chemistry

• Chemical symbols

  • Used so that scientists from many nations can communicate with each other, regardless of the language
  • Symbols in the periodic table represent atoms
• Periodic table

  • Elements arranged in columns called groups
  • "Staircase" separates metals and non-metals
• Atoms cannot be created or destroyed
• Total mass of products formed in a reaction equal to total mass of reactants (conservation of mass)
• Compounds

  • Have a fixed composition (ratio of elements is always the same in any particular compound)
  • Chemical reactions must be done to separate the elements in a compound
  • Chemical bonds between the atoms of different elements in the compound
• Mixtures

  • Have no fixed composition
  • Different elements and compounds can be separated more easily
  • No chemical bonds between atoms of the different substances in a mixture
  • Can be separated through filtration, crystallisation, distillation or chromatography
  • This depends on different solubilities and boiling points of the substances
• Filtration

  Used to separate substances that are insoluble in a particular solvent from those that are soluble in the solvent e.g. separating sand/salt from water
• Crystallisation

  1. To obtain pure salt (NaCl) from the salt solution following filtration, you would need to separate the sodium chloride in the solution (filtrate) from the water

  2. This can be done by evaporating the water from the sodium chloride solution

  3. The best way to do this is by heating it in an evaporating dish or a water bath (is a gentler way of heating than heating the evaporating dish directly on a tripod and gauze)

  4. Heating should be stopped when the solution is on the point of crystallisation- this is when small crystals first appear on the edge of a solution or when crystals appear in a drop of a solution extracted from the dish with a glass rod

  5. The rest of the water is then left to evaporate off the saturated solution at room temperature to get a good sample of sodium chloride crystals

  6. A flat-bottomed crystallisation dish or Petri can be used for this final step, giving a large surface area for the water to evaporate from
• Fractional distillation

  1. Distillation can also be used to separate miscible liquids (liquids that dissolve in each other)

  2. Miscible liquids have different boiling points so it is distilled and the liquid with the lowest boiling point is collected first

  3. However it is difficult to get pure liquids from mixtures of liquids with similar boiling points by simple distillation, as vapour is given off from each liquid before they can actually reach their boiling point

  4. To aid separation a fractional column can be added to the top of the apparatus- usually a tall glass column filled with glass beads

  5. The vapours must pass over and between the glass beads in the fractional column before they reach the condenser

  6. Temperature in the fractional column is highest at the bottom of the column and gets lower as the vapours rise up

  7. Substance with the higher boiling point will condense more readily on the glass beads and drip into the flask underneath
• Paper chromatography

  1. Used to mixtures of miscible liquids (substances that dissolve within each other)

  2. Capillary tube is used to dab a spot of solution on a pencil line near the bottom of a sheet of chromatography paper

  3. Paper is then placed standing in a solvent at the bottom of a beaker/tank

  4. Solvent is allowed to soak up the paper, running through the spot of the paper

  5. Relative solubility of the components making up the mixture in the solvent determines how far they travel up the paper

  6. The more soluble a substance the further it travels up the paper
• Dalton in early 1800s suggested that substances were made of atoms like tiny, hard spheres
• Dalton also suggested that each chemical element had its own atoms that differed from others in their mass
• Dalton suggested atoms rearranged themselves and combined with other atoms in other ways
• Evidence for electrons in atoms

  • At the end of the 1800s JJ Thompson discovered the electron (tiny, negatively charged particle)
  • Found by experimenting by applying high voltages to gases at low pressure
  • Had experiments done on the beams of particles- were attracted to a positive charge showing they must be negatively charged
  • Thompson suggested that negatively charged electrons must have been embedded in a cloud of positive charge- knew that atoms must balance each other out
• Evidence for nucleus

  • Geiger and Marsden doing an experiment with radioactive particles- fired dense, positively charged particles (alpha particles) at a thin piece of gold foil
  • Expected particles to pass straight through the gold atoms but were shocked when some deflated
  • Rutherford suggested that Thompson's atomic model was not possible as the positive charge must be concentrated at a tiny spot in the centre of the atom
  • Proposed then that the electrons must be orbiting around the nucleus which contained dense positively charged protons
• Evidence for electrons in shells

  • Niehls Bohr revisited the atomic model again and noticed that the light given out when atoms were heated had specific amounts of energy
  • Suggested that the electrons must be orbiting the nucleus at set distances in certain fixed energy levels (or shells)
  • Energy must be given out when excited electrons fall from a high to low energy level
• Evidence for neutrons

  • Scientists at the time suspected that there were two types of subatomic particles inside the nucleus to explain the missing mass that had been noticed inside atoms
  • The neutron was then proposed- a type of particle that has no charge and is the same mass as a proton
  • Proposed by Chadwick in 1932
• Electrons so light their mass can be ignored when finding out relative mass of an atom
• Atomic number


  Number of protons in each atom
• Mass number


  Number of protons + number of neutrons in the nucleus of an atom
• Ions

  If an atom gains one or more electrons it gains an overall negative charge and is called a negative ion, as there are more electrons than protons
• Isotopes

  • Atoms of the same element with different numbers of neutrons
  • Isotopes always have the same atomic number but different mass numbers
  • Sometimes extra neutrons makes the atom unstable so it is radioactive but this is not always the case- isotopes are sometimes simply just atoms of the same element of different masses
  • Samples of different isotopes may have different physical properties e.g. they will always be a different density and may/may not be radioactive
  • However they will always have the same chemical properties because their reactions depend on their electronic structures- as their atoms will have the same amount of protons and electrons the electronic structure will be the same for all isotopes of the same element
• Electronic structures


  • An energy level (or shell) can only hold a certain amount of electrons
  • The first and lowest energy level (nearest the nucleus) can hold two electrons
  • Second energy level can hold eight electrons
  • Once there are eight electrons in the third energy level the fourth begins to fill up
• Periodic table and electronic structure

