Exploring Our Fluid Earth

Water is Everywhere

People use water on a daily basis to hydrate, to clean themselves and their clothes, and to cook food. Because people use water so frequently, it is easy to forget how important and unique it is. Water is so common, it makes an excellent case study for learning about the properties of matter.

Definition of Water

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Fig. 1.4.  A diagram of a water molecule. The circles represent atoms of elements.

Image by Byron Inouye

Water is a compound composed of atoms of two elements, hydrogen and oxygen, bonded together. A molecule of water is composed of two hydrogen atoms (H) and one oxygen atom (O). This relationship is expressed in the chemical formula for water, H₂O (Fig. 1.4). A chemical formula shows the number of atoms of each element that combine to make a molecule of that compound. By itself, water is considered a pure substance. Seawater, however, is a mixture of many substances, the most abundant of which is water.

States of Matter

There are four fundamental states of matter: solid, liquid, gas, and plasma. On earth, solid, liquid, and gas are the most common states of matter. Not only is water the most common substance on earth, but it is also the only substance that commonly appears as a solid, a liquid, and a gas within the normal range of earth’s temperatures. This makes water a good model for discussing the solid, liquid, and gas states of matter.

On earth water exists in three states:

• As a liquid, water flows and takes the shape of the ocean, lakes, and river basins. In a liquid, the molecules of water are relatively close together and are free to move around each other (Fig. 1.5 A).

• As a solid, water takes a definite shape, as in a snowflake, ice cube, or glacier. In a solid, the molecules of water are relatively close together, but are held in a distinct shape. Water is a unique substance because its molecules a

• re actually farther apart in the solid state than in the liquid state. In most other substances, the molecules of the solid are closer together than the molecules of the liquid (Fig. 1.5 B).

• As a gas, water vapor is free to move great distances through the atmosphere. In a gas, the molecules of water are relatively far apart and are free to move at high speeds. It is difficult to capture a photograph of water in a gaseous form. Many people think steam or clouds are water vapor, but they are actually tiny droplets of liquid water, not gaseous water.

Fig. 1.5 (A) Water in a liquid state.

Image by Alyssa Gundersen

Fig. 1.5 (B) Water in a solid state.

Image by Alyssa Gundersen

Plasma is a gas that is electrically charged. Plasma is very common in the universe, making up the stars and the space between the planets in our solar system. Plasma can also be found on earth in thing like fluorescent lights and lightening. However, plasma is not a state of matter that is common or persistent in the natural world on earth.

Physical and Chemical Changes

Changes between states of matter can be physical changes or chemical changes. In a physical change, the chemical formula of the substance remains the same, even though the physical properties may change. Physical properties include taste, smell, texture, and color. Physical changes can generally be reversed. Ice can melt to become a liquid, which can re-freeze back to a solid state. In each of the states, the chemical formula of water, H₂O, remains the same. Other examples of physical change are filtering, cutting, melting, and coloring the surface of something with paint, crayons, or markers. Examples of physical change are shown in Figs. 1.6 A and B.

Fig. 1.6. (B) Removing the tomatoes from a salad is an example of a physical change.

Image by Kanesa Seraphin

Fig. 1.6.  (A) Coloring is an example of a physical change.

Image by Alyssa Gundersen

In a chemical change, the end product is chemically different than the starting substance and the chemical formula changes. When a sugar cane plant biologically converts carbon dioxide (CO₂), water (H₂O), and oxygen gas (O₂) into glucose (C₆H₁₂O₆), this is a chemical change. Because chemical changes produce new substances, they cannot be easily reversed. Glucose can only be broken back down into its components by another chemical change, such as burning. Other common types of chemical change are cooking, rusting, and ripening. Examples of chemical change are shown in Figs. 1.6 C and D.

Fig. 1.6. (D) Rusting is an example of a chemical change.

Image by Alyssa Gundersen

Fig. 1.6. (C) Sugar cane making sugar is an example of a chemical change.

Image Courtesy of Mariordo

Chemical change is often accompanied by a physical change, such as a change in color. This can make it difficult to distinguish between a physical change that is purely physical and a physical change that is due to a chemical change. In a purely physical change, the substances after the change are the same substances they were before the change. When coloring a piece of paper with a crayon, crayon has been put on top of the paper, but the substances are still crayon wax and paper. The substances can be separated by scratching the wax off of the paper. In a chemical change, such as rusting, the substances after the change are chemically different than the substances that were there before the change. When a piece of metal rusts, its color changes, because iron is chemically combining with oxygen from the air to form a new substance, rust. There is no physical way to separate the rust back into iron and oxygen gas.

The concepts of physical and chemical change can be used to understand the differences between mixtures and compounds. In a mixture, two or more substances, elements, or compounds are combined physically. The properties of the mixture usually reflect the properties of the individual substances. For example, if you mix salt with water, you get salty water. Many times, a mixture can be separated by a physical change into its individual substances. For example, a mixture of tomatoes, lettuce, croutons, carrots, and raisins in a salad can be separated into its individual components.

Fig. 1.7. (A) glucose

Image adapted from Wikipedia, courtesy of Umberto Salvagnin

Fig. 1.7. (B) coal

Image adapted from Wikipedia, courtesy of Nostrifikator

Fig. 1.7. (C) graphite

Image by Jordan Wang

Fig. 1.7. (D) diamond

Image by Alyssa Gundersen

In a compound, two or more elements are joined chemically. The properties of the compound are often very different than the properties of the individual elements. For example, the sugar in cereal is likely to be glucose, C₆H₁₂O₆. Glucose is a sweet, white crystal (Fig. 1.7 A), very different than carbon, hydrogen, or oxygen, the elements that make it up. Carbon (C) exists naturally as either a brittle black solid (coal), a soft gray solid (graphite), or a hard clear solid (diamond), shown in Figs. 1.7 B, C, and D. Hydrogen (H₂) and oxygen (O₂) both exist naturally as colorless, odorless gases. It is not possible to separate a compound by physical methods. Compounds can only be broken down into their constituent elements by a process of chemical change.

Table 1.3 describes how the properties of the elements that compose the compounds copper sulfate, sodium chloride, and sucrose are different from the resulting compounds and mixtures. You will complete the Matter Category column in the Properties of Matter Question Set.