Heating & Cooling Graphs

Heating and Cooling Graphs Basics

Heating and cooling graphs show how the temperature of a substance changes over time as energy is added or removed. The most common type is a temperature vs time graph, where the y-axis represents temperature (6C) and the x-axis represents time (seconds or minutes). Sometimes, graphs may also plot temperature against the amount of energy supplied.

When a substance is heated or cooled, the graph typically has two types of sections:

• Sloped sections: Temperature changes steadily as the substance gains or loses thermal energy.
• Plateaus (flat sections): Temperature remains constant even though energy is still being transferred. These occur during phase changes where latent heat is involved.

For example, when ice is heated, the temperature rises until it reaches 06C, then stays constant while the ice melts. After melting, the temperature rises again until it reaches 1006C, where it stays constant during boiling.

Interpreting Plateaus

Plateaus on heating or cooling graphs represent phase changes where the substance changes state (solid to liquid, liquid to gas, or vice versa). During these periods:

• The temperature remains constant because all the energy supplied is used to break or form bonds between particles, not to increase kinetic energy.
• Energy is used to overcome the forces holding particles together (melting or boiling) or to form bonds (freezing or condensing).
• No temperature change occurs despite energy input (heating) or output (cooling).
• This energy absorbed or released during a phase change is called latent heat.

This explains why the graph has horizontal sections during melting and boiling.

Energy Transfers in Graphs

The energy supplied to or removed from a substance changes its internal energy. Internal energy is the total kinetic and potential energy of the particles.

• When energy is added, it either raises the temperature (increasing kinetic energy) or changes the state (increasing potential energy).
• The slope of the temperature change sections depends on the specific heat capacity of the substance: a steeper slope means less energy is needed to change the temperature.

For example, water has a high specific heat capacity, so its temperature rises slowly compared to metals when the same energy is added.

For instance, if a metal block is heated and its temperature rises from 206C to 406C in 5 minutes, the graph shows a steady upward slope during this time. The slope reflects how quickly the temperature changes with energy input.

Changes of State on Graphs

Heating and cooling graphs clearly show the melting and boiling points of a substance:

• Melting point: The temperature at which a solid turns into a liquid (e.g., 06C for water).
• Boiling point: The temperature at which a liquid turns into a gas (e.g., 1006C for water).

The graph is divided into regions representing solid, liquid, and gas phases:

• Sloped regions show temperature changes within a single phase.
• Plateaus show phase changes where latent heat is involved.

During the plateaus, latent heat is the energy absorbed or released without temperature change. This energy changes the state but does not affect temperature.

Example inline: If 500 J of energy is added to a substance and its temperature rises from 106C to 206C, the slope of the graph during this time reflects the specific heat capacity of the substance.