Very good question – and very important in chemistry. In my job, I sometimes make things by accident. I would like to know what they were – what the materials were made up of. I would take a sample a use a variety of machines, and from the results I can understand what the material is made up of:
Spectroscopy: This is a fancy term for what colour is the material. Remember there are some colours we cannot see, we call them infra-red (given of when something is heated) and Ultra Violet (like the rays of the sun).
Mass Spectroscopy – this “weighs” the molecules in the material,
Melting point: What temperature does the material melt at
X-ray Crytalography: If I am lucky to grow a crystal of the material, I could fire X-rays at it, the X-rays bend around the molecule, and using a computer program, the structure and chemical formula of the molecules which make up the material can be found.
Chromatography: If you were to take a strip of paper, and mark a line width ways about 2 cm from the bottom, then dip it in a jar of water about 1 cm deep, the water would rise up, and the ink would separate into different colours. This is because (sometimes) there are many coloured components that make up a dye. – This is chromatography.
I use a more fancy machine to do this, that can separate out mixtures within the material. Where it separates can give an indication of what the material is.
Most of the ways that I use to find out what materials are use light. We shine light on a material and then watch to see what light it reflects back to us or what light passes through it. This type of experiment can tell us a lot about a material! For special cases where we want to find out what the material is made of and how the atoms/molecules are joined together we shine X-rays (another form of light that is high in energy) at the material and measure the angles that the light is scattered away from the sample. Using some clever maths we can use that information to figure out what makes up the material and how the atoms/molecules are joined together.