Sunday, June 15, 2008

Why Does Ice Float?

It is almost officially summer! And that means plenty of sunshine and hot temperatures. With all the sun and heat, you will likely be drinking more water to keep hydrated. Most people prefer their water to be "ice cold", which just means there is ice in the glass to keep it cold. But, you may have noticed that ice doesn't just sink to the bottom of the glass - have you ever wondered why? This month we will explore that very question! The meaning behind this mystery lies in the different properties of solid and liquid water. Nearly every solid, if placed in its liquid form, will sink to the bottom. Luckily for us, the properties of water are different. Unlike most other substances on Earth, the solid form of water floats on the liquid form. This is caused by the change in density, which is defined as the amount of mass in a volume. With the exception of water, most substances on Earth become denser as they become colder. The solid ice will float because its density is lower than that of water. It is about 9% less dense than water. The denser water sinks to the bottom forcing the less dense ice to the surface. What makes water molecules different from other molecules is that they attract each other in an organized fashion. As the water cools, the molecules begin to bind to each other, forming a hexagonal pattern (shape that has six sides). Water is at its densest point at 4 degrees C. After that point, the water molecules move very slowly and attract to each other. In most substances, the molecules are more tightly packed together in solid form. But in ice, the hexagonal pattern of the attracting water molecules leaves empty spaces. This is why water expands when making ice cubes. The empty space between the hexagonal shapes makes the solid form less dense than the liquid form so that it floats to the top. Thanks to this oddity of physics, the water in our oceans and seas remain in liquid state. If the solid form of ice happened to be denser than water, the ice would sink to the bottom. If this happened, the ice on the bottom would begin to freeze up toward the surface. Eventually, nearly all the water on Earth would become solid ice and never melt. Luckily, ice floats and remains on the surface so that the water underneath remains in liquid form.

Little Lion Experiment:

This experiment will demonstrate that ice does float in most liquids, but you will also test other solid materials to see if they float, too.

Items Needed

  • 4 ice cubes
  • 4 small rocks
  • 4 small magnets
  • 4 quarters
  • 4 glasses
  • 4 different liquids (e.g., water, soda pop, salt water, and milk)


  1. Put each liquid in its own glass.
  2. Drop one of each solid material (i.e., ice cube, small rock, small magnet, quarter) into each glass.
  3. Observe what happens. Did any of the materials sink to the bottom or float to the top of all the glasses? Did some sink to the bottom of one glass but float to the top of another glass? What can you conclude about the solid materials? What can you conclude about different liquids used? Keep trying different solid materials and different liquids to see how they compare!