Effects of Heat What effects can heat have on an object or substance?

Effects of Heat Cause a change in temperature Cause a change of state Cause a chemical reaction Cause a change in density Cause a change in shape

Change in Temperature Temperature is a measure of the average kinetic energy of an object. To raise the temperature of an object, you have to put energy into the object. The amount of energy needed to raise 1kg of a substance by 1°C is called its “specific heat” (c). The higher the specific heat, the more energy it takes to heat up the substance.

Change in State Enough heat can change a substance’s state (melt a solid, evaporate a liquid) Once a substance reaches its melting or boiling point, more heat is needed to make it change state. This is called latent heat of fusion (for melting point) and latent heat of vaporization (for boiling).

Chemical Reactions Heat can start chemical reactions (think of a match). This can be caused by breaking bond between atoms, starting a reaction with nearby substances, or changing the valence of the substance.

Change in Density As a substance gets hotter, its atoms vibrate more strongly and take up more space. Most substances get bigger as they get hotter. Because their mass stays the same, they become less dense. We usually measure this by how much a substance expands in one dimension (length or width or height) for each °C it increases in temperature. This it called the coefficient of thermal expansion (α).

Then why does ice float? Water is an exception to the rule that colder is denser. Water obeys the rule most of the time, but between 0° and 4°C something unusual happens. Water’s shape and dipole (remember Mickey?) forces the water molecules to line up before they can freeze. When they line up they take up more space than they did while moving freely, causing their density to go down. Water is most dense at 4°C.

How does a mercury thermometer work? Metals and gasses have relatively high coefficients of thermal expansion (they get bigger quickly as they are heated). Most liquids do not. It would be difficult to get just the right amount of most metals into a thermometer and gasses can increase in pressure instead of temperature when heated in a closed space (like the inside of a thermometer). Mercury expands like a metal, but is a liquid so it can easily be put into a thermometer. As the thermometer heats up, the mercury inside expands. Marks on thermometer show you how much it has expanded and what temperature that represents.

Change in Shape If an object contains two substances with differing α’s, its shape can change when it is heated. The side with the larger α expands more, causing it to bend. Examples include thermostats and katana. It can also cause nuts and bolts to loosen if they are not made of the same material.

How does a thermostat work? A thermostat contains a bimetallic strip (a long thin piece of metal made of two different metals— usually brass and steel). Brass has a higher α than steel, so as it gets warmer the strip tends to bend toward the steel side. The reverse happens as it cools. This can be used to open or close an electrical circuit to control a heater, air conditioner, or other devices.