All around us are pure substances combining together to create endless mixtures.
Ordinary table salt is called sodium chloride. It is considered a pure substance because it has a uniform and definite composition, and it is composed of ONLY one types of particle. All samples of sodium chloride are chemically identical. Samples of pure substances can either be elements or compounds, and in either case, the particles that make them up are identical. Water is also a pure substance. When added to water, salt easily dissolves in water, but salt water cannot be classified as a pure substance because its composition can vary. You may dissolve a small amount of salt or a large amount into a given sample of water. A mixture is a physical blend of two or more components, each of which retains its own identity and properties. Only the form of the salt is changed when it is dissolved into water. It retains its composition and chemical properties. This can be proven -- taste the mixture. Will it still have the properties of both pure substances? |
Figure 2 - Dissolved salt (NaCl)
Dissolved salt in water is a physical change, which creates a mixture. The salt has changed form by dissolved (or breaking up into smaller components) but the salt still retains its properties as does the water. |
The confusion with mixtures
Matter can be divided into two categories: pure substances and mixtures. Pure substances can be either elements or compounds. Mixtures can be either homogeneous or heterogeneous. Another term for a homogeneous mixture is a solution. Sometimes it can be difficult to tell the difference between a compound and a homogeneous mixture based solely on appearance. Consider gasoline. It is certainly homogeneous and seems a likely candidate for a compound, meaning that it would have a fixed composition. However, as you may know, gasoline can be purchased at a station in a variety of grades, such as regular or premium grade. Premium grades have a higher octane rating and cost more than regular grades. Gasoline is thus a blend of different compounds, so it is a mixture. The particle diagrams to the right show this idea. In the compound, the particles are touching, which represent them being chemically combined. Each particle is made up of the same two types of particles, and the composition is uniform. In the homogeneous mixture, there are two of the different types of particles, however, they are NOT touching-- this represents physically being mixed and together, but NOT chemically combined. Stop! Answer the questions in your handout and check with your teacher. |
Figure 5 - Particle diagrams
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Figure 6 - Varying particle representations
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Different particle representations
In the particle diagrams you saw before, the elements were made up of only single atoms. This type of elemental sample is called a monatomic element. But are all elements monatomic? See the image to the left. In the middle sample, the description shows that it is a "diatomic element". But how can this be? The particles shown are touching, and are therefore chemically bound. To the untrained eye, these particles look like compounds. However, because the particle is 2 of the SAME element atoms bonded together, they are still only one element. This is called a diatomic element. The elements chemically binding together create a molecule. Anytime two or more atoms are chemically bound, this is called a molecule. You can see, some elements can even be triatomic... and there are many other variations! STOP - answer the questions and ask your teacher to check your answers. |
Writing out elements and mixtures
Chemists use symbols and formulas as an abbreviated way of writing chemical reactions and the elements and compounds involved in them. A chemical symbol is a unique one- or two-letter designation of an element. Some examples of chemical symbols are O for oxygen, Zn for zinc, and Fe for iron. The first letter of a symbol is always capitalized. If the symbol contains two letters, the second letter is lower case. The majority of elements have symbols that are based on their English names. However, some of the elements that have been known since ancient times have maintained symbols that are based on their Latin or Arabic names, as shown to the right. |