A simple molecule with odd behavior
A water molecule contains one oxygen atom and two hydrogen atoms, bent into a V shape. The oxygen pulls on the shared electrons more strongly than the hydrogens do, so the molecule has a slightly negative side and a slightly positive side. Chemists call this a polar molecule.
Because of that polarity, neighboring water molecules attract one another: the positive hydrogen side of one is drawn to the negative oxygen side of another. This attraction is a hydrogen bond. Each bond is weak, but there are enormous numbers of them, and they explain almost all of the properties of water that make it unusual.
The properties of water that stand out
- High heat capacity. Water takes a lot of energy to warm up and releases a lot as it cools. About 4.2 joules are needed to warm one gram by one degree Celsius, which is high compared with most common materials. Oceans therefore absorb and store huge amounts of heat, and this smooths out temperatures near coasts. The same property is part of why body temperature regulation works well in a body that is mostly water.
- High boiling and melting points for its size. Hydrogen bonds hold the liquid together, so water stays liquid across the range of temperatures found on most of Earth’s surface. Without hydrogen bonds, a molecule this small would be expected to be a gas at room temperature.
- Ice floats. For most substances, the solid is denser than the liquid. In water, the hydrogen bonds lock molecules into an open, hexagonal pattern as it freezes, and ice is roughly 9 percent less dense than liquid water. So lakes freeze from the top down, leaving liquid water below where aquatic life can survive.
- Maximum density above freezing. Fresh water is densest at about 4 degrees Celsius, not at 0. That is one reason lake water can layer in winter.
- Surface tension and cohesion. Molecules at the surface are pulled inward and sideways, creating a thin skin that lets small insects walk on water and makes drops round.
- Excellent solvent. Its polarity lets water pull apart many salts and surround dissolved particles. This is why the ocean contains so many dissolved substances, and why water carries nutrients inside living things.
Why the ocean is salty and how that matters
Average ocean salinity is about 3.5 percent by weight, most of it ordinary table-salt ions of sodium and chloride, together with magnesium, sulfate, calcium and others. The salt arrived over a very long time through rivers carrying dissolved minerals from rock, along with vents and volcanic activity on the seafloor.
Salt makes seawater denser than fresh water, and also lowers its freezing point slightly below 0 degrees Celsius. Density in the ocean depends on both temperature and salinity, with colder, saltier water being denser and sinking beneath warmer, fresher water.
Density and the layered ocean
Ocean water is arranged in layers. Warm, lighter water sits near the surface and is stirred by wind, while cold, dense water lies below. Between them is a zone where temperature changes quickly with depth, called the thermocline. Because it resists mixing, the thermocline affects how heat, nutrients and oxygen move through the sea.
Currents
Currents in the sea come in two broad kinds.
- Surface currents are driven mainly by winds, and shaped by Earth’s rotation and the layout of continents. They often form large circular loops called gyres.
- Deep currents are driven by differences in density. In polar regions, seawater becomes colder, and when sea ice forms, it leaves salt behind in the water below, making that water denser. It sinks and flows slowly along the ocean floor. This density-driven circulation is sometimes called the thermohaline circulation, and it is part of a global loop that takes hundreds to around a thousand years to complete, according to common estimates.
How ocean currents shape climate by carrying warmth toward higher latitudes is a topic in its own right. The key physics here is that moving water, with its large heat capacity, is an efficient carrier of heat around the planet.
Tides
Tides are the regular rise and fall of sea level. They are caused mainly by the gravity of the Moon, with a smaller contribution from the Sun. The Moon pulls most strongly on the side of Earth facing it, less on the center, and least on the far side. The difference in pull stretches the ocean into two bulges, one facing the Moon and one on the opposite side. As Earth rotates, a coastline passes through both, which is why most places have two high tides and two low tides in about 24 hours and 50 minutes.
When the Sun, Earth and Moon line up, at new and full Moon, the effects combine to give larger spring tides. At the quarter Moons they partly offset one another, giving smaller neap tides. Local coastline shape matters a great deal, so tidal range varies from a few tens of centimeters in some places to many meters in narrow bays and funnels. Tide tables for a given location are more reliable than general rules.
Waves and other motion
Most ordinary waves are produced by wind transferring energy to the water surface. The water moves in small circles while the wave energy travels forward, so a floating object bobs more than it travels. Much longer waves, such as tsunami waves, are caused by sudden displacement of the seafloor and carry energy through the full depth of the water.
Water in motion beyond the sea
Water also cycles between the ocean, air and land, as covered in the water cycle. Ocean heat content also matters for sea level rise, since water expands slightly as it warms, and melting land ice adds more water. Those are separate issues from the physics of water itself, but they build on the same properties: heat capacity, thermal expansion and density.
Key points
- Hydrogen bonds explain water’s high heat capacity, its tendency to stay liquid, and why ice floats.
- Salinity and temperature set seawater density, which drives deep ocean circulation.
- Winds drive surface currents; the Moon and Sun drive tides.