Classification of Matter

A.    State of Matter

A sample of matter can be a gas, a liquid, or a solid. These three forms of matter are called the states of matter. The states of matter differ in some of their simple observable properties. A gas (also known as vapor) has no fixed volume or shape; rather, it conforms to the volume and shape of its container. A gas can be compressed to occupy a smaller volume, or it can expand to occupy a larger one. A liquid has a distinct volume independent of its container but has no specific shape: It assumes the shape of the portion of the container that it occupies. A solid has both a definite shape and a definite volume: It is rigid. Neither liquids nor solids can be compressed to any appreciable extent.

The properties of the states can be understood on the molecular level (Figure 1.4). In a gas the molecules are far apart and are moving at high speeds, colliding repeatedly with each other and with the walls of the container. In a liquid the molecules are packed more closely together, but still move rapidly, allowing them to slide over each other; thus, liquids pour easily. In a solid the molecules are held tightly together, usually in definite arrangements, in which the molecules can wiggle only slightly in their otherwise fixed positions. Thus, solids have rigid shapes.

B.  Pure Substance

A  pure substance is matter that has distinct properties and a composition that doesn't vary from sample to sample. 

1. Atoms, are the basic units of matter and the defining structure of elements. Atoms are made up of three particles: protons, neutrons and electrons.
2. Elements  is a substance made from only one type of atom. For example, Oxygen is an element made up of ONLY oxygen atoms. To understand this better, let us see the how atoms behave. Every element is made up of atoms. Atoms are the smallest piece that can exist in an element. You will need to put millions of atoms together to get an element of about half millimeter in size. An atom is made up of ‘Electrons, Protons and Neutrons’.  The diagram on your left is an illustration of an atom. The center part is the nucleus. Atoms in some elements do not join up with other atoms of the same element. An example is Helium. Helium atoms exist alone and can look like this: 
   
3. Compounds, A compound is a substance made from two or more elements that have reacted chemically with each other. The elements in the compound can NOT be separated by physical means. Let us see this example below: 
   
   
4. Mixtures, is formed of little bits of one or more substances mixed together. Usually, the parts can be separated from each other by physical means, because it does not involve any chemical reactions or bonds.  A mixture can involve two or more substances of the same phase or different phases. For example you can mix water and sand (liquid and solid), sugar and salt (solid and solid), water and oil (liquid and liquid) or nitrogen and oxygen (gas and gas). Clearly, mixtures can vary a lot and can be homogeneous or heterogeneous.
   ·         Homogeneous mixture: Mixtures involve mixing substances, so let us first be clear what a homogenous substance is. When a sample of matter has the same composition throughout, we call that substance a homogeneous substance. A cup of water will have the same chemical composition throughout . That makes it a homogeneous substance. A piece of gold will also have the same chemical composition, making it a homogenous substance. Homogeneous Mixtures behave in a similar way — the substance formed appear to have the same chemical composition. 
   ·         Heterogeneous Mixture,  A mixture can also result in two or more phases clearly separated by boundaries. Very often, the separation can be clearly seen by the eye. A heterogeneous mixture is one that does not have uniform properties and composition. Take a look at a bowl of cereal with nuts. A spoon full will surely have a different number of nuts than a second spoonful taken at random. Another example—take some sea-sand into your palms. Look at it closely and you will notice that some sand particles are bigger than others, and the colors of some particles may be different too. Heterogeneous mixtures include colloids, emulsions or suspensions.