1.9 Chemical Analysis Flashcards
What is an element, compound, pure substance, and a mixture?
Element- a single pure substance that cannot be broken down into anything simpler by chemical means.
Compound- a pure substance made when two or more elements are chemically combined.
Pure substance- a single element or compound that is not mixed with any other substances.
Mixture- two or more substances mixed together, which are usually easy to separate.
Describe what happens to an solid as the temperature is increased
-When a solid is heated, the temperature of the solid increases until it reaches its melting point.
-The temperature remains constant while the substance melts, even though it is being heated.
-Once all the solid has changed to liquid, the temperature of the liquid increases until it reaches its boiling point.
-The temperature remains constant while the substance boils, even though it is still being heated.
-Once all the liquid has changed to a gas, the temperature of the gas increases.
Describe the melting and boiling points of pure and impure substances
Pure substances (elements and compounds) melt and boil at specific temperatures. The melting and boiling points can be used to distinguish substances from mixtures. An impure substance will have a lower melting point and will melt over a range of temperatures. The melting point is lower since the impurities disrupt the regular lattice arrangement and so the bonds between the particles will be weaker. The greater the amount of impurity the lower the melting point.
What is a formulation?
A formulation is a mixture that has been designed as a useful product and is formed by mixing together several different substances in carefully measured quantities to ensure the product has the required properties
What are some examples of formulations?
-Alloys (e.g. 22 carat gold)- an alloy is a mixture of two or more elements one of which is a metal. The resulting mixture has metallic properties.
-Medicine (e.g. calpol)- tablets are formulations of the active drug along with ingredients such as corn starch. Liquid medicines are also formulations.
-Fertiliser (e.g. NPK fertiliser)- fertilisers contain mixtures of nitrogen, phosphorus and potassium compounds. Different formulations are suitable for different plants.
What is meant by the terms: soluble, insoluble, solute, solvent, solution, evaporation, condensation, miscible and immiscible?
Soluble- can be dissolved in the liquid.
Insoluble- cannot be dissolved in the liquid.
Solute- the substance that dissolves in a solvent.
Solvent- the liquid in which the solute dissolves.
Solution- the mixture of the solute and solvent.
Evaporation- the change of state from liquid to gas.
Condensation- the change from a vapour to a liquid.
Miscible- two liquids mixing together to form a single solution.
Immiscible- two liquids that do not mix together but instead form two layers.
Describe the different methods of seperation
- Filtration- used to separate an insoluble substance from a liquid or solution.
The filtrate is the liquid or solution that passes through the filter paper and the residue is the insoluble substance trapped in the paper. - Evaporation- this is used to obtain a solute from a solution. The liquid (solvent) is allowed to escape as vapour.
- Crystallisation- another method of separating a dissolved solid from a solvent which involves heating the solution to boil off some of the solvent. This creates a saturated solution. A saturated solution is one in which no more solid can dissolve at that temperature. The saturated solution is then cooled. The dissolved solid becomes less soluble and consequently crystallises out of the solution. The crystals may be separated by filtration.
- Simple distillation- this is used to separate a liquid from an involatile solute: to obtain the solvent from a solution.
- Fractional distillation- used to separate two or more miscible liquids. The liquids are separated due to their different boiling points. The liquids with the lower boiling points reach the top of the fractionating column in advance of the ones with the higher boiling points.
Give examples for the different methods of seperation
-Filtration: sand from water
-Evaporation: salt (NaCl) from salt water mixture.
-Crystallisation: N/A
-Simple distillation: water from salt water; water from fountain pen ink.
-Fractional distillation: crude oil; liquid air.
Describe paper chromatography
-This is used to separate and often identify substances in mixtures.
-There is a stationary phase (a solid or a liquid supported on a solid) and a mobile phase (a liquid or gas). The mobile phase flows over the stationary phase and carries the substances in the mixture with it. Different substances travel at different rates.
-Paper chromatography is most often used to separate coloured substances such as different dyes in inks or food dyes used in processed foods. Another example is the separation of natural dyes in plants and flowers. The stationary phase is paper and the mobile phase is a solvent or mixture of solvents.
-A line is drawn in pencil (the origin) a few cm from the bottom of the paper and a sample of the mixture is spotted on the line. The paper is then placed in a container with the solvent that the solvent level is below the origin. A lid is placed over the container and the allowed to travel up the paper. The paper is removed when the solvent has travelled close to the top of the paper (the solvent front). Different coloured substances will separate out at different heights above the origin. The more soluble ones will travel further. The distance travelled can be used to identify them. Two substances that travel to the same height above the origin are the same substance.
What is the calculation to find the Rf value of a substance and what does this tell us about the substance?
Rf = X/Y = distance moved by solute A/ distance moved by solvent
Rf values are between zero and one. A zero value means the spot has not moved. The higher the value the further the spot has moved on the chromatogram. The distance moved by the spot is taken from the origin to the centre of the spot. If the spot has an irregular appearance the approx. centre of the spot is taken.
What is potable water?
Water which is safe to drink.
Describe the 3 processes in making potable water from fresh water
Filtration- removes insoluble solids- the water is sprayed onto specially prepared layers of sand and gravel called filter beds. Different sized particles are removed as the water trickles through the filter beds; and sand and gravel act like filter paper. The filter beds are cleaned every so often by pumping clean water backwards through the filter.
Sedimentation- clumps tiny particles together into large particles which precipitate out of solution- in the sedimentation tank, aluminium sulfate is added to clump tiny particles together to make large particles, which settle out more easily (flocculation). The water is then passed through a fine filter, such as carbon granules, to remove very small particles.
Chlorination- kills microbes- chlorine gas is bubbled into the water to kill microbes and sterilise the water.
Describe the process of making potable water from sea water
Seawater is made potable by distillation. The process of making sea water potable is known as desalination. Desalination is the removal of dissolved salts from sea water. Desalination is often carries out by distillation. The sea water is boiled, the water evaporates and turns to steam, which is condensed to give pure water. The process requires large amounts of energy and so is expensive. Solar power is often used in hot countries to reduce costs.
What is the test for water?
Anhydrous copper (II) sulfate does not contain any water of crystallisation and is a white powdery solid. On addition of water the anhydrous copper (II) sulfate turns to blue hydrated copper (II) sulfate crystals. This reaction is exothermic and is used as a chemical test for water. It does not test the purity of water only the presence of water.
What is the equation of the test for water?
CuSO4 (s) + 5H2O (l) —> CuSO4.5H2O (s)
