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Heat and temperature

What heat means in thermodynamics, and how we can calculate heat using the heat capacity.

Key points

  • Heat, start text, q, end text, is thermal energy transferred from a hotter system to a cooler system that are in contact.
  • Temperature is a measure of the average kinetic energy of the atoms or molecules in the system.
  • The zeroth law of thermodynamics says that no heat is transferred between two objects in thermal equilibrium; therefore, they are the same temperature.
  • We can calculate the heat released or absorbed using the specific heat capacity start text, C, end text, the mass of the substance start text, m, end text, and the change in temperature delta, start text, T, end text in the equation:
start text, q, end text, equals, start text, m, end text, times, start text, C, end text, times, delta, start text, T, end text

Heat in thermodynamics

What contains more heat, a cup of coffee or a glass of iced tea? In chemistry class, that would be a trick question (sorry!). In thermodynamics, heat has a very specific meaning that is different from how we might use the word in everyday speech. Scientists define heat as thermal energy transferred between two systems at different temperatures that come in contact. Heat is written with the symbol q or Q, and it has units of Joules (start text, J, end text).
Three melting ice cubes in a puddle of water on a mirrored surface.
Heat is transferred from the surroundings to the ice, causing the phase change from ice to water. Photo of ice cubes from flickr, CC BY 2.0.
Heat is sometimes called a process quantity, because it is defined in the context of a process by which energy can be transferred. We don't talk about a cup of coffee containing heat, but we can talk about the heat transferred from the cup of hot coffee to your hand. Heat is also an extensive property, so the change in temperature resulting from heat transferred to a system depends on how many molecules are in the system.

Relationship between heat and temperature

Heat and temperature are two different but closely related concepts. Note that they have different units: temperature typically has units of degrees Celsius (degrees, start text, C, end text) or Kelvin (start text, K, end text), and heat has units of energy, Joules (start text, J, end text). Temperature is a measure of the average kinetic energy of the atoms or molecules in the system. The water molecules in a cup of hot coffee have a higher average kinetic energy than the water molecules in a cup of iced tea, which also means they are moving at a higher velocity. Temperature is also an intensive property, which means that the temperature doesn't change no matter how much of a substance you have (as long as it is all at the same temperature!). This is why chemists can use the melting point to help identify a pure substanceminusthe temperature at which it melts is a property of the substance with no dependence on the mass of a sample.
On an atomic level, the molecules in each object are constantly in motion and colliding with each other. Every time molecules collide, kinetic energy can be transferred. When the two systems are in contact, heat will be transferred through molecular collisions from the hotter system to the cooler system. The thermal energy will flow in that direction until the two objects are at the same temperature. When the two systems in contact are at the same temperature, we say they are in thermal equilibrium.

Zeroth law of thermodynamics: Defining thermal equilibrium

The zeroth law of thermodynamics defines thermal equilibrium within an isolated system. The zeroth law says when two objects at thermal equilibrium are in contact, there is no net heat transfer between the objects; therefore, they are the same temperature. Another way to state the zeroth law is to say that if two objects are both separately in thermal equilibrium with a third object, then they are in thermal equilibrium with each other.
The zeroth law allows us to measure the temperature of objects. Any time we use a thermometer, we are using the zeroth law of thermodynamics. Let's say we are measuring the temperature of a water bath. In order to make sure the reading is accurate, we usually want to wait for the temperature reading to stay constant. We are waiting for the thermometer and the water to reach thermal equilibrium! At thermal equilibrium, the temperature of the thermometer bulb and the water bath will be the same, and there should be no net heat transfer from one object to the other (assuming no other loss of heat to the surroundings).

