Physics and Compounds Help (page 2)
Different elements can join together, sharing electrons. When this happens, the result is a chemical compound . One of the most common compounds on Earth is water, the result of two hydrogen atoms joining with an atom of oxygen. There are thousands of different chemical compounds that occur in nature.
Compounds: Not Just A Mixture!
A compound is not the same thing as a mixture of elements. Sometimes, however, when elements are mixed (and, if necessary, given a jolt of energy), compounds result because the elements undergo chemical reactions with each other. If hydrogen and oxygen are mixed, the result is a colorless, odorless gas. A spark will cause the molecules to join together to form water vapor. This reaction will liberate energy in the form of light and heat. Under the right conditions, there will be an explosion because the two elements join eagerly. When atoms of elements join together to form a compound, the resulting particles are molecules . Figure 9-3 is a simplified diagram of a water molecule.
Fig. 9-3 . Simplified diagram of a water molecule.
Compounds often, but not always, appear different from any of the elements that make them up. At room temperature and pressure, both hydrogen and oxygen are gases. But water under the same conditions is a liquid. The heat of the reaction just described, if done in the real world, would result in water vapor initially, and water vapor is a colorless, odorless gas. However, some of this vapor would condense into liquid water if the temperature got low enough for dew to form. Some of it would become solid, forming frost, snow, or ice if the temperature dropped below the freezing point of water.
A note of caution: Do not try an experiment like this! You could be severely burned. In the extreme, if enough of the hydrogen-oxygen air is inhaled, your lungs will be injured to the point where you may die of asphyxiation. We sometimes read or hear news reports about home experimenters who blew themselves up with chemistry sets. Don’t become the subject matter for one of these stories!
Another common example of a compound is rust. This forms when iron joins with oxygen. Iron is a dull gray solid, and oxygen is a gas; however, iron rust is a maroon-red or brownish powder, completely unlike either of the elements from which it is formed. The reaction between iron and oxygen takes place slowly, unlike the rapid combination of hydrogen and oxygen when ignited. The rate of the iron-oxygen reaction can be sped up by the presence of water, as anyone who lives in a humid climate knows.
Compounds Can Be Split Apart
The opposite of the element-combination process can occur with many compounds. Water is an excellent example. When water is electrolyzed , it separates into hydrogen and oxygen gases.
You can conduct the following electrolysis experiment at home. Make two electrodes out of large nails. Wrap some bell wire around each nail near the head. Add a cupful (a half-pint) of ordinary table salt to a bucket full of water, and dissolve the salt thoroughly to make the water into a reasonably good electrical conductor. Connect the two electrodes to opposite poles of a 12-volt (12-V) battery made from two 6-V lantern batteries or eight ordinary dry cells connected in series. (Do not use an automotive battery for this experiment.) Insert the electrodes into the water a few centimeters apart. You will see bubbles rising up from both electrodes. The bubbles coming from the negative electrode are hydrogen gas; the bubbles coming from the positive electrode are oxygen gas (Fig. 9-4). You probably will see a lot more hydrogen bubbles than oxygen bubbles.
Fig. 9-4 . Electrolysis of water, in which the hydrogen and oxygen atoms are split apart from the compound.
Be careful when doing this experiment. Don’t reach into the bucket and grab the electrodes. In fact, you shouldn’t grab the electrodes or the battery terminals at all. The 12 V supplied by two lantern batteries is enough to give you a nasty shock when your hands are wet, and it can even be dangerous.
If you leave the apparatus shown in Fig. 9-4 running for a while, you will begin to notice corrosion on the exposed wire and the electrodes. This will especially take place on the positive electrode, where oxygen is attracted. Remember that you have added table salt to the water; this will attract chlorine ions as well. Both oxygen and chlorine combine readily with the copper in the wire and the iron in the nail. The resulting compounds are solids that will tend to coat the wire and the nail after a period of time. Ultimately, this coating will act as an electrical insulator and reduce the current flowing through the saltwater solution.
Always In Motion
Figure 9-3 shows an example of a molecule of water, consisting of three atoms put together. However, molecules also can form from two or more atoms of a single element. Oxygen tends to occur in pairs most of the time in Earth’s atmosphere. Thus an oxygen molecule is sometimes denoted by the symbol O 2 , where the O represents oxygen, and the subscript 2 indicates that there are two atoms per molecule. The water molecule is symbolized H 2 O because there are two atoms of hydrogen and one atom of oxygen in each molecule. Sometimes oxygen atoms are by themselves; then we denote the molecule simply as O. Sometimes there are three atoms of oxygen grouped together. This is the gas called ozone that has received attention in environmental news. It is written O 3 .
Fig. 9-3 . Simplified diagram of a water molecule.
Molecules are always moving. The speed with which they move depends on the temperature. The higher the temperature, the more rapidly the molecules move around. In a solid, the molecules are interlocked in a sort of rigid pattern, although they vibrate continuously (Fig. 9-5a). In a liquid, they slither and slide around (see Fig. 9-5b). In a gas, they are literally whizzing all over the place, bumping into each other and into solids and liquids adjacent to the gas (see Fig. 9-5c).
Fig. 9-5 . Simplified rendition of molecules in a solid (a), in a liquid (b), in a gas (c). The gas molecules are shown smaller for illustrative purposes only.
Practice problems of these concepts can be found at: Particles of Matter Practice Test
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