It’s Friday afternoon and you’re finally home from school after a long week. You bound into the house and ask the question that’s on every student’s mind: what’s for dinner? Your parents ask if you want to help prepare dinner with them tonight. You get excited, because you know what that means: spaghetti time!
You get out the big metal spaghetti pot and fill it with water from the sink. You place it on the stove and turn the burner on high. After what seems like forever, the water finally begins to boil, bubbling a spray of steam into the kitchen air.
You put the noodles into the water while you heat the spaghetti sauce in a separate saucepan. Before long, you have tender noodles ready to be covered in delicious sauce. The only thing you’re missing is a cool drink. You fill a glass with ice cubes and pour lemonade into the glass. Time to eat!
You probably weren’t thinking about school subjects, like science, while you were helping prepare dinner. However, you were a firsthand witness to some interesting scientific principles. If you think about it, you used water in each of its states: liquid water went into the pot, gaseous water vapor filled the air when the liquid water began to boil, and solid water in the form of ice cubes cooled your lemonade!
All of these changes in the state of water involved transfers of heat and thermal energy. When scientists study the amount of thermal energy and its movement in and out of systems, we call that field of study thermodynamics.
The scientific processes you witnessed in the kitchen also reflected a couple of scientific truths known as laws of thermodynamics. We’ll take a look at two of these, known as the first and second laws of thermodynamics.
The first law of thermodynamics describes the conservation of energy. It states that when heat is added to a system, it transforms to an equal amount of another form of energy. It does this by doing one or both of the following things: (1) increasing the amount of internal energy within the system; and/or (2) doing external work by leaving the system, thereby affecting the area around the system.
For example, after you added water to your pot to boil, you added energy to the system by heating it. The added heat caused the temperature and energy of the water to increase. The system also released some of that energy, thereby heating the air around the water.
The measure of the total energy in a thermodynamic system is called enthalpy. Enthalpy is usually measured in either joules (International System of Units) or calories (British Thermal Units). Scientists always measure the change in enthalpy of a system, because a system’s total enthalpy can’t be directly measured.
The second law of thermodynamics states that no reaction is completely efficient and that heat is always transferred from warmer objects to colder objects. In other words, some energy is always lost to heat in any reaction, and you’ll never find heat moving from colder objects to warmer objects in a system.
The second law of thermodynamics is also sometimes known as the law of disorder, because it describes the concept of entropy. Scientists use the word entropy to describe the amount of freedom or randomness in a system. In other words, entropy is a measure of the amount of disorder or chaos in a system.
In any system, the energy present is inherently active and will act spontaneously to scatter or minimize thermodynamic forces. The more energy present in a system, the more disorder or entropy there will be. For example, when a substance changes states from a solid to a liquid to a gas (such as water going from ice to liquid water to water vapor), the atoms and molecules have more freedom to move at each new stage. That increase in freedom that corresponds to an increase in disorder is entropy.
Entropy is thus a measure of the random activity in a system, whereas enthalpy is a measure of the overall amount of energy in the system. We bet you didn’t realize that fixing spaghetti involved so many laws of thermodynamics!