Lunar and Planetary Institute

Explore! Ice Worlds - All About Water Background

All About Water Background

Water Is Unique

Ice Is One of Three States of Water

Ice is but one state that water can exist in on Earth. As far as our exploration of the solar system has discovered, Earth is the only planet to have all three of these states together:

Solid - Ice - A solid is the state of matter that is rigid; they resist changes to shape or volume. Solids have distinct boundaries. The atoms or molecules that make a solid are fixed in position relative to each other; they (essentially) are not moving.

Liquid - Water - A liquid is the state of matter that is a fluid that changes shape to fill a confining container. Liquids have distinct surfaces at their boundaries. The atoms or molecules making up a fluid move freely around the volume of the liquid.

Gas - Water Vapor or Steam - A gas is the state of matter that does not have a definite shape or volume. A gas is the least dense state of matter. The atoms or molecules making up a gas move around freely — and often energetically! in the space that the gas occupies.

We interact with these states every day. While typically invisible, water vapor is commonly seen as steam. Clouds are concentrations of water droplets or ice crystals. We are familiar with the solid form of water as ice cubes; hail, sleet, and snowflakes falling from the sky; icicles hanging from roofs and trees in winter; glaciers winding down mountain valleys; sea ice covering polar seas; and ice caps and ice sheets covering large stretches of continents at the north and south ends of our globe. Water covers just over 70% of Earth's surface. Most of this is in our oceans as salt water (97%), with glaciers and ice caps holding 2.4% of our water in a frozen state. The remaining 0.6% is the freshwater in our rivers and lakes. Liquid water is the one substance required by all life as we know it.

Ice is a mineral! A mineral is a naturally occurring solid that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. Ice has all of these attributes.

Changes in temperature cause water to change state, and these changes occur at certain key temperatures. Pure water transitions between the solid and liquid states at 32°F (0°C) at sea level. This temperature is referred to as the melting point when rising temperatures are causing ice to melt and change state from a solid to a liquid (water). It is referred to as the freezing point when temperatures decrease, causing water to change state from a liquid to a solid (ice). (For most substances, the melting and freezing points are about the same temperature.) At the boiling point, water transitions from its liquid to gas (vapor) state. Increasing the temperature above the boiling point, 212°F (100°C), causes water to change from liquid to gas (water vapor).

The melting/freezing and boiling points change with pressure. The boiling point of water varies with atmospheric pressure; at lower pressure or higher altitudes, the boiling point is lower. On the top of Mount Everest, where atmospheric pressure is less, pure water boils at about 154 °F (68°C). In the deep oceans, under immense pressure, water remains liquid at temperatures of 750°F (400°C) around hydrothermal vents, where delicate hot-water ecosystems thrive. Pressure also alters the melting/freezing point. Ice at the base of thick glaciers is under such pressure that it melts at temperatures a few degrees Celsius below zero.

A diagram showing the pressures and temperatures at which the different states of water exist.

A diagram showing the pressures and temperatures at which the different states of water exist. As your experience tells you, water occurs in the solid state — ice — at colder temperatures. Water is a liquid at higher temperatures and pressures, and a gas at the highest temperatures. At sea level (1 atmosphere of pressure) water freezes at 32 °F (0 °C), changing from a liquid to a solid. Water boils — changes from a liquid to a gas — at 212 °F (100 °C) at sea level.

Notice that ice can go directly from solid to gas. This process is called sublimation. It is very common in cold regions, such as Antarctica, where it never warms up sufficiently for the ice to melt.

Image courtesy of

Another way to alter the melting/freezing (and boiling) point of water is to add salt. (Actually, any substance that dissolves in water will alter the melting/freezing point.) Pure water and ice, kept insulated from the warm outside world, come to equilibrium over time. On a molecular level, water molecules are freezing onto the ice at the same rate as they are melting off of it. The entire water/ice solution is at the melting/freezing point, 32°F (0°C). Adding salt disrupts this equilibrium. Fewer water molecules are interacting with the ice at any given moment, so the freezing rate is slowed. The salt has no effect on the melting rate, so more melting occurs than freezing melting "wins" and the ice melts. In doing so, heat energy is used to break the hydrogen bonds that hold the molecules in the ice together. In other words, the ice "uses up" some warmth from the solution and the temperature drops. Melting and freezing again match rates ("tie") once the temperature has dropped to the new melting point.

The greater the amount of salt, the lower the freezing point (to a point; once there is sufficient salt that no more will dissolve, the freezing point no longer decreases). Ocean water is about 3.5% salt; sea water freezes at about 28°F (-2°C). A 10% salt solution freezes at about 20°F (-6°C), and a 20% solution freezes at 2°F (-16°C).

This changing of the melting/freezing point is used to clear icy roads. Putting salt on an icy road causes the ice to melt by lowering its freezing point. The salt begins to dissolve when it comes into contact with the ice; this makes salt water with a very high concentration of salt! The salty water continues to interact with the ice, lowering its freezing point and melting it. The temperature would have to drop below the freezing point of pure water (32°F, 0°C) in order to freeze the salty water. Many different salts are used for deicing roads. Sodium chloride (NaCl) is a common road salt table salt is a higher grade sodium chloride. At colder temperatures, salts like magnesium chloride or calcium chloride are often used because they reduce the melting point even further.

