Based on the kinetic-molecular theory of matter, explain what happens to water molecules and to its properties during a phase change.
Answer (2)
The kinetic-molecular theory describes how water molecules move and interact during phase changes, such as melting ice into water or condensing vapor back into liquid. These changes involve energy transfer, alterations in molecular motion, and shifts in intermolecular forces, leading to changes in water's physical properties. For example, ice has a rigid structure, while liquid water is more fluid, reflecting the different states of matter.
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The kinetic-molecular theory of matter helps us understand what happens to molecules, like those in water, during phase changes. Let’s break it down step by step:
Solid to Liquid (Melting):
When ice (solid water) melts into liquid water, heat energy is absorbed. This energy increases the kinetic energy of water molecules, causing them to vibrate faster.
As the molecules gain energy, they break free from their fixed positions in the solid structure. This is why the rigid structure of ice transitions into the fluid form of water.
The properties change as the solid becomes less rigid and starts to flow.
Liquid to Gas (Evaporation/Boiling):
When water becomes steam (gas), further heat energy is absorbed. This energy gives water molecules enough kinetic energy to overcome intermolecular forces completely and move freely.
As a result, the molecules spread apart, forming water vapor, which is much less dense than liquid water.
During this change, the properties of water change from a liquid that can be poured to an invisible gas that expands to fill its container.
Gas to Liquid (Condensation):
When steam cools down and condenses into liquid water, it releases energy into the surroundings.
The decreased kinetic energy allows molecules to slow down and come closer together, forming liquid water.
During this process, water changes from a vapor back to a liquid state where it takes up less space and has a defined volume.
Liquid to Solid (Freezing):
As liquid water freezes into ice, it loses energy.
The decrease in kinetic energy allows molecules to settle into fixed positions to form a solid.
This results in the formation of ice, which is rigid and maintains a fixed shape unlike a liquid.
These phase changes illustrate the relationship between temperature (a measure of kinetic energy) and the state of matter. By understanding the kinetic-molecular theory, we can predict how changes in energy affect the properties of water and other substances.