Heat capacity: Converting between heat and change in temperature

How can we measure heat? Here are some things we know about heat so far:
  • When a system absorbs or loses heat, the average kinetic energy of the molecules will change. Thus, heat transfer results in a change in the system's temperature as long as the system is not undergoing a phase change.
  • The change in temperature resulting from heat transferred to or from a system depends on how many molecules are in the system.
We can use a thermometer to measure the change in a system's temperature. How can we use the change in temperature to calculate the heat transferred?
In order to figure out how the heat transferred to a system will change the temperature of the system, we need to know at least 2 things:
  • The number of molecules in the system
  • The heat capacity of the system
The heat capacity tells us how much energy is needed to change the temperature of a given substance assuming that no phase changes are occurring. There are two main ways that heat capacity is reported. The specific heat capacity (also called specific heat), represented by the symbol start text, c, end text or start text, C, end text, is how much energy is needed to increase the temperature of one gram of a substance by 1, space, degrees, start text, C, end text or 1, start text, K, end text. Specific heat capacity usually has units of start fraction, start text, J, end text, divided by, start text, g, r, a, m, s, end text, dot, start text, K, end text, end fraction. The molar heat capacity, start text, C, end text, start subscript, start text, m, end text, end subscript or start text, C, end text, start subscript, start text, m, o, l, end text, end subscript, measures the amount of thermal energy it takes to raise the temperature of one mole of a substance by 1, space, degrees, start text, C, end text or 1, start text, K, end text, and it usually has units of start fraction, start text, J, end text, divided by, start text, m, o, l, end text, dot, start text, K, end text, end fraction. For example, the heat capacity of lead might be given as the specific heat capacity, 0, point, 129, start fraction, start text, J, end text, divided by, start text, g, end text, dot, start text, K, end text, end fraction, or the molar heat capacity, 26, point, 65, start fraction, start text, J, end text, divided by, start text, m, o, l, end text, dot, start text, K, end text, end fraction.

Calculating start text, q, end text using the heat capacity

We can use the heat capacity to determine the heat released or absorbed by a material using the following formula:
start text, q, end text, equals, start text, m, end text, times, start text, C, end text, times, delta, start text, T, end text
where start text, m, end text is the mass of the substance (in grams), start text, C, end text is the specific heat capacity, and delta, start text, T, end text is the change in temperature during the heat transfer. Note that both mass and specific heat capacity can only have positive values, so the sign of start text, q, end text will depend on the sign of delta, start text, T, end text. We can calculate delta, start text, T, end text using the following equation:
delta, start text, T, end text, equals, start text, T, end text, start subscript, start text, f, i, n, a, l, end text, end subscript, minus, start text, T, end text, start subscript, start text, i, n, i, t, i, a, l, end text, end subscript
where start text, T, end text, start subscript, start text, f, i, n, a, l, end text, end subscript and start text, T, end text, start subscript, start text, i, n, i, t, i, a, l, end text, end subscript can have units of either space, degrees, start text, C, end text or start text, K, end text. Based on this equation, if start text, q, end text is positive (energy of the system increases), then our system increases in temperature and start text, T, end text, start subscript, start text, f, i, n, a, l, end text, end subscript, is greater than, start text, T, end text, start subscript, start text, i, n, i, t, i, a, l, end text, end subscript. If start text, q, end text is negative (energy of the system decreases), then our system's temperature decreases and start text, T, end text, start subscript, start text, f, i, n, a, l, end text, end subscript, is less than, start text, T, end text, start subscript, start text, i, n, i, t, i, a, l, end text, end subscript.

Example problem: Cooling a cup of tea

Let's say that we have 250, start text, m, L, end text of hot tea which we would like to cool down before we try to drink it. The tea is currently at 370, start text, K, end text, and we'd like to cool it down to 350, start text, K, end text. How much thermal energy has to be transferred from the tea to the surroundings to cool the tea?
A cup of black tea with a slice of lemon in a white teacup with a saucer.
The hot tea will transfer heat to the surroundings as it cools. Photo from Photozou, CC BY-NC-ND 2.5
We are going to assume that the tea is mostly water, so we can use the density and heat capacity of water in our calculations. The specific heat capacity of water is 4, point, 18, start fraction, start text, J, end text, divided by, start text, g, end text, dot, start text, K, end text, end fraction, and the density of water is 1, point, 00, start fraction, start text, g, end text, divided by, start text, m, L, end text, end fraction. We can calculate the energy transferred in the process of cooling the tea using the following steps:

