9.6 Temperature, Pressure & Ice

Density isn’t fixed forever — heat it or squeeze it and the density changes, because the volume changes while the mass stays the same. This explains hot-air balloons and one of nature’s most important quirks: floating ice.

Heat lowers density

Why Hot Things Are Less Dense Recall Chapter 7: heating makes particles move apart and spread out, so the volume increases while the mass is unchanged. Since density = mass ÷ volume, a bigger volume means a lower density. So heating generally lowers density; cooling raises it.

The Hot-Air Balloon Heat the air inside a balloon and it expands, becoming less dense than the cooler air around it — so it rises, carrying the balloon up. The same reason warm air rises off a hot road, and why the warm low-pressure regions of Chapter 6 form. One idea, many sights.

Pressure and the states of matter

Pressure Squeezes Gases Most Pressure changes density too — but how much depends on the state (Chapter 7’s spacing!):

  • Gases: lots of space → pressure pushes particles closer → volume drops → density rises noticeably.
  • Liquids: nearly incompressible → small effect.
  • Solids: particles already packed → effect is negligible.

The wonderful exception: floating ice

Why Ice Floats Almost everything gets denser as it cools — but water is special. Water’s density is highest at 4 °C. As it cools further to 0 °C and freezes, its particles arrange into a structure that takes up more space (it expands). The same mass now fills a larger volume, so ice is less dense than liquid water — and floats.

A Quirk That Saves Lives Because ice floats, it forms a layer on top of a frozen lake, insulating the water beneath and keeping it liquid and warm enough for fish and other creatures to survive the winter. If ice sank, lakes would freeze solid from the bottom up — and aquatic life couldn’t make it through the cold.

Think Like a Scientist — The Sinking Egg Drop a raw egg into a glass of tap water and it sinks (the egg is denser than water). Now here’s the puzzle: what could you add to the water to make the egg float? (Hint: dissolving lots of salt makes the water itself denser — try it and see!)