All About Water and Ice Background
Ice Is Important on Earth
In our everyday experiences, we encounter water typically in its liquid state. Most of our fresh water, however, exists in its frozen form. About three-quarters of it is found in snow, sea ice, icebergs, ice shelves, glaciers, ice sheets, and soils that remain frozen for two or more years (permafrost). Snow and ice may appear only seasonally at mid-latitudes, but at high altitudes and in the polar regions, frozen water persists year-round as glaciers and ice sheets. Glaciers form in regions where more snow accumulates than melts, such as in high mountain valleys. In the extremely cold polar regions, the glaciers grow to form continent-sized ice sheets. The largest ice sheets cover Antarctica, and smaller ice sheets cover Greenland and part of Iceland. Some of the ice in the Antarctic ice sheet represents the build-up of nearly a million years of snow.
Ice Is One of Three States of Water
Ice is just one state that water can exist in on Earth.
Solid - Ice - A solid is the rigid state of matter. Solids resist changes to shape or volume, and 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 a fluid state of matter 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 a 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. 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.
Changes in temperature cause water to change state. 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.
- At sea level, pure water boils at 212 °F (100°C).
- At the lower atmospheric pressure on the top of Mount Everest, 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.
Pressure also alters the melting/freezing point. The incredible pressure at the base of thick glaciers melts the ice at temperatures a few degrees Celsius below zero.
Another way to alter the melting/freezing (and boiling) point of water is to add salt (or any substance that will dissolve in water). The greater the amount of dissolved salt, the lower the freezing point. 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 temperature would have to drop even further in order to freeze the salty water. Many different salts are used for de-icing roads. 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.
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 has 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 atoms make loose bonds, called hydrogen bonds, with the oxygen atoms of other water molecules. This causes water to "clump" into drops, rather than spread across a surface.
Each molecule of water is made of two hydrogen atoms (white) and one oxygen atom (red). Image modified from Wikimedia Commons.
Their positively-charged "ears" and negatively-charged "faces" make water molecules appealing to many types of 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. Salts, sugars, acids, alkalis, and gases dissolve in easily in water.
Ice Crystals Need Their Space
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 creates more space between the molecules. 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.
This decreased density allows ice to float on water, a critical property for our lives on Earth. If solid ice were denser than water, it would sink. In the most extreme conditions, there would be no way to melt the ice in deep lakes and oceans, which would thicken from the bottom to the surface, forming permanently frozen bodies of ice and disrupting Earth's water cycle.
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 Keeps Our Temperatures Constant
Water has a high specific heat capacity—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 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 have more gradual temperature shifts.
Ice Isn't Just Water
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 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. Read more about Ice in the Solar System.