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Why does ice float?

Have you ever thought about why ice floats on water? Both are water, one is a solid and one is a liquid. Now, this is not something we think about on a daily basis but what if ice did not float? Rivers and lakes would freeze from the bottom up instead of on the surface and all the species that live within those aquatic biomes would now be exposed to cold from the ice below and the cold air above. It is unlikely that those species would not be able to survive. Fortunately, we know this is not the case and ice does float over liquid water. This floating ice creates an insulating layer between the cold air of winter and the species within the aquatic biome which allows organisms to survive through winter seasons. But why does ice float? To understand this we have to understand why some objects float and some objects sink.

To understand that we need to understand density. If an object has a density less than that around it it will float, if that object has a greater density it will sink. Density is the amount of matter within a given space. Understanding the densities of materials helps us predict how things will move and interact with other objects. Air is less dense than human cells which is why we can pass through it. Granite is denser than human cells so we can not pass through it without incredible force to blast a hole through it. Oil spills on the ocean, the oil floats because it is less dense than the saltwater. Helium balloons rise because helium is less dense than the surrounding air. Over time the helium escapes and air replaces the helium inside the balloon and causes the balloon to sink.

So the answer to why does ice float is that ice is less dense than liquid water. But how is that possible when they are both made of exactly the same thing! Typically we think that all solids are denser than liquids but that is not always the case! Pumice is a solid rock and it floats in water. Styrofoam is a solid man-made product and it floats. Polypropylene plastic floats and other plastics like nylon and acrylic sink. Some woods float and others sink.

Most liquids contract on cooling, decreasing their volume. The same mass but less volume would typically increase an object’s density but that is not the case with water and ice. Because of its molecular structure water behaves differently than other liquids. Water consists of one oxygen molecule and two hydrogens. As water cools its molecules contract and get closer and closer until they reach a temperature of 4 degrees Celsius, this is the point where the science magic, otherwise known as chemistry, happens. The water as it continues to cool actually increases in volume and decreases its density. The oxygen attracts electrons more strongly than hydrogen does, causing the electrons in the water molecule to hang out closer to the oxygen than to the hydrogen. This causes the Oxygen end to be slightly negatively charged and the hydrogen ends being slightly positively changed, making water a polar molecule. When water is in its liquid phase, its molecules are randomly organized, as the water is cooled the molecules get closer together and at 4 degrees Celcius they begin to align in a specific way with the positive and negative sides of the molecule start attracting each other and they snap into a rigid structure. Due to the shape of the molecule, because the hydrogens are attached at angles to the water molecule when the molecules snap together, they leave holes between the individual molecules, thus increasing the volume of water. As the volume increases, but the mass stays the same, our density decreases, making ice less dense than its liquid counterpart and thus floats in liquid water.

In these videos, I demonstrate what happens when cold water in a test tube in placed in hot water and vice versa when hot water in a test tube is placed in cold water. You can see the density column setup immediately.

We apply the concept of density when we design and build ships and submarines, dams, and airplane weight distribution. Density is important in the design of plumbing systems. Engineers need to know the densities of fluids so that they can move from one point to another. Denser fluids are harder to move and need more powerful pumps, less dense fluids are easier to move and need less powerful pumps. The condiments in your fridge got into their bottles through pumps and for that to happen, engineers need to know and understand their densities.

Additionally, we can use the concept of density to help explain other aspects of processes on Earth. Density is a factor in convection currents in the ocean that drive Earth’s weather and climate system. Warm water is less dense than cold water. The density of water vapor drives the water cycle on Earth. As seas warm, they increase in volume thus increasing sea-level rise in warmer areas of the globe.

Earth’s layers are denser as you move from the crust of the Earth to Earth’s inner core. The density of continental and oceanic plates determines if a plate will collide and produce a mountain range like the Himalayas or subduct and form a trench such as the Marianas Trench. Density is not just a simple equation or relationship between mass and volume. The concept of density helps us understand so many aspects of life.


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