OCR GCSE Combined Science A Gateway revision notes
Elements, compounds and mixtures Revision Notes
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Elements, compounds and mixtures
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Understanding Changes of State and Energy Transfers Revision Notes
Energy and Changes of State
Changes of state, such as melting, boiling, freezing, and condensation, involve energy transfers. When a substance is heated, its particles gain kinetic energy, causing them to move faster. At specific temperatures, known as melting and boiling points, the energy supplied is no longer used to increase temperature, but instead, it is absorbed to overcome the forces between particles, leading to a change in state. This absorbed energy is known as latent heat.
Particle Forces and Melting Point
The melting point of a substance is the temperature at which it changes from a solid to a liquid. In a solid, particles are held in fixed positions by strong forces. To melt, enough energy must be supplied to overcome these forces, allowing the particles to move past one another. The stronger the forces holding the particles in the solid state, the more energy is required to break them, resulting in a higher melting point.
Particle Forces and Boiling Point
The boiling point of a substance is the temperature at which it changes from a liquid to a gas. In a liquid, particles are still relatively close but can move randomly. To boil, even more energy is needed to completely overcome the intermolecular forces, allowing particles to become widely separated and move freely as a gas. Substances with stronger intermolecular forces will require more energy to boil, leading to a higher boiling point.
Strength of Bonds/Forces and Temperature
The relative strength of chemical bonds (in giant structures or within molecules) and intermolecular forces (between simple molecules) directly influences melting and boiling points. Substances with strong covalent or ionic bonds (e.g., giant covalent structures, ionic compounds) require a large amount of energy to overcome these bonds, leading to very high melting and boiling points. Simple molecular substances, which only have weak intermolecular forces between molecules, require much less energy to overcome these forces, resulting in lower melting and boiling points.
Understanding Relative Formula Mass (Mr) Revision Notes
What is Relative Formula Mass (Mr)?
The relative formula mass (Mr) is the sum of the relative atomic masses (Ar) of all the atoms present in the chemical formula of a substance. It applies to all types of substances, whether they are made of molecules (like water, H₂O) or ionic compounds (like sodium chloride, NaCl). The Mr is a way to express the 'mass' of a compound relative to the mass of a carbon-12 atom, and it does not have units. You can find the relative atomic mass of each element on the periodic table. For example, Ar(H) = 1, Ar(C) = 12, Ar(O) = 16, Ar(Na) = 23, Ar(Cl) = 35.5.
Calculating Mr for Simple Compounds
To calculate the Mr for a simple compound, you identify the number of each type of atom from its chemical formula and multiply that number by the element's relative atomic mass. Then, sum these values. For instance, to calculate the Mr of carbon dioxide (CO₂): there is one carbon atom and two oxygen atoms. Mr(CO₂) = (1 × Ar(C)) + (2 × Ar(O)) = (1 × 12) + (2 × 16) = 12 + 32 = 44. This straightforward process is applied to all compounds without complex sub-structures like brackets.
Calculating Mr for Compounds with Brackets
When a chemical formula contains brackets, such as calcium hydroxide, Ca(OH)₂, the subscript number outside the bracket indicates that everything inside the bracket is multiplied by that number. For Ca(OH)₂, there is one calcium atom, two oxygen atoms (because the (OH) group is taken twice), and two hydrogen atoms. So, Mr(Ca(OH)₂) = (1 × Ar(Ca)) + (2 × Ar(O)) + (2 × Ar(H)). Using Ar(Ca) = 40, Ar(O) = 16, and Ar(H) = 1, we get Mr(Ca(OH)₂) = (1 × 40) + (2 × 16) + (2 × 1) = 40 + 32 + 2 = 74. This method ensures all atoms in the compound are accounted for correctly.
Mr in Balanced Chemical Equations
The concept of relative formula mass is essential for interpreting balanced chemical equations. For example, in the reaction 2H₂ + O₂ → 2H₂O, the coefficients (the numbers in front of the formulas) tell us the ratio of reacting particles. If we want to consider the total relative formula mass of reactants or products, we multiply the Mr of each species by its coefficient. For the reactants in the example: Total Mr of 2H₂ = 2 × Mr(H₂) = 2 × (2 × 1) = 4. Total Mr of O₂ = 1 × Mr(O₂) = 1 × (2 × 16) = 32. Total Mr of reactants = 4 + 32 = 36. For the product: Total Mr of 2H₂O = 2 × Mr(H₂O) = 2 × (2 × 1 + 1 × 16) = 2 × 18 = 36. This demonstrates the conservation of mass in chemical reactions, where the sum of the Mr of reactants equals the sum of the Mr of products.