Salt water, and its decreased melting/freezing point, is also the secret to homemade ice cream. Rock salt is added to the ice that surrounds a container holding the ice cream ingredients to create a solution colder than pure ice water could. The super-cold brine causes the ice cream ingredients to freeze.

Thanks to a property of ice called the "latent heat of melting," adding more ice to your drink on a hot summer's day will do nothing to lower the temperature. Whenever ice and water exist together, they are either in the process of melting or freezing. The warmth of a summer day goes into melting the ice rather than raising the temperature. Once all the ice has melted, that energy can begin to increase the water's temperature.

Water Molecules Get Along Well with Others

Water molecules are unique in that they get along with so many other molecules and substances. Their shape makes them in the molecular world, at least attractive. A water molecule is similar in outline to Mickey Mouse, with two hydrogen atoms placed on one end of an oxygen atom. The oxygen atoms are positively charged and hydrogen atoms negatively charged, so water molecules stick to each other like tiny magnets. The hydrogen "ears" make loose bonds, called hydrogen bonds, with the oxygen "faces" of other water molecules. This gives water its high surface tension and causes water to "clump" into drops, rather than spread across a surface.

Water molecule

Each molecule of water is made of two hydrogen atoms (white) and one oxygen atom (red). The hydrogen atoms are not placed on opposite sides of the oxygen atom; they occur at one end. A water molecule looks a little like an outline of Mickey Mouse. Image modified from Wikimedia Commons.

Their positively-charged "ears" and negatively-charged "faces" make water molecules appealing to not only to all those Minnie Mouse-shaped water molecules out there, but many types of substances. This is especially true with other polar molecules and substances made of charged atoms or molecules (ionic substances). A water molecule's polarized nature and ability to form hydrogen bonds makes it easy for other substances to get up and personal with it or in other words, dissolve. When a substance is placed in water, the polarized ends of the water molecules work to break the bonds holding the molecules of the substance together. Positively charged atoms (ions) of the substance are pulled toward the negatively charged oxygen atoms of the water molecules surrounding the substance, and negatively charged atoms (ions) are pulled toward the positively charged hydrogen atoms of water molecules. Salts, sugars, acids, alkalis, and gases dissolve in easily in water. While water dissolves a large number of substances, it is not the universal solvent. If it was, you would dissolve when you took a shower an unpleasant prospect!

Ice Crystals Need Their Space

Floating iceberg
Ice is less dense than water, enabling this huge iceberg to float on the ocean. Image courtesy of the National Science Foundation's U.S. Antarctic Program.

Water molecules in the liquid state like hanging out together as droplets, but in the solid (frozen) state, they need their space. Their hydrogen bonds link them together like acrobats, stacked one on top of the other, with their arms and legs outstretched. This provides more space between the molecules in the crystal structure of ice than the molecules of liquid water. The solid form of water - ice - is less dense than the liquid form. Water is the only known non-metallic substance that expands when it freezes; its density decreases and it expands approximately 9% by volume. This decreased density allows the solid form of water to float on the liquid form. Such a minor detail has many far-reaching consequences (beyond the tragedy of the Titanic). If solid ice were denser than water, it would sink, potentially displacing or killing bottom-dwelling aquatic life. In the most extreme conditions, there would be no way to melt the ice in deep lakes and oceans and it would thicken from the bottom to the surface, forming permanently frozen bodies of ice and disrupting Earth's water cycle.

Water Keeps Our Temperatures Constant

Water has a high specific heat capacity. This is the amount of heat energy required to increase the temperature of a certain volume (usually a gram or a kilogram) of a substance by a certain amount, usually one degree Celsius. Water can absorb a lot of heat before its temperature goes up. This property allows the water in our atmosphere and ocean to help regulate or buffer the rate at which our temperatures change. This is why deserts, with little water vapor in the air, get cold very quickly at night and get hot quickly during the day. This is also why areas very close to oceans or large bodies of water have more gradual temperature shifts day to day or season to season. Water even helps regulate the temperatures of living things; all organisms are made of a significant amount of water by weight! For example, while the exact percentage varies, water makes up about 60% of the body weight of humans.

Scientists Look for Ice in Our Solar System

Ice has been discovered across our solar system from Earth to Mars, to the rings of Saturn, to the orbiting comets. Recent discoveries suggest it exists at Mercury's poles! Future exploration of our Moon encompasses the search for water, which would be a valuable resource for future human outposts.

Ice Isn't Just Water

When you think of ice, you probably think of the ice cubes that you put in drinks, or of snow, or even of the ice on which the polar bears and penguins live (but at opposite poles!). In our solar system there are different types of ice. Planetary scientists consider ice to be substances that, although we normally find them as liquids or gases at room temperature on Earth, occur as solids, usually crystalline, in the colder temperatures of our solar system. Water is one of these substances! Ammonia, methane, and carbon dioxide also occur as ices on other planets and moons.


Last updated
October 19, 2009


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