Figure 6.1.46.1.4\PageIndex{4}: Temperature versus heat. The system is constructed so that no vapor evaporates while ice warms to become liquid water, and so that, when vaporization occurs, the vapor remains in the system. The long stretches of constant temperatures at 0oC0oC0^oC and 100oC100oC100^oC reflect the large amounts of heat needed to cause melting and vaporization, respectively.

Heat and work have signs (positive or negative), and the sign of each depends on whether the system we are considering is gaining or losing energy. In this class, if a process makes the system gain energy, qqq and/or www are positive; if the process makes the system lose energy, qqq and/or www are negative. We can put this information into four formal statements: If heat flows into a system, qqq is positive. If heat flows out of a system, qqq is negative If the surroundings do work on the system, www is positive. If the system does work, www is negative.

In order to measure energy, we need a unit for it. In the metric system, the standard unit of energy is the joule. The formal definition of a joule is: A joule is the amount of energy expended when an object is moved 1 meter against a resisting force of 1 newton. (You can learn all about the concept of force in a physics class.) As for the joule, here are some statements that may help you visualize this unit.  A joule is… …enough energy to lift a one kilogram object 10.2 centimeters. …enough energy to heat one milliliter of water from 20ºC to 20.24ºC. …enough energy to keep a 60 watt light bulb glowing for 0.0167 seconds. Obviously, a joule is a very small amount of energy, and in fact it is an inconveniently small amount when we describe chemical reactions. Chemists usually report energies for reactions in kilojoules (1 kJ = 1000 J).