chemical processes coming from, exactly?

Different states of matter at different temperatures is a difficult and expansive topic in chemistry that is mostly understood to be governed by the individual intermolecular forces that dominate in each chemical that you study. For the case of water, it is liquid around room temperature because the average kinetic energy that the water molecules have at room temperature is around the same energy as the intermolecular interactions that bind the water together, allowing water molecules to move around each other but still have forces that hold them together. As you heat higher and higher, the average kinetic energy of the water molecules will increase, till it is great enough to break all of the intermolecular bonding (i.e.) at 100 degrees Celsius and 1atm, you will boil water because all of the hydrogen bonding is broken.

Other sources online also say that heat is a change in temperature and temperature is not energy, only a measurement, so I don't think it's just my textbook. I feel like I'm missing something.

Another important thing to think about is the fact that temperature is an intensive property, so if you take a tank of hot water, divide that water in half, and none of the heat is lost, then both tanks will be the same temperature hot water, meaning that their molecules will still have the same average kinetic energy, but they can only give a certain amount of total heat to another substance using the formula, where Q is the amount of heat transferred between two samples at different temperatures (the difference in energy between hot water and cold water), m is the total mass of both samples when mixed, c is the specific heat capacity which for water is $$4.184\frac{J}{g ^\text{o}C}$$. This means that 1 gram of water takes exactly 4.184 Joules of heat to raise that sample 1 degree Celsius in temperature.

$$Q=mc\Delta T$$

I hope that my previous answers also help this question, but if you're looking for more help, I would suggest looking up Khan Academy videos on Youtube of heat transfer problems.

TL:DR (Here's a YouTube video of someone explaining the simulator and much of what I just explained) Heat vs. Temperature - Chem Academy

• Heat is energy transferred from one 'body' to another, you cannot say that a substance has 'an amount of heat ' as you do in your first paragraph, any more than you can say that a substance has an amount of work. – porphyrin Apr 4 '19 at 9:49

Heat is a form of energy. According to the kinetic theory of matter, particles in solids vibrate about their mean position with mean kinetic energy. Particles in fluid travel with a mean velocity. In either case, the kinetic energy is proportional to the temperature. If there is a medium possessing a temperature gradient, particles at a higher temperature level will possess higher kinetic energy. Upon collision with the adjacent particle with lower temperature and hence lower kinetic energy, some kinetic energy is transferred from the higher to the lower kinetic energy particle causing it to move further than its mean position. The kinetic energy of the lower temperature particle increases while that of the higher energy particles decreases. With the increase in the kinetic energy of the lower temperature particle, there is an increase in the temperature of the particle and hence change in its mean position. In this manner, heat is transmitted from one point to another and this gives the microscopic description of the heat conduction process. Generally, in liquids and electrically non-conducting solids, heat transmission is caused by longitudinal oscillation of the lattice structure. In metals, thermal conduction is as a result of free electrons while in gases, conduction is as a result of the elastic collision of molecules. Particles are more densely packed in solids than in liquids and in liquids than in gases. Liquids have shorter mean free path than gases. Thus, the probability of collision occurring in solids is higher than in liquids which in turn is higher than in gases. Consequently, solids conduct heat faster than liquids which in turn conduct heat faster than in gases. I hope this might help you to understand your current and probable future doubts.

Please read the following links for better understanding: https://nptel.ac.in/courses/112108149/pdf/M1/Student_Slides_M1.pdf

Also kindly take a look at the following Link as well: https://sciencing.com/role-heat-play-chemical-reactions-13455.html

Please ask for any further clarifications or suggestions. Keep learning and have fun. :)