Pattern Recognition
1. All slanted lines on warming or cooling curves indicate changes in kinetic energy (changes in average velocity of the particles). The slopes of these lines indicate the magnitude of the specific heat (the greater the slope, the lower the specific heat).
2. All horizontal lines on warming or cooling curves indicate changes in potential energy (significant changes in attractive forces). The length of these lines reflects the strength of the attractive forces; long horizontal portions suggest strong attractive forces.
3. All portions of a warming curve indicate an increase in energy for the substance being heated. As time goes on during the warming process, energy is being stored in the substance.
4. All portions of a cooling curve indicate a decrease in energy for the substance being cooled; energy is released by the substance as it cools.
5. Substances with low vapor pressures have strong attractive forces in the liquid phase. These substances will have high boiling points and will also evaporate slowly.
6. Substances with high vapor pressures have weak attractive forces in the liquid phase. These substances will have low boiling points and will evaporate rapidly.
1. At the melting point of a crystalline substance, the particles that have turned liquid are moving faster than the particles that are still in the solid phase.
In fact, the difference between the solid and liquid particles at the melting point is a difference in the forms of kinetic energy (translation, rotation, vibration). The liquid particles also have added potential energy due to the fact that they have been moved a distance apart.
2. The phase in which substances exist at normal temperatures is the phase in which they always exist.
For example, students find it difficult to picture iron in the gaseous state or mercury in the solid state. They usually think of water only as a liquid, and need to be jogged to think of ice or water vapor. At liquid nitrogen temperatures, it is possible to drive a rubber nail into a wooden board with a mercury hammer.
3. Materials which are more viscous are necessarily more dense.
For example, ask students which is more dense, milk or cream. They will frequently conclude that cream is more dense because it is thicker. Yet, cream floats on milk and thus is less dense. They may also draw the same faulty conclusion when comparing oil and water or vinegar and oil. We can think of viscosity as a rate process where increasing energy (temperature) allows molecules to move past their neighbors more rapidly. Viscosity is partially determined by intermolecular forces of attraction, and thus parallels energies of vaporization.
4. The freezing point and boiling point of substances are fixed points and never vary. (Water always boils at 100 °C.)
The freezing and boiling points are dependent upon many things, especially the prevailing pressure and the purity of the substance.
5. Substances are always cold when they freeze.
Students never seem to think of iron freezing at a very high temperature, but that is exactly what any material does as it passes from the liquid to solid phase, no matter what the temperature is when this process takes place.
6. Ice never gets colder than 0 °C.
This is a very common idea held by students. To deal with this misconception, place water in a test-tube. Place a thermometer in the water. Place the whole system in a beaker with an ice/salt/water mixture. Have the students watch the temperature. This would also work nicely as a computer/temperature probe demonstration
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