Interpreting Chromatograms and Rf Values Revision Notes
Understanding Chromatograms
A chromatogram is the visual output of a chromatography experiment, typically a piece of paper or a plate with separated spots. The baseline is where the mixture was initially applied. The solvent front is the maximum distance the solvent travelled from the baseline. Each separated spot represents a different component of the original mixture. The number of spots indicates the number of pure substances in the mixture. If a single substance forms multiple spots, it suggests that the substance is not pure or has reacted with the stationary phase. If a known pure substance is spotted alongside an unknown, and their spots travel the same distance, it suggests they are the same compound if the solvent and stationary phase are the same.
Calculating Rf Values
The Rf (retardation factor) value is a ratio used to identify substances in chromatography. It quantifies the movement of a substance relative to the movement of the solvent. The formula is: Rf = (distance travelled by spot) / (distance travelled by solvent front). Both distances must be measured from the starting line (baseline). For example, if a spot travels 6 cm and the solvent front travels 10 cm, the Rf value is 6 cm / 10 cm = 0.6. Rf values are always unitless because it's a ratio of two distances, and they are always less than or equal to 1.0. An Rf value of 0 means the substance did not move from the baseline, while an Rf value of 1 means it moved with the solvent front.
Interpreting Rf Values
Rf values are specific to a particular substance under specific chromatographic conditions (solvent, temperature, stationary phase). They can be used to identify components of a mixture by comparing them to known Rf values. If an unknown substance has the same Rf value as a known substance in the same solvent, it suggests they are the same compound. A higher Rf value indicates that the substance is more soluble in the mobile phase (solvent) and/or has less affinity for the stationary phase (paper/plate), causing it to travel further. Conversely, a lower Rf value indicates lower solubility in the mobile phase and/or higher affinity for the stationary phase.
Purity Assessment Using Chromatography
Chromatography can be used to assess the purity of a substance. A pure substance should ideally produce only a single spot on a chromatogram. If multiple spots are observed, it indicates that the substance is a mixture of different compounds. However, it is important to note that two different compounds could have very similar Rf values in a particular solvent, appearing as a single spot. To confirm purity or identity, it is often necessary to run the chromatography experiment using different solvents or different types of stationary phases. This helps to resolve components that might not separate well in all conditions.
Separating Mixtures: Filtration, Crystallisation, Simple and Fractional Distillation Revision Notes
Filtration: Separating Insoluble Solids from Liquids
Filtration is a technique used to separate an insoluble solid from a liquid. The mixture is poured through a filter funnel lined with filter paper. The liquid, called the filtrate, passes through the filter paper and is collected. The insoluble solid, known as the residue, remains on the filter paper. This method works because particles of the insoluble solid are too large to pass through the pores in the filter paper, whereas the liquid molecules and any dissolved substances are small enough. An example is separating chalk powder from water.
Crystallisation: Obtaining Soluble Solids from Solutions
Crystallisation is a method to separate a soluble solid from a solution. The solution is gently heated to evaporate most of the solvent, creating a concentrated solution. The hot, concentrated solution is then allowed to cool slowly. As it cools, the solubility of the dissolved solid decreases, causing it to form pure crystals. The crystals can then be collected by filtration and dried, often using absorbent paper. This technique is suitable for obtaining pure salt from saltwater, provided the salt does not decompose upon heating.
Simple Distillation: Separating Liquids from Non-Volatile Solids or Liquids with Large Boiling Point Differences
Simple distillation is used to separate a liquid from a solution containing a dissolved non-volatile solid, or to separate two liquids with significantly different boiling points. The mixture is heated in a flask, causing the liquid with the lower boiling point to vaporise. The vapour then passes into a condenser, where it is cooled by circulating water and condenses back into a liquid (the distillate). This purified liquid is collected. The non-volatile solid or higher boiling point liquid remains in the distillation flask. This process is used to obtain pure water from seawater.
Fractional Distillation: Separating Liquids with Close Boiling Points
Fractional distillation is an enhanced version of distillation used to separate two or more liquids that have boiling points that are relatively close to each other. The apparatus includes a fractionating column positioned between the distillation flask and the condenser. This column is usually packed with glass beads or rings to provide a large surface area for repeated vaporisation and condensation cycles. As the mixture is heated, vapours rise. The fraction with the lowest boiling point travels furthest up the column before condensing and being collected, achieving a better separation than simple distillation. This method is crucial for separating crude oil into its useful fractions or separating ethanol from water.
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