Phase Change: Properties & Causes

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Matter exists in different phases, and can change from one phase to another. But how and why does this occur? In this lesson we will explore these concepts, looking at the transitions from solid, to liquid, to gas, and back.

Matters Exists in Different Phases

You are likely already familiar with the three major phases of matter: solid, liquid, and gas. In solids, the particles are held together so tightly by attractions that they form a fixed shape. The particles in a liquid are held together a little less tightly, so they can move around to take the shape of their container (up to the surface of the liquid). Gas particles aren't really held together at all, so they move around all over and spread out to fill the entire space.

The three main phases of matter: solid, liquid, and gas.
phases of matter

You may also know that a substance can exist in all three different phases. For example, ice is the solid form of water, and there's liquid water and water vapor, which is the gaseous form. But how do we get form one phase to another?

Adding & Removing Heat

We do this by adding or removing energy as heat. When we remove heat from liquid water we freeze it and get ice (cold is really only an absence of heat!). And when we add heat to liquid water we boil it and turn it into a gas. But we can also melt ice back into a liquid by adding heat and condense gaseous water into a liquid by removing heat.

The temperature at which a substance melts from solid to liquid is called its melting point. But this is also the same temperature as that substance's freezing point. This is because that specific temperature is where the phase change occurs for that substance. The same is true for the boiling point and condensation point of a substance. This is the temperature where a substance will vaporize into a gas when heat is added, and condense into a liquid when heat is removed.

Melting/freezing and boiling/condensation points are unique to each substance. For example, the melting/freezing point of water is 0°C (32°F) and the boiling/condensation point is 100°C (212°F). But for nitrogen, the boiling/condensation point is -196°C (-320°F!) and the melting/freezing point is -210°C (-346°F).

An important note here is that liquid particles on the surface can evaporate below the boiling point. Think about it: if you leave a glass of water out eventually all of the water will be gone - it will have evaporated even though you didn't boil the water. We also experience evaporative cooling when we sweat. When our bodies get hot we sweat out water that evaporates off of our skin, taking heat with it and cooling us in the process.

Sweating helps our bodies to cool. Water comes to the surface where it evaporates and takes heat with it as it goes.

Expanding & Contracting

Ok, so now that we know that heat is involved in phase changes let's take a look at why substances change phase with heat. On a warm sunny day you feel energized and want to go run around outside. But when it's cold you would rather stay inside and not do much. Substances do the same thing! Removing heat from a substance means the particles have less energy so they don't move around as much. But they also contract or get closer together. There is so little energy in fact that the particles have barely any motion at all - they are only able to vibrate in their fixed positions.

When we add head to get a liquid the particles have more energy and feel like moving around more. Kind of like the first few days of spring weather after a cold winter. Because the particles have more energy and can move around more the substance expands so that it takes up more space.

Add even more heat (energy) to the substance and we're in full-blown summer. The particles have so much energy that they are moving around all over the place, which is why gases fill their entire space. The gas expands as far outward as it can because those particles are moving around so much. This change in size of the substance turns out to be very useful. For example, thermometers that are filled with mercury or alcohol expand and 'rise' as the temperature increases because of the addition of energy to the particles.

The mercury in a thermometer expands as heat is added to it, which in turn tells us the temperature.
mercury thermometer

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