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Question Video: Describing Why Ice Floats on the Surface of Water Science

Why does ice float on the surface of water?

04:24

Video Transcript

Why does ice float on the surface of water? (A) Ice is less dense than water and stays on the surface. (B) Ice is denser than water and stays on the surface. (C) Ice is held at the surface of the liquid by water tension. (D) Ice is too dense for air pressure to force it under the surface of the water. Or (E) ice always contains pockets of air, which make it more dense than water.

We can see from the image given that ice floats to the surface of water. Ice is water in its solid state. Water has the chemical formula of H2O. Its molecules are each composed of one oxygen atom and two hydrogen atoms. Water molecules are polar molecules.

A polar molecule is a covalently bonded molecule where one side is partially positive and one side is partially negative. In water, the side of the molecule with oxygen is partially negative, which we can represent using a 𝛿 minus symbol. This is due to the difference in the electronegativity between oxygen and hydrogen. Electronegativity is the measure of how strongly an atom attracts an electron pair, or pairs, from a chemical bond.

In the structure of water, oxygen forms one covalent bond with each hydrogen. Each covalent bond involves the sharing of one electron pair. Because oxygen has a greater electronegativity, the shared electrons are more strongly drawn towards the oxygen atom. Thus, the side of the molecule with oxygen becomes partially negative. The side of the molecule with hydrogen atoms becomes partially positive, which we can represent using 𝛿 plus symbols.

Due to their polar nature and structure, water molecules can engage in a special type of intermolecular force called hydrogen bonding. Hydrogen bonding is the attraction between hydrogen atoms and the partially negative sides of adjacent molecules, particularly between the fluorine, nitrogen, or oxygen atoms of the adjacent molecule. This is because atoms of fluorine, nitrogen, and oxygen have very high electronegativities and would likely be partially negative and attractive to the partially positive hydrogen atoms.

All water molecules in the glass of ice water are engaging in hydrogen bonding. Let’s have a closer look at how the hydrogen bonding differs in the water and ice. In liquid water, adjacent water molecules are attracted to each other and are forming hydrogen bonds. These interactions are represented by the green dotted lines in the diagram.

In the liquid state, water molecules and their hydrogen bonds cause a relatively random and close formation. In the solid state of ice, the hydrogen bonding results in a different formation of molecules. Upon freezing, the hydrogen bonds between molecules cause a rigid pattern and form the crystals of ice.

A hexagonal pattern is formed that has a larger volume than the same mass of water as a liquid. Solid water takes up more space than liquid water. With the same mass and larger volume, water has a lower density as a solid than as a liquid. This is why ice floats to the top when in liquid water. We can see that the answer choice that matches this description is answer choice (A). Therefore, the reason that ice floats on the surface of water is that ice is less dense than water and stays on the surface.

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