OCR GCSE Chemistry A Gateway revision notes
Elements, compounds and mixtures Revision Notes
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Elements, compounds and mixtures
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Periodic Table, Electron Arrangement, and Atomic Number Revision Notes
Atomic Number and Electrons
The atomic number of an element is the number of protons in the nucleus of an atom. In a neutral atom, the number of electrons is equal to the number of protons. Therefore, the atomic number defines the total number of electrons in an atom and its electrical neutrality. This fundamental number dictates an element's identity and its placement in the Periodic Table.
Groups and Valence Electrons
Elements arranged in the same vertical column, known as a group, have atoms with the same number of electrons in their outermost shell (valence electrons). The number of valence electrons primarily determines an element's chemical properties and how it will react with other elements. For example, Group 1 elements all have one valence electron, leading to similar reactive metallic behaviour.
Periods and Electron Shells
Elements in the same horizontal row, called a period, have atoms with the same number of occupied electron shells. The period number directly corresponds to the number of electron shells containing electrons in an atom of that element. Moving across a period, the atomic number increases, and electrons progressively fill the available shells.
Relating Position to Electron Arrangement
The Periodic Table is structured to reflect electron arrangement. The group number indicates the number of outer shell electrons (for main group elements), while the period number indicates the number of occupied electron shells. This arrangement allows chemists to predict chemical properties and reactivity based solely on an element's position.
Understanding Chemical Bonds and Structures Revision Notes
Ionic Compounds
Ionic compounds form between metal and non-metal atoms. Electrons are transferred from the metal atom to the non-metal atom, creating positively charged metal ions and negatively charged non-metal ions. These oppositely charged ions are attracted to each other, forming strong electrostatic forces. This results in a giant ionic lattice structure, where ions are arranged in a regular, repeating pattern. Ionic compounds typically have high melting and boiling points because a large amount of energy is required to overcome these strong electrostatic forces. They conduct electricity when molten or dissolved in water, as the ions are free to move and carry charge, but not as solids because the ions are fixed in the lattice.
Simple Molecules and Giant Covalent Structures
Simple molecules involve non-metal atoms sharing electrons to form strong covalent bonds within the molecule. Examples include water (H₂O) and carbon dioxide (CO₂). While the bonds within the molecules are strong, the forces between individual molecules (intermolecular forces) are weak. This means simple molecular substances have low melting and boiling points, as only a small amount of energy is needed to overcome these weak intermolecular forces. They generally do not conduct electricity because there are no free-moving charged particles. In contrast, giant covalent structures, such as diamond or silicon dioxide, consist of a vast network of atoms held together by many strong covalent bonds throughout the entire structure. This extensive bonding network gives them very high melting and boiling points, making them hard and often insulators, as electrons are localised in bonds.
Polymers
Polymers are long-chain molecules formed from many smaller repeating units called monomers, which are joined together by strong covalent bonds. The atoms within the polymer chain are held by strong covalent bonds. However, the forces between different polymer chains (intermolecular forces) can vary. Some polymers have weak intermolecular forces, making them flexible and having lower melting points (e.g., poly(ethene)). Others have stronger intermolecular forces, leading to higher melting points and increased rigidity. Polymers are generally electrical insulators because their electrons are usually localised in covalent bonds and cannot move freely to carry charge.
Metals
Metals consist of a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons. These outer shell electrons are not associated with any specific atom but are free to move throughout the entire structure. The strong electrostatic attraction between the positive metal ions and the delocalised electrons constitutes metallic bonding. This structure gives metals their characteristic properties: high melting and boiling points, good electrical and thermal conductivity (due to the mobile delocalised electrons), and malleability and ductility (the layers of ions can slide over each other without disrupting the metallic bond). Metals do not dissolve easily.
Understanding "Purity": Scientific vs. Everyday Meaning Revision Notes
Scientific Definition of Pure Substance
In science, a pure substance is defined as a single chemical element or a single chemical compound. This means it contains only one type of atom (for an element) or only one type of molecule/formula unit (for a compound). For instance, pure copper contains only copper atoms, and pure water contains only H₂O molecules. A key characteristic is a fixed chemical composition throughout the substance.
Everyday Definition of Pure Substance
The everyday understanding of 'pure' is different from the scientific definition. In daily language, 'pure' often means something is natural, unadulterated, or free from harmful substances or unwanted additives. For example, 'pure milk' might simply mean it contains no artificial colourings or preservatives, even though milk is a complex mixture of water, fats, proteins, and sugars.
Physical Properties of Pure Substances
Pure substances have distinct and fixed physical properties. These include a precise melting point and a precise boiling point. For example, pure water always boils at 100 °C and freezes at 0 °C at standard atmospheric pressure. Other fixed properties include density and solubility. These specific properties are used by scientists to identify and confirm the purity of a substance.
Effect of Impurities on Physical Properties
The presence of impurities (other substances) significantly alters the physical properties of a substance. Impurities generally lower the melting point and raise the boiling point of a substance. Crucially, instead of melting or boiling at a single, sharp temperature, impure substances will melt or boil over a range of temperatures. The extent of this change depends on the amount and type of impurity present. This is a practical way to detect impurities.
Using Chromatography to Distinguish Pure from Impure Substances Revision Notes
What is Chromatography?
Chromatography is a separation technique used to separate components of a mixture. It works on the principle that different substances have different solubilities in a solvent (the mobile phase) and different attractions to a stationary phase (like paper or a thin layer of silica gel). As the mobile phase moves through the stationary phase, it carries the components at different rates, leading to their separation. This allows for the identification of substances and the assessment of their purity.
Identifying Pure vs. Impure Substances
To determine if a substance is pure or impure using chromatography, you observe the resulting chromatogram. A pure substance will typically produce only one spot on the chromatogram, regardless of the solvent used, provided it's a single compound. This single spot shows that all the material is identical and travels at the same rate. An impure substance, which is a mixture, will separate into two or more distinct spots on the chromatogram. Each spot corresponds to a different component within the mixture. The more spots, the more components are present.
How Chromatography Works (Practical Application)
In paper chromatography, a spot of the substance is applied to a baseline on a piece of chromatography paper (stationary phase). The bottom edge of the paper is then dipped into a suitable solvent (mobile phase), ensuring the solvent level is below the baseline. The solvent travels up the paper by capillary action, dissolving the sample and carrying its components with it. Components that are more soluble in the solvent and less adsorbed by the paper travel further up, resulting in separation. After the solvent has travelled a sufficient distance, the paper is removed, and the solvent front is marked. The separated spots reveal the composition of the sample.
Interpreting Chromatograms
When interpreting a chromatogram for purity, look for the number of distinct spots. If a substance yields only one spot, it is likely pure. If it yields multiple spots, it is impure. For comparison, known pure substances can be run alongside the unknown sample. If a spot from the unknown aligns with a spot from a known pure substance, it suggests the presence of that substance in the unknown. The height each spot travels is characteristic of that component in that particular solvent and stationary phase.
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