BRAINCAKEElements, compounds and mixtures
Revision notes for Elements, compounds and mixtures in OCR Combined Science A (J250). Every lesson in BrainCake comes with R.E.C.I.P.E. recall steps, a quick check and spaced reviews.
- Purity of a substanceIn everyday life the word pure is used loosely.
- Melting point and purityEvery pure substance melts at one fixed temperature, called its melting point.
- Relative formula massThe relative atomic mass (Ar) of an element is the average mass of its atoms compared with one twelfth of the mass of a carbon-12 atom.
- Empirical formulaeThe empirical formula of a compound shows the simplest whole number ratio of the atoms of each element in it.
- Formulations and alloysMany of the useful materials we use every day are mixtures.
- Separating mixturesThe substances in a mixture are not chemically joined, so they can be separated by physical methods.
- Paper and thin layer chromatographyChromatography separates the substances in a mixture, such as the dyes in an ink.
- Stationary and mobile phases in chromatographyEvery type of chromatography has two phases.
- Interpreting chromatograms and calculating Rf valuesOn a finished chromatogram, each spot shows a substance.
- Choosing a suitable purification techniqueTo choose a technique you look at the substances in the mixture and ask what is different about them.
- Using chromatography to tell pure from impure substancesA chemically pure substance contains only one substance.
- Metals and non-metals: physical and chemical propertiesElements are either metals or non-metals, and the two groups behave very differently.
- Atomic structure and the position of metals and non-metalsThe Periodic Table is split by a stepped line.
- Electron arrangement, group, period and atomic numberThe elements in the Periodic Table are arranged in order of increasing atomic number.
- Bonding in ionic compounds, simple molecules and giant covalent structuresThere are three main types of bonding structure in compounds.
- Bonding: electrostatic forces and transfer or sharing of electronsAtoms bond to reach a full outer shell of electrons.
- Dot and cross diagrams for covalent and ionic substancesA dot and cross diagram shows only the outer shell electrons.
- Limitations of bonding models and diagramsScientists use models to show how atoms bond, but every model leaves something out or shows something that is not real.
- Electron arrangement and the reactions of elementsThe chemical reactions of an element depend on the arrangement of the electrons in its atoms.
- From Mendeleev's table to the modern Periodic TableIn the 1860s Mendeleev arranged the elements in order of atomic weight, which is now called relative atomic mass, and he put elements with similar…
- Carbon forms four covalent bondsCarbon is in Group 4 of the periodic table, so a carbon atom has four electrons in its outer shell.
- Carbon chains, rings and families of compoundsCompounds built around carbon are called organic compounds.
- Diamond, graphite, fullerenes and grapheneDiamond, graphite, fullerenes and graphene are all forms of carbon, but their structures and bonding give them different properties.
- Energy, bonds and changes of stateMelting and boiling are changes of state.
- Predicting states from dataThe state of a substance at a given temperature depends on where that temperature sits compared with its melting point and boiling point.
- Bonding and bulk properties of materialsThe bulk properties of a material, such as melting point, hardness and conductivity, come from the bonds in it and how its particles are arranged.