Matter and Mixtures

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Matter

For chemists, the most elemental building block of matter is the atom. While it is certainly true that the atom can be divided into even more elemental building blocks, it is at the level of the atom that the first distinctive "chemical" properties start appearing. There are many different types of atoms, as you can see in the periodic table of the elements, each with their own distinctive chemical properties. From these atoms molecules can be assembled. Molecules are groups of atoms held together by forces called chemical bonds.

States of Matter

Chemists classify matter into three different states:

  • SOLID - has a definite volume, a definite shape, and is rigid.
  • LIQUID - has a definite volume but no definite shape.
  • GAS - has no distinct volume, no distinct shape and can be easily compressed to occupy a smaller volume.
Demo:
  1. Cl2(g), Br2(l), I2(s)
  2. Dip Br2(l) into liquid nitrogen and make Br2(s)

Mixtures

A substance containing only one type of atom or one type of molecule is a pure substance. Most of the matter around us, however, consists of mixtures of pure substances. Air, wood, rocks and dirt are examples of such mixtures. Mixtures can be classified as Homogeneous and Heterogeneous.

Homogeneous Mixtures

Homogeneous Mixtures are uniformly mixed on an atomic or molecular level. These types of mixtures are also called solutions. Below are a few examples of homogeneous mixtures.

Air is a homogeneous mixture (gaseous solution) of N2, O2, H2O, and CO2 gases. In contrast, a container of each gas by itself would be a pure substance. Only when they are mixed on an molecular level are they a homogeneous mixture (or gaseous solution).

Brass is a homogeneous mixture (solid solution) of copper and zinc. Again each metal by itself is a pure substance. Only when they are mixed on an atomic level are they a homogeneous mixture (or solid solution).

Beer is a homogeneous mixture (liquid solution) of H2O, C2H5OH, and a few other substances. (Sorry, there is no beer molecule!)

Demo:
  1. Mix water and ethanol to make homogeneous solution - also show molar volume concept.

Heterogeneous Mixtures

Heterogeneous mixtures are not uniformly mixed on an atomic or molecular level. For example,

Salt and pepper, chocolate chip cookies, or a Twix™ candy bar, ...

are all examples of heterogeneous mixtures, where substances are not mixed on a molecular level.

Demo:
  1. Separate Iron Filings and Sulfur with a Magnet.
  2. Separate Grape Soda into Orange and Blue Solutions using Chromatography.

All mixtures, heterogeneous and homogeneous, can be separated into pure substances using physical methods.

  • Mixtures, Pure Substances:

Physical Change

Any change of matter that does not change the type of atoms and molecules within the matter is called a physical change. Water boiling is an example of a physical change. When water boils it is changing from a liquid state to a gaseous state. Chemists would represent this process as follows:

H2O(l) H2O(g)

Here (l) stands for liquid and the (g) stands for gas. As this is a physical change, the H2O molecule does not change.

Demo:
  1. CO2(s) CO2(g)
  2. H2O2(l) H2O2(g)

Chemical Change

Molecules, on the other hand, can split or combine together to make other types of molecules. The process where a molecule is transformed into a different molecule is called a chemical change.
Demo:
  1. C2H5OH + 2 O2(g) 2 CO2(g) + 3 H2O2(g)

To help us understand the concept of chemical change let's examine why light bulbs are made the way they are. A lightbulb works by passing electrical current through a tungsten wire inside the bulb. The tungsten wire is sealed inside a glass bulb since if you did this in air the light bulb would burn out very quickly. This is because the tungsten wire undergoes a chemical reaction with the O2 gas in the air to form tungsten oxide.

2W + 3O2 2WO3

To prevent this reaction from happening, all oxygen is removed from the air sealed inside the bulb.

Chemical Change
If oxygen leaks into the bulb, then the tungsten wire reacts to become tungsten oxide and the oxidized wire will no longer pass electrical current readily. If you try to pass electrical current through the oxidized wire it will quickly heat up and break. That is, the lightbulb burns out.
Demo:
  1. Lightbulb with hole drilled in bulb.

Homework from Chemisty, The Central Science, 10th Ed.

1.1, 1.2, 1.9, 1.19, 1.21