Heating and Cooling Graphs

Heating and cooling graphs provide a visual representation of temperature changes in a substance as energy is either ADDED to or REMOVED from it. They indicate the points at which CHANGES OF STATE occur.

These graphs typically follow a similar pattern: during heating, energy is transferred to the substance, increasing the molecules' KINETIC ENERGY, while cooling involves energy being transferred away, resulting in a decrease in the molecules' KINETIC ENERGY.

 

Heating Graphs:

This graph depicts the relationship between temperature and time for a substance as it changes state from solid to liquid to gas:

 

• Starting on the left, the initial upward slope indicates an increase in temperature as a SOLID. The substance's temperature rises steadily as it absorbs heat, causing its particles to vibrate more energetically.
• The first HORIZONTAL line, represents the MELTING. Here, the temperature remains CONSTANT despite the continued input of heat. This is because the energy is used to BREAK BONDS (forces) that hold the solid particles in place, rather than increasing their KINETIC ENERGY. This phase change from solid to liquid occurs at a specific temperature known as the MELTING POINT.
• After the substance has completely melted into a LIQUID, the temperature begins to rise again with further heat input, shown by the upward slope following the horizontal section. The liquid's particles gain KINETIC energy and move more freely.
• The second HORIZONTAL line signifies the BOILING point where the liquid begins to change into a GAS. The temperature does not increase during this phase change because the energy goes into BREAKING the BONDS (forces) that hold the liquid particles together. This is the BOILING point where the substance transitions from liquid to gas.
• As the phase change completes, the temperature rises once more, as shown by the final upward slope indicating the substance is now in the GASEOUS STATE. The particles are moving rapidly due to the high kinetic energy from the absorbed heat.

 

Cooling Graphs:

Cooling graphs are the opposite of heating graphs and illustrate the changes in temperature over time as a substance transitions from gas to solid:

 

• Starting from the left, the substance begins as a GAS. As it loses energy, the temperature decreases, shown by the downward slope.
• The first HORIZONTAL line, indicates the process of CONDENSING, where the gas turns into a liquid. During this phase change, the temperature remains constant despite the continuous loss of energy. This is because the energy removed is being used to change the state of the substance and MAKE BONDS (forces) between the particles rather than lowering the temperature.
• Following condensation, the temperature of the now liquid substance continues to decrease with further energy loss, as indicated by the next downward slope.
• The second HORIZONTAL line represents FREEZING, where the liquid turns into a solid. Similar to condensation, the temperature does not change during this phase transition. The energy is being used to change the state, in this case by CREATING BONDS (forces) that turn the liquid into a solid.
• After the substance has completely frozen into a SOLID, any further energy loss will once again result in a decrease in temperature, continuing the downward trend on the graph.