Three Properties of the Element H2O: Water's Unique Traits

Water is often called the “universal solvent,” but that is only one of its unusual behaviors. The element H₂O (two hydrogen atoms bonded to one oxygen atom) has physical and chemical properties that are very different from those of other small molecules. Three properties stand out as the most consequential for life on Earth: its high specific heat capacity, its solid form being less dense than its liquid form, and its strong polarity that enables hydrogen bonding. These traits follow directly from the bent shape of the water molecule and the electronegativity difference between oxygen and hydrogen.

High Specific Heat Capacity

Water can absorb or release a large amount of heat energy without changing temperature very much. Its specific heat capacity (the energy needed to raise one gram of water by one degree Celsius) is about 4.18 joules per gram per degree Celsius National Institute of Standards and Technology. That is roughly ten times higher than the specific heat of iron and about four times higher than that of dry air.

This property comes from the hydrogen bonds between water molecules. Heat energy first goes into breaking those bonds before it can increase the kinetic energy of the molecules (that is, the temperature). Because water has many hydrogen bonds to break, it takes more energy to raise its temperature.

Large bodies of water, like oceans and lakes, heat up and cool down slowly. This moderates coastal climates, keeping them cooler in summer and warmer in winter than inland areas at the same latitude. For living organisms, the high specific heat of water means that cells and body fluids resist sudden temperature changes. Enzymes and metabolic processes can operate within a stable thermal environment, even when external temperatures shift.

Ice Floats on Liquid Water

Most substances are denser as solids than as liquids. Water is the opposite. When water freezes, the molecules arrange into a hexagonal crystal structure held open by hydrogen bonds. This open lattice takes up more volume than the same number of molecules in the liquid state. Ice is about 9 percent less dense than liquid water, so it floats.

In liquid water, molecules move past each other and pack more closely together. As the temperature drops toward 4°C, water reaches its maximum density. Below that, the molecules begin to align into the ice structure, and the density decreases. At 0°C, the solid is less dense than the liquid.

This property has direct ecological consequences. When a lake freezes in winter, ice forms on the surface and insulates the water below. Fish and other aquatic organisms survive in the liquid water under the ice. If ice sank, lakes would freeze from the bottom up, killing most life in them.

Strong Polarity and Hydrogen Bonding

Water is a polar molecule. The oxygen atom pulls electrons toward itself more strongly than the hydrogen atoms do, giving the oxygen end a partial negative charge and each hydrogen end a partial positive charge. The molecule is bent at about 104.5 degrees, so these partial charges do not cancel out.

This polarity makes water an excellent solvent for ionic and polar substances. Salt (sodium chloride) dissolves because the positive sodium ions are attracted to the negative oxygen ends of water molecules, and the negative chloride ions are attracted to the positive hydrogen ends. The same principle applies to sugars, amino acids, and many other biological molecules.

Each water molecule can form up to four hydrogen bonds with neighboring water molecules (two through its hydrogen atoms and two through lone pairs on the oxygen). These bonds are weaker than covalent bonds but strong enough to give water its cohesion, surface tension, and capillary action. Surface tension allows water striders to walk on the surface of a pond. Capillary action pulls water upward through the narrow tubes in plant stems, against gravity. Cohesion (water molecules sticking to each other) combines with adhesion (water molecules sticking to other surfaces) to transport water from roots to leaves.

Because water dissolves so many substances, it acts as the medium for nearly all biochemical reactions. Nutrients, gases, and waste products move through cells and blood dissolved in water. The polarity of water also helps proteins fold correctly and stabilizes the structure of DNA.

Key Takeaways

• Water has a high specific heat capacity because hydrogen bonds absorb heat before temperature rises, which stabilizes climates and body temperatures.
• Ice is less dense than liquid water because the hydrogen-bonded crystal structure is more open, allowing ice to float and insulate aquatic life.
• Water is a polar molecule that forms hydrogen bonds, giving it strong solvent properties, surface tension, and cohesion that are essential for biological transport and chemical reactions.