chap 1 - experimental chemistry

• burette --> 2 d.p --> units = cm³

  pipette --> 1 d.p --> units = cm³

  measuring cylinder --> 1 d.p --> units = cm³

  electronic balance --> 1 or 2 d.p --> units = g

  ruler( half metre ruler or meter ruler) --> 1 d.p --> units = cm

  stopwatch (digital) --> whole no. --> units = s

  thermometer --> 1 d.p --> units = ⁰C
• significant figures

  • if addition/subtraction , follow least decimal place
  • if multiplication/division , follow least significant figure
• how are gases formed?

  • depends on two physical properties : solubility in water

  : density

  • 4 ways to collect gas

  > gas syringe

  > water displacement

  > downward delivery

  > upward delivery
• ways to collect air

  • gas syringe

  - used to collect small and accurate volume of gases

  • water displacement

  - insoluble to slightly soluble in water

  - density does not affect gas collection
• ways to collect air

  • downward delivery

  - can be soluble or insoluble

  - denser than air

  • upward delivery

  - can be soluble or insoluble

  - less dense than air (lighter than air)
• difference between a mixture and pure substance

  • mixture is made up of 2 or more substance that are not chemically combined together
  • pure substance is made up of one substance/component. it is not mixed with any other substance
  • air is a mixture of 78% nitrogen, 21 % oxygen and 1% other gases and carbon dioxide
• separating a solid from a liquid

  • filtration

  - used to separate insoluble particles from a liquid

  - insoluble and a solvent are usually used (eg: sand and water)

  - upon filtration, the solid that remains on the filter paper is called residue

  - large insoluble solid particles are trapped by the filter paper in the filter funnel. small particles of liquid pass through the filter paper

  - the liquid that passes through the filter is called filtrate
• separating solid from a liquid

  • evaporation to dryness

  - obtain soluble solid from a solution by heating the solution until all the water has boiled off

  - solid obtained by evaporation to dryness is not always pure. when all the water is evaporated, any soluble impurities will be left together with the solid

  - salt remains as a white residue in the evaporating dish, water in the solution is lost to the atmosphere
• separating solid from a liquid

  • crystallisation

  - obtain pure solid sample from its solution

  - heat solution to remove most of the solvent. heating is stopped when a saturated solution is formed

  - the hot, saturated solution is allowed to cool and the dissolved salt appears as pure crystals (rapid cooling produces small crystals while slow cooling produces large crystals

  - the crystals formed are removed by filtration. the residue of pure crystals is washed with cold distilled water. the crystals are then dried by pressing them between pieces of filter paper
• separating solid from liquid

  • using a suitable solvent

  - separate mixture of two solids, use a solvent in which one solid is soluble and the other is insoluble

  - different solid dissolves in different solvents
• solute --> substance that dissolves in a solvent

  solvent --> substance that dissolves other substance
• separating liquid from a solution

  • simple distillation

  - used to separate a pure solvent from a solution

  - boiling chips are placed in the flask to ensure smooth boiling

  - thermometer bulb should be just beside the side arm leading to the condenser --> this is to ensure that the thermometer measures the boiling point of the substance that is being distilled

  - water enter the condenser from the bottom and leaves from the top --> this ensures that the condenser is cool enuf to cool and condense more efficiently
• separating liquid from a solution

  • fractional distillation

  - used to separate a mixture of miscible (mix tgt completely) liquids with different boiling points

  - water & ethanol vapour rise up the column as the solution is heated. the water-vapour condense in the fractionating column and falls back into the flask (higher boiling point)

  - ethanol will reach the upper part of the column go in the condenser. in the condenser, hot ethanol vapour condenses as running water cools it down and flows down the inner tube and into the receiver to be collected as distillate
• separating liquid from a solution

  • paper chromatography

  - separate components

  - draw a line with a pencil ( insoluble in water)

  - place a spot of colouring on the pencil line and wait for it to dry

  - dip paper into solvent and ensure coloured spot is above the solvent level( prevent dye from dissolving into solution)

  - wait for the dye to separate as the solvent travels up the paper

  - the more soluble a dye is in the solvent, the further it will move up the paper
• separating liquid from a solution

  • paper chromatography

  - interpretation of a chromatogram

  > if a dye consists of 2 components , it is not pure; it is a mixture

  > if a dye consists of 1 component, it is a pure substance

  - use of chromatography

  > separate the components in a sample

  > identify the components present in a sample

  > identify banned substance

  > determine its purity
• determining purity

  • check for fixed melting / boiling point of substance
  • performing chromatography
• determining purity

  • effect of impurities on melting and boiling point

  - melting point : pure substance

  > fixed and constant (melts completely at one temp)

  : impure substance

  > lowers the melting point ( melts over a range of temp)

  - boiling point : pure substance

  > fixed and constant ( boils completely at one temp)

  : impure substance

  > increase the boiling point (boils over a range of temp)