1. Calculate the mass of the substance

We can calculate the mass of the tea/water using the volume and density of water:
start text, m, end text, equals, 250, start cancel, start text, m, L, end text, end cancel, times, 1, point, 00, start fraction, start text, g, end text, divided by, start cancel, start text, m, L, end text, end cancel, end fraction, equals, 250, start text, g, end text

2. Calculate the change in temperature, delta, start text, T, end text

We can calculate the change in temperature, delta, start text, T, end text, from the initial and final temperatures:
ΔT=TfinalTinitial=350K370K=20K\begin{aligned}\Delta \text T&=\text T_{\text{final}}-\text T_{\text{initial}}\\ \\ &=350\,\text K-370\,\text K\\ \\ &=-20\,\text K\end{aligned}
Since the temperature of the tea is decreasing and delta, start text, T, end text is negative, we would expect start text, q, end text to also be negative since our system is losing thermal energy.

3. Solve for start text, q, end text

Now we can solve for the heat transferred from the hot tea using the equation for heat:
q=m×C×ΔT=250g×4.18JgK×20K=21000J\begin{aligned}\text q &= \text {m} \times \text C \times \Delta \text T\\ &=250\,\cancel{\text g} \times4.18\,\dfrac{\text J}{\cancel{\text g} \cdot \cancel{\text K}} \times -20\,\cancel{\text K}\\ &=-21000\,\text J\end{aligned}
Thus, we calculated that the tea will transfer 21000, start text, J, end text of energy to the surroundings when it cools down from 370, start text, K, end text to 350, start text, K, end text.

Conclusions

In thermodynamics, heat and temperature are closely related concepts with precise definitions.
  • Heat, start text, q, end text, is thermal energy transferred from a hotter system to a cooler system that are in contact.
  • Temperature is a measure of the average kinetic energy of the atoms or molecules in the system.
  • The zeroth law of thermodynamics says that no heat is transferred between two objects in thermal equilibrium; therefore, they are the same temperature.
  • We can calculate the heat released or absorbed using the specific heat capacity start text, C, end text, the mass of the substance start text, m, end text, and the change in temperature delta, start text, T, end text in the following equation:
start text, q, end text, equals, start text, m, end text, times, start text, C, end text, times, delta, start text, T, end text

Want to join the conversation?

  • piceratops seedling style avatar for user Tu Huynh
    For I know that Kelvin is always positive, but why in the example, why Kevin degree is negative?
    (7 votes)
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  • blobby green style avatar for user Nanananananananana
    Why is it in my book it is indicated that q = C x change in T, where C is heat capacity (J/C). C is then equal to (m x s x change in T)hot + (m x s x change in T)cold/-change in T, where s is the specific heat capacity (J/gC). Can you please explain to me? I'm quite confused.
    (5 votes)
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    • spunky sam blue style avatar for user Ernest Zinck
      q = CΔT and q = msΔT, so C = ms.
      C is the total heat capacity of the object.
      s is the specific heat capacity, i.e. the heat capacity per gram.
      If you multiply the heat capacity per gram (s) by the number of grams, you get the total heat capacity (C).
      Usually you know the specific heat capacity for water, but you don't know the specific heat capacity of the calorimeter.
      But it is quite easy to do a separate experiment to determine the total heat capacity of the calorimeter.

