GCSE Chemistry

• Protons have a positive charge and a relative mass of 1. Neutrons have a neutral charge and a relative mass of 1. electrons have a negative charge and a relative mass of around 1/2000.
• Isotopes are atoms with different numbers of neutrons but the same number of protons
• The nucleus contains protons and neutrons, while the shells contain electrons
• The atomic number is equal to the number of protons, which determines what element it is. It also happens to be the number of electrons.
• Mass number = number of protons + number of neutrons
• Relative formula mass (Mr) is calculated by adding up the masses of all the atoms present in one molecule
• Atoms are neutral even though they contain positive and negative particles because the number of protons and electrons are balanced so they cancel out.
• The chemical reactivity of an element can be determined by the number of electrons in the outer shell. Elements in group 0, such as Helium, are unreactive because they have a full outer shell of electrons (noble gases).
• To seperate two liquids with different boiling points, you would use distillation.

  The liquid with the lower boiling point boils and is condensed before the second liquid boils.
• Two gases that fan be collected by downwards displacement of air are Hydrogen (H₂) and Helium (He).
• Word equation for the formation of carbon dioxide from HCl and marble (calcium carbonate) 


  Hydrochloric acid + calcium carbonate -> Carbon dioxide + calcium chloride
• We should not react sodium with hydrochloric acid as it would cause a highly vigorous reaction and could explode
• Aqueous solutions are made when a solid dissolves in water to form a solution. The solute is the substance being dissolved and the solvent is the substance doing the dissolving.
• Nitric acid is formed when nitrogen dioxide dissolves in water.
• A saturated solution is when a solvent has dissolved the maximum amount of solute at a certain temperature and can't dissolve any more.
• The formula of potassium manganate (VII) is KMnO4. This means there are 4 different elements and 6 different atoms.
• Solubility (g per 100g of solvent) = mass of solid (g) / mass of water removed (g) x 100
• Atoms can join together to form molecules.

  Atoms in gases often go around in pairs (for example: Oxygen - O₂); a molecule with two atoms in it is called a diatomic molecule.
• Metals have high melting and boiling points. This is because there are strong electrostatic forces of attraction between metal atoms and the sea of delocalised electrons (metallic bonds) which require a lot of energy to break.
• Metals are malleable because the layers of metal ions can slide over one another, which allows the structure to bend.
• Particles of ammonia are smaller than hydrochloric acid, meaning they diffuse faster.
• More reactive metals will lead to larger changes in temperature. The greater the temperature change, the more reactive the metal must be (given that surface area and mass of two or more metals is the same when comparing).
• Hydrocarbons with shorter chains are collected at the top of the fractionating column.
• Paper chromatography is used to seperate a mixture of substances based on their solubility in a solvent.
• A higher Rf value means the substance has moved further along the paper. A lower Rf value means it hasn't moved as far.
• Ions with opposite charges will attract each other.

  This force of strong electrostatic attraction between oppositely charged ions forms an ionic bond.
• Rf = distant travelled by solute / distance travelled by solvent
• Isotopes are different atomic forms of the same element, which have the same number of protons but different numbers of neutrons.
• Relative atomic mass (Ar) = total mass of all atoms / total number of atoms
• The group number of an atom tells us the number of electrons in its outer shell.
• To figure out the number of neutrons in an element, you subtract the atomic number from the mass number.
• Filtration is used to seperate an insoluble solid from a liquid, whereas crystallization seperates a soluble solid from a solution.

  You can use both filtration and crystallization to seperate rock salt,

  which is a mixture of salt and sand.

  • The sand is insoluble and stays as large grains, so it collects on the filter paper.
  • The salt is dissolved in solution, so it will go through the filter paper. But, when the water is evaporated, the salt is left behind as crystals in the evaporating dish.
• How to separate out dyes using paper chromatography

  1. Draw a line near the bottom of a sheet of filter paper - this is the baseline. Use a pencil as it won't dissolve in the solvent.

  2. Add spots of different inks to the baseline at regular intervals.

  3. Place the sheet into a beaker of solvent, e.g. water. The solvent used depends on what's being tested. Some compounds will dissolve well in water but sometimes solvents like ethanol are needed.

  4. Make sure the level of solvent is below the baseline so the inks don't dissolve into it.

  5. Place a lid on top of the container to stop the solvent evaporating.

  6. The solvent should seep up the paper, carrying the inks with it. Each different dye will travel up the paper at a different rate and form a spot in a different place.

  7. When the solvent has almost reached the top of the paper, remove the paper from the beaker and leave it to dry.

  8. The end result is a pattern of spots called a chromatogram.
• Simple distillation is used to seperate out a liquid from a solution


  The solution is heated. The part of the solution with the lowest boiling point evaporates.

  The vapor is cooled, condenses and is collected in a seperate container. The rest of the solution is left behind in the original flask.

  The issue with simple distillation is it can only be used to seperate substances with very different boiling points. If you have a mixture of liquids with similar boiling points, you should use fractional distillation instead.
• If atoms have covalent bonds, they shair pairs of electrons. Each covalent bond provides one extra shared electron for each atom. In covalent bonding, there's a strong electrostatic attraction between the negatively charged shares electrons an the positively charged nuclei of the atoms involved.