Water is the most common substance on Earth, covering almost three quarters of the planet’s surface. Known by its chemical symbol, H2O, water is the only known substance on Earth that naturally exists as a gas, liquid, and solid. The vast majority of water, about 97%, is in the oceans.
The liquid form of water also exists in lakes, rivers, streams, and groundwater (water beneath Earth’s surface that is held between soil particles and rock, often supplying wells and springs). In its solid form, water makes up sheets of ice on the North and South Poles, and permanent snow.
Water also exists as water vapor (gas) in the atmosphere. The hydrosphere is the whole body of water that exists on or around Earth, which includes all the bodies of water, ice, and water vapor in the atmosphere. All life needs water to survive and the cells of all living things contain water.
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Chemistry is the science of the composition, structure, and properties of all substances, called matter, that have mass and occupy physical space. On Earth, the unique chemistry of water determines, in large part, not only the chemistry of the hydrosphere, but also the chemistry of the solid Earth (geochemistry), the atmosphere (atmospheric chemistry), and the living Earth (biochemistry).
The water molecule
Water is made up of the elements hydrogen and oxygen. An element is a substance that cannot be divided by ordinary chemical means. Hydrogen, oxygen, nitrogen, silicon and iron are all common elements on Earth. Atoms are the building blocks of elements and all matter. An atom is the smallest particle that has the characteristics of an element. Water is composed of groups of atoms called molecules.
A group of atoms arranged in a particular way makes up a molecule, which is the smallest unit of a substance that has the properties of that substance. Atoms and small molecules like water are so small that they cannot be seen with even the most powerful microscopes. Much of what is known about water molecules has been inferred from indirect observations and chemical experiments.
A water molecule is a group of three atoms arranged in a shape similar to Mickey Mouse’s head; Mickey’s face is a larger oxygen atom (symbolized by the letter O) and his ears are two smaller hydrogen atoms (symbolized by H2).
Strong chemical bonds, called covalent bonds, hold the hydrogen and oxygen atoms together. To form covalent bonds, atoms share subatomic particles (particles smaller than atoms) called electrons, which have a negative charge. Atoms also have subatomic particles that have a positive charge, called protons.
In a water molecule, H2O, more of the shared electrons collect around the oxygen atom than around the hydrogen atoms. This gives the oxygen end of the molecule a negative electrical charge and the hydrogen ends a positive electrical charge. This property of the water molecule, called dipolarity, gives water many of its chemical and physical characteristics.
Chemical properties of water
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In chemistry, positive and negative electrical charges attract each other, like charges (two positive charges) repel each other. The negative ends of dipolar water molecules are attracted to molecules and atoms with positive charges, and vice versa.
The molecules within a raindrop, storm cloud, and ice cube are arranged with positive and negative poles (opposite sides of the atom) attached to one another. The positive charge near the hydrogen atoms and the negative charge near the oxygen results in the formation of a hydrogen bond.
The many hydrogen bonds between the liquid water molecules cause these molecules to stick together. Water molecules at the surface form even stronger hydrogen bonds with their neighboring molecules, causing the formation of a surface film (layer). This phenomenon is called surface tension. Water’s high surface tension makes it more difficult for solid objects to penetrate the water surface than for submerged objects to move through water.
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Certain water bugs can walk on the film due to water’s surface tension. Water forms bubbles and drops because surface tension pulls the shape of unconfined liquids into a ball. (Without deformation by forces like gravity, all raindrops and bubbles would be perfectly sphere-shaped.) A person washes cleaner in a hot bath than a cold one because hot water has lower surface tension than cold, making it better able to get into openings. Soaps and detergents also lower surface tension.
Surface tension is also partly responsible for capillary action (water’s ability to rise in a small narrow tube called a capillary). Water molecules stick, or adhere, to the sides of the capillary, and surface tension forms a curved bridge, called a meniscus, across the opening.
Adhesion at the capillary walls creates an upward force and cohesion holds the water surface together. The whole meniscus moves up or through the capillary. Water moves up through plant roots to leaves by capillary action. Capillaries carry blood through the human body. (Human blood is about 83% water.)
Surface tension and other properties of the water molecule allow nutrients to enter and wastes to leave plant and animal cells. Surface tension aids in the exchange of oxygen and carbon dioxide in the human lungs. Groundwater moves through soil and rock openings by capillary action.
Water, the universal solvent
Water is called the universal solvent because many solid substances dissolve easily into water. Water molecules form hydrogen bonds with electrically charged atoms called ions and dipolar molecules other than water molecules. Table salt, for example, is composed of a positive ion, sodium (Na+), and a negative ion, chlorine (Cl–).
In a salt molecule, the sodium and chlorine ions bond to one another. When table salt is dropped in water, the positive ends of the water molecules surround the chlorine and the negative ends surround the sodium. The salt molecule disappears, but its ions are still in the water.
A liquid that contains dissolved ions is called a solution. When conditions in the solution change in some instances, the dissolved ions bond to one another and turn back into a solid, a process called precipitation. When the water in salt water evaporates, salt molecules reform.
Water on Earth is an ever-changing solution that dissolves and precipitates substances as it flows. Pure water has no smell, taste, or color. Most water on Earth, however, contains many dissolved materials. Seawater, for example, is a complex solution that contains traces of almost every naturally occurring element. Falling rainwater contains dissolved carbon dioxide.
“Hard” water (water that contains minerals) that forms scale (crusty deposits) on the hot water heater and makes it hard to lather up for a shower contains dissolved magnesium and calcium. Water that stains a porcelain sink red contains dissolved iron. Water that smells like rotten eggs contains sulfur. Many cities and neighborhood water districts add fluoride to the tap water because it prevents tooth decay.
Some dissolved materials such as lead, mercury, arsenic, petroleum, and pesticides (chemicals used to kill insects, rodents, and other pests) are hazardous to human and animal health, and can be absorbed by food crops that are irrigated with contaminated water. The United States has set drinking water standards to prevent harmful chemicals from entering the drinking water supply.