Chemistry and Solids Help (page 2)
Introduction to Solids
A brick, a leaf, and a knee cap, what do these three things have in common? Can you guess? They are all solids. They have different functions and a much different make-up, but they are all members of the solid club.
Solids are, well, solid. They are made to last. When archeologists excavate ancient sites, solids are still there, but liquids and gases are long gone. We base 90% of what we know of past civilizations on the solids they left behind like bits of pottery, metal tools, and weapons. But why are some types of matter members of the solid club and not others?
The main difference between solids and other forms of matter is density. The more compact or dense the atoms or molecules of a sample, the more tightly packed and the more solid it is.
Solids are denser than liquids or gases, but there is a lot more to it than that. Solids have a lot of quirks that make the club fairly interesting.
Compared to liquids, the atoms of a solid are a lot more compressed. The atoms that make up a solid are bonded very tightly and have very little room to move around. They are like people on a packed commuter train at rush hour on a Monday morning. They are basically stuck in one spot until some outside force allows them to move more freely. These outside forces are commonly, changes in bonding, crystallization, heat, and pressure.
Depending on their location in the Periodic Table, different elements can have very different densities. For example, the density of balsa wood is approximately 0.13 g/cm 3 and the density of water (20°C) is 0.998 g/cm 3 . It makes sense then that porous balsa wood floats easily in water since its density is so much less. The density of copper is 8.96 g/cm 3 , compared to the density of mercury at 13.55 g/cm 3 . If a copper penny is dropped into a container of mercury, would it sink or float? Well, look at the densities. Mercury is much denser, so the copper penny would float. If the same penny were dropped into a beaker of water, what would happen then?
Solids are normally found in one of two types, amorphous and crystalline. The first group, amorphous, is made up of shifting members that really can’t make up their minds if they like being a solid or not. These are things like wax, rubber, glass, and polyethylene plastic.
Amorphous solids have no specific form or standard internal structure.
Brittle, non-crystalline solids tend to shatter every which way into sharp pieces when broken. These amorphous solids are less dense than other crystalline club members and have no definite melting point. When heated, they slowly soften and become very flexible. Have you ever heated glass tubing in a flame to make a 90° angle in a straight rod? This is an example of the shape changing of non-crystalline solids.
Most people are a lot more familiar with crystalline solids. These are solids like quartz, diamond, salt, and different gemstones. The atoms of crystalline solids go together into specific crystal patterns of an ordered lattice or framework.
Crystalline solids are arranged into regular shapes based on a cube; simple, central, and face-centered.
Figure 15.1 shows the three ways the atoms of a crystalline solid can be arranged. As a molecule goes from a simple cubic structure to a face-centered cubic structure, the density increases. The less space between the atoms, the more tightly packed the entire molecule, and the harder and less flexible. Unlike amorphous solids, a lattice structure provides for predictable breaks along set lines. This is the reason why diamonds and gemstones can be cut into facets. The round, oval, pear, emerald cut, and diamond-shaped cuts used in jewelry can be cut by different gem cutters all over the world due to their characteristic lattice structures.
Solids have regular parts of their personality that can be used to classify them. These can be found by their placement in the Periodic Table as well. The metals found in the middle of the Periodic Table share a lot of the same characteristics, like brothers and sisters in the same family that have the same hair or eye color. Some of these characteristics are hardness and structure (crystalline or amorphous), melting point, and electrical conductivity. Table 15.1 lists these personality traits of solids.
Cannot be compressed
Held together by molecular, ionic, metallic, or covalent bonds
Do not expand when heated
Amorphous or crystalline in form
One of the major players in the solid club is temperature. Temperature controls a lot of who gets along with whom and whether they play nice. Temperature also decides which form club members take to be a member.
Look at water, normally a member of the liquid club, but exposed to very low temperatures and pow! Ice. Not only a solid, but see through as well. Thanks to this special gift of H 2 O, we enjoy popsicles, ice hockey, and snow balls.
What happens when we raise the temperature a few degrees? No more ice. It’s back to the liquid club. Without ice, we have skiing on rocks (which is very hard on skis), no polar ice caps, and soupy chocolate chip ice cream. In other words, the end to life as we know it on this planet!
Generally, every element can be a solid if the temperature is dropped low enough, even gases. This helps us to understand the conditions on planets like Jupiter and Saturn in our solar system. They are mostly made up of frozen gases.
Practice problems for these concepts can be found at – Solids Practice Test
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