      You often set up your calculations like this:
      heat for water + heat for object + heat for calorimeter = 0
      q₁ + q₂ + q₃ = 0.
      q₁ = m₁s₁ΔT₁ for the water
      q₂ = m₂s₂ΔT₂ for the object
      q₃ = CΔT for the calorimeter.
      If you then know everything but one of the variables, you can calculate it from the above equations .
      (14 votes)
  • blobby green style avatar for user Fatima Naz
    Why do kelvins don't have degree?
    (3 votes)
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    • blobby purple style avatar for user Abdülrezzak Bostani
      In general, 'degrees' are found in units which are mainly arbitrary. Fahrenheit and Celsius are meant to be convenient instead of absolute. The same goes for angle degrees, since that also divides the circle into 360 degrees arbitrarily. Like, there is no 'degrees' in radians, which is the 'natural' way of measuring angles.

      The kelvin is the SI base unit of temperature. it is the absolute temperature scale. By absolute we mean that the zero on the Kelvin scale, denoted by 0 K, is the lowest temperature that can be attained theoretically. On the other hand, 0 F and 0 C are based on the behavior of an arbitrarily chosen substance.

      For Kelvin. Temperature is defined in terms of the average energy of particles in a system, and Kelvin is directly proportional to that -- the zero in the Kelvin scale corresponds to absolute zero, and not any arbitrary temperature, and Kelvin is the 'natural' unit to measure temperature.
      (12 votes)
  • piceratops seed style avatar for user Suraj Singh
    Why the Zeroth Law of thermodynamics is called so,is it the most basic law?
    (4 votes)
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  • blobby green style avatar for user danishsa314
    i did not understand how chemists use the melting point to identify the purity of a substance
    (3 votes)
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    • leaf green style avatar for user Rohan Bassi
      Chemists can heat up substances to the point when they start to melt. At this temperature, the chemists can compare this value to a table of standard results from a data table/source/reference. The standard results are the true results for that particular substance. However, when a substance has an experimental melting point significantly higher than the standard data book value, then we know there must be impurities present. Impurities may have a higher melting point than the substance we're interested in, so the overall melting point for the impure substance is higher than expected.
      (9 votes)
  • blobby green style avatar for user ac4444122
    Can somebody give me an example that shows the difference between heat and temperature?
    (2 votes)
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    • hopper jumping style avatar for user Lucas De Oliveira
      A thermometer shows the temperature because it measures (compare) the degree of agitation of its molecules in relation to the degree of agitation of the molecules of the environment, but this is only possible because you transfer heat to the thermometer. This is because 2 bodys at different temperatures tend to get in the thermal balance between them with the one that is hot by giving energy to the coldest in the form of heat, so on a cold day you wear a jacket not to heat yourself but to reduce the loss of thermal energy to the environment in the form of heat.
      (6 votes)
  • blobby green style avatar for user Kay Johnson
    Why is it often not possible to directly measure the heat energy change of the reactants and products?
    (3 votes)
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  • leaf green style avatar for user Ayudh Saxena
    Difference between work and heat?
    (2 votes)
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  • duskpin ultimate style avatar for user aricohens13
    Underneath the picture of the ice cubes, the caption reads, "As ice melts, heat is transferred from the water to the surroundings." Shouldn't it read, "As ice melts, heat is transferred from the surroundings to the ice" or something similar?
    (2 votes)
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  • blobby green style avatar for user Starlight Ray
    So when I say that I feel hot when I touch boiling, water what actually happens (in the atomic level) is the molecules of H2O hitting my hand with a good deal of force (causing my nerves to "feel"the burn) and the energy transformation (I think I am wrong here) causing the damage to the cells of my hand results in the wounds. Am I right?
    Please answer!
    Thanks!
    (1 vote)
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    • piceratops seed style avatar for user RogerP
      What you say is essentially correct. The thermal energy in the boiling water is transferred to compounds in your hand which causes damage to cells. A whole variety of chemicals in the cells in your hand will be heated up, including the water in the cells. Locally, temperatures will increase to above the levels at which cells are intended to work, so cells will rupture, proteins will denature, etc.

      Nerve cell receptors are also triggered by the thermal energy and quickly transfer an electrical signal to the brain which you interpret as pain.
      (4 votes)