Navigation
An introduction to how the motion of tiny particles defines the states of matter and thermal energy.
Why does the smell of fresh cookies travel across the house in minutes, even if there is no breeze? The answer lies in a hidden, microscopic dance happening in every object you touch.
The Kinetic Molecular Theory (KMT) states that all matter is made of tiny particles that are constantly in motion. In a solid, particles are packed tightly together and only vibrate in fixed positions. In a liquid, particles have enough energy to slide past one another, allowing the substance to flow. In a gas, particles move rapidly and randomly, filling whatever container they are in. The state of matter depends entirely on how much energy these particles have to overcome the attractions pulling them together.
Quick Check
In which state of matter do particles have the least amount of movement, only vibrating in place?
Answer
Solids
Imagine two identical cups of tea. Cup A is steaming hot, and Cup B is iced tea. 1. In Cup A, the water molecules have high thermal energy, meaning their average (velocity) is high. 2. In Cup B, the molecules have low thermal energy, so their is much lower. 3. If you drop food coloring into both, the coloring in Cup A spreads faster because the fast-moving water molecules kick the dye particles around more aggressively.
Quick Check
If you increase the temperature of a gas, what happens to the average velocity () of its particles?
Answer
The average velocity increases.
When we add thermal energy (heat) to a system, the particles absorb that energy and convert it into kinetic energy. This causes them to vibrate more violently or move faster. Eventually, they may gain enough energy to break the bonds holding them together, leading to a phase change (like melting or boiling). Conversely, removing energy (cooling) causes particles to slow down. As they slow, attractive forces pull them closer together, turning gases into liquids and liquids into solids.
Engineers must leave gaps in bridges called 'expansion joints.' 1. On a hot summer day, the sun adds thermal energy to the metal atoms in the bridge. 2. These atoms vibrate more vigorously, requiring more space to move. 3. This causes the entire bridge to physically expand in length. 4. Without the joints, the bridge would buckle under the pressure of its own vibrating atoms!
Scientists have a theoretical limit called Absolute Zero ( Kelvin or ). 1. Since temperature is the measure of particle motion, Absolute Zero is the point where all molecular motion stops entirely. 2. At this point, . 3. Because it is impossible to remove all energy from a system perfectly, we have never actually reached Absolute Zero in a lab, though we have come within billionths of a degree!
According to the Kinetic Molecular Theory, what happens to the particles of a liquid as it freezes into a solid?
If two objects have the same mass, but Object A has a higher temperature than Object B, which is true?
Particles in a gas move slower than particles in a solid of the same substance.
Review Tomorrow
In 24 hours, try to explain to a friend how the 'vibration' of atoms explains why a hot metal spoon feels different than a cold one.
Practice Activity
Observe a pot of water as it begins to boil. Identify the moment the particles gain enough kinetic energy to break free from the liquid and become a gas.