Which statement helps to explain why ice is less dense than liquid water?

Note that the dashed lines represent hydrogen bonds. Liquid water actually has a similar "open" structure also due to hydrogen bonding. But in the case of liquid water, the hydrogen bonds are not rigid and semi-permanent as in ice. So imagine that in the image above, the hydrogen bonding network collapses. This is what happens when enough thermal energy is present to break the rigid hydrogen bonds resulting in melting. Clearly, once this crystaline structure is no longer forced into place by the rigid hydrogen bonding in ice, it can collapse into itself, resulting a greater density of water molecules.

Thus the liquid form of water, although engaged in transient hydrogen bonding, is not as open and expanded as when held into it's solid form by the rigid, semi-permanent hydrogen bonding.

Why does ice float on top of the water, rather than sink like most solids? There are two parts to the answer to this question. First, let's take a look at why anything floats. Then, let's examine why ice floats on top of liquid water, instead of sinking to the bottom.

A substance floats if it is less dense, or has less mass per unit volume, than other components in a mixture. For example, if you toss a handful of rocks into a bucket of water, the rocks, which are dense compared to the water, will sink. The water, which is less dense than the rocks, will float. Basically, the rocks push the water out of the way or displace it. For an object to be able to float, it has to displace a weight of fluid equal to its own weight.

Water reaches its maximum density at 4°C (40°F). As it cools further and freezes into ice, it actually becomes less dense. On the other hand, most substances are most dense in their solid (frozen) state than in their liquid state. Water is different because of hydrogen bonding.

A water molecule is made from one oxygen atom and two hydrogen atoms strongly joined to each other with covalent bonds. Water molecules are also attracted to each other by weaker chemical bonds (hydrogen bonds) between the positively-charged hydrogen atoms and the negatively charged oxygen atoms of neighboring water molecules. As the water cools to below 4°C, the hydrogen bonds adjust to hold the negatively charged oxygen atoms apart. This produces a crystal lattice commonly known as ice.

Ice floats because it is about 9% less dense than liquid water. In other words, ice takes up about 9% more space than water, so a liter of ice weighs less than liter water. The heavier water displaces the lighter ice, so ice floats to the top. One consequence of this is that lakes and rivers freeze from top to bottom, allowing fish to survive even when the surface of a lake has frozen over. If ice sank, the water would be displaced to the top and exposed to colder temperature, forcing rivers and lakes to fill with ice and freeze solid.

However, not all water ice floats on regular water. Ice made using heavy water, which contains the hydrogen isotope deuterium, sinks in regular water. Hydrogen bonding still occurs, but it's not enough to offset the mass difference between normal and heavy water. Heavy water ice sinks in heavy water.

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.

Ice is less dense

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.

Although it seems heavy, an iceberg is less dense than water. Image credits: NOAA’s National Ocean Service.

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.

The structure of water molecules as they form ice, notice all the empty space. Image credits: NIMSoffice.

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.

Important for our Earth

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.

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