States of Matter
Solids (s), liquids (l) and gases (g) can be represented using particle diagrams. In these diagrams, particles are represented by solid spheres.
Particles in solids have less energy than liquids, which in turn have less energy than gases. To get from a solid to a liquid, or a gas, energy has to be supplied - usually through heating. During these changes the particles gain energy, which is used to break or overcome the intermolecular forces of attraction.
The amount of energy required to change state depends on the strength of the intermolecular forces between the particles of each substance. Some of the forces need to be broken during melting, whereas all of the forces must be broken during evaporating/boiling. Changing from one state to another is a physical change.
Solids have a fixed volume and shape, and cannot flow as the particles are unable to move. They cannot be compressed as the particles are already too close together - with no space to be able to move into.
Liquids have a fixed volume, but no fixed shape, and can flow as the particles are able to move. They are able to take the shape of their container, however are unable to be compressed as their particles are already too close together, with no space to move into.
Gases do not have fixed volume, or fixed shape, and can flow as the particles are able to move. They are able to take the shape of their container, and can be compressed as their particles are far apart and have space to move into.
A pure substance is either a single element, or compound, that is not mixed with any other substance.
A mixture consists of two or more elements/compounds, that are not chemically combined together and can be separated by physical processes.
Impure substances (mixtures) will melt over a range of temperatures. If a substance is impure it will generally also have a lower melting point and a higher boiling point.
Pure substances will melt and boil at specific temperatures, so we can use the temperature that a substance melts/boils at to tell how pure it is.
Filtration is a method for separating an insoluble solid from a liquid. A mixture of liquid and solid is passed through filter paper into a flask below. The solid stays on the filter paper (residue), and the liquid passes through to the container below (filtrate).
Crystallisation/evaporation is a method that separates a soluble solid from a liquid. A solution of liquid and dissolved solid is heated, causing the solvent to evaporate and leaves solid crystals behind.
Simple distillation is a method for separating the solvent from a solution, leaving behind the solute. This method works because the solvent has a much lower boiling point than the dissolved solute. Heating the solution allows the solvent to evaporate, and then it passes through a condenser, where it is cooled and condensed into a separate container. The solute does not evaporate and so it stays behind.
Fractional distillation is a method of separating multiple liquids from each other. The technique works in the same way as distillation, but is used to separate mixtures of liquids (and normally on a much larger scale).
Chromatography is a method for separating dissolved substances from one another. It works because some of the coloured substances dissolve in the solvent better than others, so they travel further up the paper. This technique is often used to separate out inks, or food colourings.
Paper chromatography consists of two stages:
the stationary phase (a.k.a. the phase that doesn't move - this is the paper)
the mobile phase (a.k.a. the phase that moves - this is the solvent)
Separation by chromatography produces a chromatogram, and can be used to distinguish between a pure substance (that produces one spot), and a mixture (impure) which produces multiple spots. Different substances will move at different rates through the paper.
When setting up the experiment you must make sure to draw the baseline in pencil, otherwise the ink from a pen would also dissolve in the solvent. Also ensure that the baseline is above the solvent line (to begin with) - otherwise the inks will just dissolve into the solvent below.
The Rf value of a spot can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value will always be the same for each substance (when the same solvent is used). It can be calculated by using:
Rf value = distance moved by substance ÷ distance moved by solvent
Core Practical 1: Investigating Inks
This core practical is in two parts; a simple chromatography practical to obtain a chromatogram of dyes in ink and using simple distillation apparatus to separate pure water from ink.
It needs to cover usage of a Bunsen burner, methods used in chromatography and distillation, and safety of handling liquids.
add about 20 ml of ink to a conical flask
connect the flask to a delivery tube and set up the apparatus as shown above
heat the flask using a Bunsen burner until boiling, then reduce the heat for a gentle boil
collect a small sample of the distilled solvent, then turn the Bunsen Burner off
draw a pencil line across chromatography paper (about 1 cm above the bottom)
use a pipette, or capillary tube, to add small spots of each ink to the line on the paper (equally spread across the paper)
place the paper into a container with a suitable solvent (e.g. water) in the bottom - making sure the pencil line is above the water line
allow the solvent to move through the paper, but remove the chromatogram before it reaches the top
allow the chromatogram to dry, then measure the distance travelled by each spot and by the solvent to calculate Rf
All life requires water to survive. The water they need has to be of good enough quality, which for humans means it needs low levels of dissolved salts and microbes. Water that is safe to drink is called potable water. Potable water is not pure water, because it contains dissolved substances.
There are many methods used to produce potable water, and which method is used will depend on the available supplies of water and local conditions.
In the United Kingdom (UK), rain provides water with low levels of dissolved substances (fresh water) that collects in the ground and in lakes and rivers. To make this water potable, it must go through these steps:
sedimentation - solids sink to the bottom and are removed
filtration - fine particles like sand are removed
chlorination - microbes are killed using chlorine
If supplies of fresh water are limited (such as on an small island), desalination of sea water (high salt levels) may be required. Desalination can be done by distillation. This process requires large amounts of energy and is costly to maintain.
When we do analysis in the lab, we normally use distilled (often called deionised) water. This is because tap water contains dissolved salts that may interfere in our readings, so we must use pure water as these salts have been removed.