Chemistry: Heat, Energy, and Phase Changes
Exploring how heat affects matter and changes of state
Chemistry: Heat, Energy, and Phase Changes
Exploring how heat affects matter and changes of state
Chemistry - Grade 6-8
- 1
A student places an ice cube on a warm plate. Explain what happens to the ice cube and describe how energy moves during this change.
Heat moves from warmer objects to cooler objects.
The ice cube melts into liquid water. Thermal energy moves from the warmer plate into the colder ice, causing the particles in the ice to move faster and break out of their fixed positions. - 2
What is the difference between heat and temperature?
Heat is the transfer of thermal energy from one object or substance to another. Temperature is a measure of the average kinetic energy of the particles in a substance. - 3
A pot of water is heated on a stove. The water temperature rises from 25 degrees Celsius to 100 degrees Celsius. What happens to the motion of the water particles as the temperature increases?
Higher temperature means faster particle motion.
As the temperature increases, the water particles move faster. Their average kinetic energy increases because temperature is related to particle motion. - 4
Name the phase change that occurs when liquid water becomes water vapor. Explain whether energy is absorbed or released.
The phase change is vaporization, which includes evaporation and boiling. Energy is absorbed because liquid water particles need energy to move far enough apart to become a gas. - 5
Name the phase change that occurs when water vapor turns into liquid water on the outside of a cold glass. Explain why this happens.
Think about what happens to gas particles when they cool down.
The phase change is condensation. It happens because water vapor in the air loses thermal energy when it touches the cold glass, causing the gas particles to slow down and form liquid droplets. - 6
Sort these phase changes into two groups: melting, freezing, condensation, evaporation, sublimation, deposition. Which absorb energy, and which release energy?
Melting, evaporation, and sublimation absorb energy. Freezing, condensation, and deposition release energy. - 7
A solid substance is heated, but its temperature stays the same while it melts. Explain why the temperature does not increase during melting.
During a phase change, energy changes particle arrangement rather than temperature.
The temperature does not increase during melting because the added energy is used to overcome the forces holding the particles in fixed positions. The energy changes the state of the substance instead of increasing particle speed. - 8
Use the terms solid, liquid, and gas to describe how particle arrangement changes as a substance gains energy.
In a solid, particles are tightly packed and vibrate in place. As energy is added, the substance can become a liquid with particles that slide past each other, and then a gas with particles that are far apart and moving freely. - 9
Dry ice is solid carbon dioxide. At room temperature, it changes directly from a solid into a gas. What is this phase change called, and does it absorb or release energy?
A direct change from solid to gas has a special name.
This phase change is called sublimation. It absorbs energy because the solid carbon dioxide particles need energy to separate and become a gas. - 10
Frost forms on a cold window when water vapor in the air changes directly into ice. What is this phase change called, and does it absorb or release energy?
This phase change is called deposition. It releases energy because gas particles lose energy and become arranged as a solid. - 11
A student says, "Boiling water is hotter than steam because water is liquid and steam is gas." Explain why this statement can be incorrect.
Phase alone does not tell you the exact temperature of a substance.
The statement can be incorrect because steam can be at the same temperature as boiling water or even hotter. A gas is not always cooler or hotter than a liquid, and temperature depends on the average kinetic energy of the particles. - 12
A cup of hot chocolate is left on a table. After 20 minutes, it is cooler. Explain what happened to the thermal energy of the hot chocolate.
Energy transfers from warmer matter to cooler surroundings.
Thermal energy moved from the hot chocolate to the cooler air, cup, and table around it. As the hot chocolate lost energy, its particles moved more slowly and its temperature decreased.