When heat is absorbed it is what?

Heat absorption refers to the heat transfer that occurs between two bodies; it can occur through conduction, convection or radiation. Heat absorption also is an endothermic reaction. In an endothermic process, a cooler object absorbs the hotter object’s heat.

Índice Show

Heats of Fusion and Solidification
What happens when heat is absorbed?
What is it called when heat is absorbed?
When heat is absorbed it is positive or negative?
When heat is absorbed is it cold or hot?

Baking bread in a oven is an endothermic reaction and an example of heat absorption. Another example of heat absorption is placing cold, raw chicken into a hot oven. As the chicken absorbs heat from the oven’s surrounding, it gets hot. The longer the chicken is in the oven, the more heat it absorbs.

Different colored objects absorb heat at different rates. For example, a black object absorbs more heat than one of a lighter color.

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

Heats of Fusion and Solidification
1. Example \(\PageIndex{1}\)
  1. Solution
  2. Step 1: List the known quantities and plan the problem.
  3. Known
  4. Unknown
  5. Step 2: Solve.
  6. Step 3: Think about your result.
Summary

Suppose that you are holding an ice cube in your hand. It feels cold because heat energy leaves your hand and enters the ice cube. What happens to the ice cube? It melts. However, the temperature during a phase change remains constant. So the heat that is being lost by your hand does not raise the temperature of the ice above its melting temperature of \(0^\text{o} \text{C}\). Rather, all the heat goes into the change of state. Energy is absorbed during the process of changing ice into water. The water that is produced also remains at \(0^\text{o} \text{C}\) until all of the ice is melted.

Heats of Fusion and Solidification

All solids absorb heat as they melt to become liquids. The gain of heat in this endothermic process goes into changing the state, rather than changing the temperature. The molar heat of fusion \(\left( \Delta H_\text{fus} \right)\) of a substance is the heat absorbed by one mole of that substance as it is converted from a solid to a liquid. Since the melting of any substance absorbs heat, it follows that the freezing of any substance releases heat. The molar heat of solidification \(\left( \Delta H_\text{solid} \right)\) of a substance is the heat released by one mole of that substance as it is converted from a liquid to a solid. Since fusion and solidification of a given substance are the exact opposite processes, the numerical value of the molar heat of fusion is the same as the numerical value of the molar heat of solidification, but opposite in sign. In other words, \(\Delta H_\text{fus} = - \Delta H_\text{solid}\). The figure below shows all of the possible changes of state along with the direction of heat flow during each process.

Figure \(\PageIndex{1}\): From left to right, heat is absorbed from the surroundings during melting, evaporation, and sublimation. From right to left, heat is released to the surroundings during freezing, condensation, and deposition. (CC BY-NC; CK-12)

Every substance has a unique value for its molar heat of fusion, depending on the amount of energy required to disrupt the intermolecular forces present in the solid. When \(1 \: \text{mol}\) of ice at \(0^\text{o} \text{C}\) is converted to \(1 \: \text{mol}\) of liquid water at \(0^\text{o} \text{C}\), \(6.01 \: \text{kJ}\) of heat are absorbed from the surroundings. When \(1 \: \text{mol}\) of water at \(0^\text{o} \text{C}\) freezes to ice at \(0^\text{o} \text{C}\), \(6.01 \: \text{kJ}\) of heat is released into the surroundings.

\[\begin{array}{ll} \ce{H_2O} \left( s \right) \rightarrow \ce{H_2O} \left( l \right) & \Delta H_\text{fus} = 6.01 \: \text{kJ/mol} \\ \ce{H_2O} \left( l \right) \rightarrow \ce{H_2O} \left( s \right) & \Delta H_\text{solid} = -6.01 \: \text{kJ/mol} \end{array}\nonumber \]

The molar heats of fusion and solidification of a given substance can be used to calculate the heat absorbed or released when various amounts are melted or frozen.

Example \(\PageIndex{1}\)

Calculate the heat absorbed when \(31.6 \: \text{g}\) of ice at \(0^\text{o} \text{C}\) is completely melted.

Solution

Step 1: List the known quantities and plan the problem.

Known

Mass \(= 31.6 \: \text{g}\) ice
Molar mass \(\ce{H_2O} = 18.02 \: \text{g/mol}\)
Molar heat of fusion \(= 6.01 \: \text{kJ/mol}\)

Unknown

The mass of ice is first converted to moles. This is then multiplied by the conversion factor of \(\left( \frac{6.01 \: \text{kJ}}{1 \: \text{mol}} \right)\) in order to find the \(\text{kJ}\) of heat absorbed.

Step 2: Solve.

\[31.6 \: \text{g ice} \times \frac{1 \: \text{mol ice}}{18.02 \: \text{g ice}} \times \frac{6.01 \: \text{kJ}}{1 \: \text{mol ice}} = 10.5 \: \text{kJ}\nonumber \]

Step 3: Think about your result.

The given quantity is a bit less than 2 moles of ice, and so just less than \(12 \: \text{kJ}\) of heat is absorbed by the melting process.

Summary

The molar heat of fusion \(\left( \Delta H_\text{fus} \right)\) of a substance is the heat absorbed by one mole of that substance as it is converted from a solid to a liquid.
The molar heat of solidification \(\left( \Delta H_\text{solid} \right)\) of a substance is the heat released by one mole of that substance as it is converted from a liquid to a solid.
Calculations of heat changes during fusion and solidification are described.

This page titled 17.10: Heats of Fusion and Solidification is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

What happens when heat is absorbed?

Heat is a type of ENERGY. When absorbed by a substance, heat causes inter-particle bonds to weaken and break which leads to a change of state (solid to liquid for example). Heat causing a phase change is NOT sufficient to cause an increase in temperature.

What is it called when heat is absorbed?

A reaction in which heat is absorbed is called an endothermic reaction .

When heat is absorbed it is positive or negative?

Note: When heat is absorbed by the solution, q for the solution has a positive value. This means that the reaction produces heat for the solution to absorb and q for the reaction is negative. When heat is absorbed from the solution q for the solution has a negative value.