  • All the elements in a group on the periodic table will have the same amount of electrons in its outermost shell
  • The way an element reacts is determined by the number of electrons in its highest energy level- as a result of this, all elements in a group act in a similar way
• Chromatography

  • Chromatography always involves a mobile (liquid or gas) and a stationary phase- mobile phase moves through the stationary phase, carrying components of the mixture under investigation with it
  • Each component in the mixture will have a different attraction for the mobile phase and the stationary phase- a substance with stronger forces of attraction between itself and the mobile phase than between itself will carry a greater distance in a given time
  • This means that in paper chromatography the mobile phase is the solvent chosen and the stationary phase can be thought of as the paper
  • Given an unknown organic solution chromatography can tell you if it is a single compound or a mixture
  • If the unknown sample is a mixture of compounds there will most likely be more than one spot formed on the chromatgram
  • A single spot suggests that there is only a pure substance
• Identifying unknown substances using chromatography

  1. Once the compounds have been separated using chromatography they can be identified

  2. You can compare spots on the chromatogram with others obtained from known substances
• Rf (retention factor)


  • A ratio calculated by dividing the distance a spot travels up the paper (measured to the centre of the spot) by the distance the solvent font travels
  • Given that the number generated in the calculation is a ratio, it does not matter how long you run your chromatography experiment or what quantities you use
  • For comparisons of the Rf values to be valid, you must ensure that the solvent and the temperature used are the same as those quoted in the database
• Finding Rf values

  Using a capillary tube, pencil, pipette, water, boiling tube and a narrow strip of chromatography paper find the Rf values of different dyes
• Chromatography

  • Chromatography is a process for separating components of a mixture. To get the process started, the mixture is dissolved in a substance called the mobile phase, which carries it through a second substance called the stationary phase.
  • The different components of the mixture travel through the stationary phase at different speeds, causing them to separate from one another. The nature of the specific mobile and stationary phases determines which substances travel more quickly or slowly, and is how they are separated. These different travel times are termed retention time.
• Dalton arranged the elements in order of their atomic weights
• Mendeleev's periodic table


  • Mendeleev arranged the elements in order of their atomic weights so that a periodic (regularly occurring) pattern could be seen in their properties
  • Left gaps for elements yet to be discovered and used his table to predict what properties they would have
  • A few years later new elements were discovered with properties that closely matched wheat Mendeleev thought
  • However not elements fit Mendeleev's pattern e.g. argon atoms have a greater average mass than potassium ones and ordering by atomic weights would mean that argon (a noble gas) being in the same group as highly reactive elements such as potassium
  • It was not until 20th century that scientists began to find out more about the structure of the atom and as a result of this, the elements are nwo in order of their atomic number (number of protons)
• Periodic table and electronic structure

  The chemical elements are placed in order of their atomic number in the periodic table- this arranges the elements so that they line up in groups (vertical columns) with similar properties
• Metals and non-metals

  • Main difference between metals and nonmetals is that metals can conduct electricity, but some exceptions include carbon, which is a non metal
  • In general metals also have higher boiling points
  • Metals are also ductile (can be drawn out into wires)
  • Metals malleable (can be hammered into shapes without smashing), whereas non-metals are brittle
• Crude oil

  • Finite resource found in rocks from the remains of tiny sea creatures that were buried in the mud
  • Over time, layer upon layer of rock was laid down on the top, creating the conditions (high pressure and temperature) to make crude oil
  • Crude oil is a mixture of many carbon compounds
  • Nearly all of the compounds in crude oil are compounds containing only hydrogen and carbon atoms (referred to as hydrocarbons
  • Before crude oil can be used it must be separated into different substances with similar boiling points- these are known as fractions
  • Because the properties of substances do not change when they are mixed the mixture of substances in crude oil can be separated using distillation
• Alkanes

  • Most hydrocarbons are alkanes- these are saturated meaning that all the carbon-carbon bonds are single covalent bonds, so no more hydrogen atoms can be added to each molecule
  • The general formula for alkanes is CnH2n+2- this means that for every carbon (n) atoms there are 2n +2 hydrogen atoms
• Fractional distillation of crude oil

  • Crude oil is separated into hydrocarbons with similar boiling points called fractions
  • This process is called fractional distillation
  • Each hydrocarbon fraction contains molecules with a similar number of carbon atoms
  • Each of these fractions boils at a different temperature range because of the different sizes of molecules innit
  • Crude oil is heated and fed in near the bottom of a tall tower (called fractionating column) as hot vapour
  • The column is very hot at the bottom and cooler at the top so the temperature decreases going up to the top of the column
  • Gases move up the column and the hydrocarbons condense when they reach the temperature of their boiling points
  • The different fractions are collected as liquids at different levels in a continuous process
  • Hydrocarbons with the smallest molecules have the lowest boiling points so they are piped out of the cooler top as gases
• Burning hydrocarbon fuels

  • The lighter fractions from crude oil are very useful as fuels
  • When hydrocarbons burn in air they transfer lots of energy to the surroundings
  • E.g.orange gas cylinders used in mobile heaters and for gas cookers in main areas without mains gas
  • These cylinders contain propane- a lighter fraction that comes out of the top of the fractionating column
  • When propane burns, the carbon and hydrogen are oxidised completely
• Burning hydrocarbon fuels

  When hydrocarbons burn in air they transfer lots of energy to the surroundings
• Lighter fractions from crude oil

  • Orange gas cylinders used in mobile heaters and for gas cookers in main areas without mains gas
  • These cylinders contain propane- a lighter fraction that comes out of the top of the fractionating column
• Combustion of propane

  The carbon and hydrogen are oxidised completely when they burn like this