Surprising Facts About Water

Water is so familiar that we forget how unusual it is. Its odd properties make life on Earth possible and still give scientists puzzles to solve.

Key takeaways

• Water covers about 71 percent of Earth's surface.
• Ice is less dense than liquid water, so it floats.
• Water has a high capacity to store heat, which moderates climate.
• It dissolves many substances, which is why it carries nutrients and minerals.
• Some water behaviours, such as the Mpemba effect, are still debated.

Earth and body

About 71 percent of Earth's surface is covered by water, mostly in the salty oceans. Only about 3 percent of the world's water is fresh, and most of that is frozen in glaciers and ice sheets or stored underground. The human body is made up of a large fraction of water, often quoted as around 50 to 65 percent in adults, depending on age, sex and body composition.

The water molecule

A water molecule contains two hydrogen atoms bonded to one oxygen atom, H2O. The molecule is bent, and the oxygen end is slightly negative while the hydrogen ends are slightly positive, making it a polar molecule. These slightly charged regions attract neighbouring molecules, forming hydrogen bonds. Many of water's unusual features come from these bonds.

Why ice floats

Most substances become denser when they freeze. Water is unusual: below about 4 degrees Celsius, it begins to expand, and ice has a more open structure, making it about 9 percent less dense than liquid water. This is why ice floats and why lakes freeze from the top down, leaving liquid water below where fish and other organisms can survive the winter.

Heat storage

Water takes a lot of energy to heat up, so oceans warm and cool slowly. This stores heat and helps moderate temperatures, making coastal areas milder than inland regions at the same latitude. Ocean currents carry heat around the globe and shape climate.

The universal solvent

Water is often called the universal solvent because it dissolves more substances than any other common liquid, particularly salts and sugars. This ability lets it carry nutrients in organisms and minerals in rivers. The name is not literally true, since many things, such as oils, do not dissolve well in water.

Surface tension and capillary action

Hydrogen bonds pull molecules at the surface together, creating surface tension strong enough for small insects such as water striders to walk on water. Water also climbs up narrow tubes, called capillary action, which helps move water in plants from roots to leaves, though other forces matter too.

States and boiling

Water exists naturally as a solid, liquid and gas on Earth. At sea level, it boils at 100 degrees Celsius, but at high altitudes, where air pressure is lower, it boils at a lower temperature, which is why cooking can take longer on mountains. Pure water can sometimes remain liquid below zero, in a state called supercooled.

The Mpemba effect

Some people claim that hot water can freeze faster than cold water, which is called the Mpemba effect, named for a Tanzanian student, Erasto Mpemba, who described it in the 1960s. Observations have been reported, but experiments are inconsistent and the effect is debated, so scientists are not sure it occurs reliably or why.

Water on other worlds

Scientists search for water elsewhere because it is linked to life as we know it. There is evidence of water ice on Mars and the Moon, and subsurface oceans are thought to exist on moons such as Europa and Enceladus.

Water in the human body

Water makes up a large share of body weight, and it plays many roles. It is the main component of blood plasma, which carries nutrients, oxygen and waste. It helps regulate temperature through sweating, lubricates joints and is the medium in which the chemical reactions of cells take place. The body loses water through breathing, sweat and urine, and replaces it through drinks and food. Even mild dehydration can affect concentration and mood, while serious dehydration is dangerous.

Hard and soft water

Rainwater is soft, but as water flows through rocks and soil it picks up dissolved minerals such as calcium and magnesium. Water with a lot of these minerals is called hard. Hard water is safe to drink, but it leaves limescale on kettles and pipes and makes soap lather less easily. Soft water has fewer minerals and lathers well. Some households use water softeners or descaling products, and the hardness of tap water varies across regions.

Water scarcity

Although Earth is rich in water, safe drinking water is not available to everyone. According to estimates from the World Health Organization and UNICEF, billions of people still lack safely managed drinking water at home. Scarcity can result from drought, pollution, lack of infrastructure and rising demand from growing cities and agriculture. Solutions include repairing leaking pipes, treating and reusing wastewater, collecting rainwater, improving irrigation efficiency and protecting watersheds.

Hidden water in what we buy

A water footprint measures the water used to produce goods. Much of it is hidden in food and products. Growing crops, raising animals and manufacturing all require water, and the amounts differ widely. Beef and some nuts need a lot of water per kilogram compared with vegetables and grains, although figures vary by region and method. Thinking about water footprints can inform choices, such as wasting less food, which saves the water used to produce it.

Water and climate

Water helps shape climate. Oceans store heat and carry it around the globe in currents, and water vapour is the most abundant greenhouse gas, amplifying warming caused by other gases. Changes in rainfall patterns, snowpack and glaciers affect supplies in many regions, which is why water management is a central part of adapting to climate change.

Frequently asked questions

Is tap water the same as distilled water?

Tap water contains dissolved minerals and sometimes disinfectants. Distilled water has most impurities removed.

Why is the sea salty and not rivers?

Rivers carry small amounts of dissolved minerals to the sea, where evaporation leaves the salts behind.

Does water have a taste?

Pure water is tasteless, but dissolved minerals can give it a flavour.

What is heavy water?

Heavy water is water in which the usual hydrogen atoms are replaced by deuterium, a heavier form of hydrogen. It looks and tastes like ordinary water but is denser, and it is used in some nuclear reactors and in scientific research.

How much of the world's water is drinkable?

Only a small fraction. About 97 percent is salty, and of the remaining fresh water, most is frozen in ice or deep underground. The share that is easily accessible in rivers, lakes and shallow groundwater is well under one percent of the total.

Does water have memory?

No. The idea that water retains an imprint of substances once dissolved in it, used to justify homeopathy, has no support in physics or chemistry. Water molecules constantly rearrange in fractions of a second.

Sources and further reading

• Properties of water (Wikipedia)
• USGS Water Science School