Why is water less dense as a solid

Share with the children that there is a difference in density between ice and water. In their experiment, the children had the same amount of material in both cups — the same amount of water - but when they froze the water, the volume changed. The amount of water was "stretched" to make a larger volume of ice.

• What would the children have had to do to get the ice to fill the cup only to the line when the water froze? Add less water!

Ice is less dense than water. If you had the same volume of ice, or ice that only went to the line on the cup, it would contain less material (water) than water of the same volume.

Facilitator's Note: Density is mass per unit volume, or the ratio of the amount of matter in an object compared to its volume, or simply put:

• Mass = the amount of "stuff"
• Weight = how heavy the "stuff"is (Weight is determined by the amount of gravitational pull on an object, which is a property of the mass of the planet the "stuff" is on. The greater the gravitational pull, the more the "stuff" weighs.)
• Density = how tightly packed the "stuff" is
• Volume = the area of space the "stuff" takes up

If we apply this information to the cups, the cup of water and the cup of ice have the same mass, i.e. amount of "stuff.” Because they had the same amount of "stuff," they have the same weight. The "stuff" (molecules) in water is more tightly packed than in ice, so water has greater density than ice. Don't let the fact that ice is a solid fool you!

As water freezes it expands. So, ice has more volume (it takes up more space, but has less density) than water.

Share with the children that this is a very special property of water. As most substances get colder, their material (molecules) scrunch closer and closer together, like penguins snuggled together to keep warm. Water molecules, however, don't like to scrunch. They spread out like acrobats in formation to form ice crystals like we see in snowflakes and frost.

Facilitator's Note: Water molecules in the liquid state like hanging out together as droplets, but in the solid (frozen) state, they need their space. A water molecule is similar in outline to Mickey Mouse, with two hydrogen atoms placed on one end of an oxygen atom. The oxygen atoms are positively charged and hydrogen atoms negatively charged, so water molecules stick to each other like tiny magnets. The hydrogen "ears" make loose bonds, called hydrogen bonds, with the oxygen "faces" of other water molecules.

Each molecule of water is made of two hydrogen atoms (white) and one oxygen atom (red). The hydrogen atoms are not placed on opposite sides of the oxygen atom; they occur at one end. Image modified from: http://commons.wikimedia.org/wiki/File:H2O_(water_molecule).jpg.

In ice crystals, their hydrogen bonds link them together like acrobats, stacked one on top of the other, with their arms and legs outstretched. This provides more space between the molecules in the crystal structure of ice than the molecules of liquid water. As a result, ice is less dense than the liquid form. Water is the only known non-metallic substance that expands when it freezes; its density decreases and it expands approximately 9% by volume.

Ice floats— that’s why the ocean has polar ice and icebergs, and why the ice in your drink floats. If you think about it, it might seem a bit strange because ice is a solid and intuitively, it should be heavier than a liquid and sink. Though this is true for most substances, water is an exception. Its hydrogen bonds and its solid state actually make it lighter than it is as a liquid.

Water is an amazing substance that basically fuels life on earth— every living organism needs it. It also has some interesting properties that enable life to be the way that it is. One of the most important properties is that water is the densest at 4 °C (40°F). Hot water and ice are both less dense than cool water. Less dense substances float on top of more dense substances. For example, when you make salad dressing oil floats on top of vinegar because it is less dense. The same is true for everything. If you have a blow-up beach ball in a pool, it floats, if you have a rock, it sinks.

The reason why ice is less dense than water has to do with hydrogen bonds. As you know, water is made up of one oxygen and two hydrogen atoms. They are attached by covalent bonds that are very strong. However, another type of bond also forms between different water molecules called a hydrogen bond, which is weaker. These bonds form because the positively-charged hydrogen atoms are attracted the negatively-charged oxygen atoms of nearby water molecules. When water is warm, the molecules are very active, move around a lot, and form and break bonds with other water molecules quickly. They have the energy to push closer to each other and move quickly.

As water gets below 4 °C, the kinetic energy decreases so the molecules don’t move around so much anymore. They don’t have the energy to move and break and form bonds so easily. Instead, they form more hydrogen bonds with other water molecules to form hexagonal lattice structures. They form these structures to keep the negatively charged oxygen molecules apart. In the middle of the hexagons, there is a lot of empty space.

Ice is actually about 9% less dense than liquid water. Therefore, ice takes up more space than water. Practically, this makes sense, because ice expands. It’s why you shouldn’t freeze a glass bottle of water and why frozen water can create bigger cracks in concrete. If you have a liter bottle of ice and a liter bottle of water, then the ice water bottle would be lighter. The molecules are further away from each other at this point than when the water is warmer. Therefore, ice is less dense that water and floats.

When ice melts, the stable crystal structure collapses and is suddenly denser. As water warms past 4 °C, it gains energy and the molecules move faster and further apart. So hot water also takes up more space than colder water and it floats on top of the cooler water because it is less dense. It’s like when you go to a lake to go swimming and the top layer is nice and warm but when you stick your legs below it is suddenly much colder.

So why does this even matter? Ice’s buoyancy has important consequences for life on earth. Lakes freeze over on the top in the winter in cold places, which allows fish and other animals to survive below. If the bottom froze, the whole lake could be frozen and almost nothing could survive the winter in the lake. In the northern or southern oceans, if ice sank, the ice caps would all be at the bottom of the ocean, preventing anything from living there. The ocean floor would be full of ice. Additionally, polar ice is important because it reflects light and keeps our planet from getting too warm.

Solid water is less dense than liquid water, which is why ice floats rather than sinks. This unusual property is due to hydrogen bonding between the molecules.