experimental techniques

Question 1

what are the units for temperature

Answer 1

the units of temperature are degrees Celsius (ºC)

Question 2

how do we measure temperature

Answer 2

Temperature is measured with a thermomete

Question 3

why are digital thermometers better than traditonal thermometers

Answer 3

Digital thermometers are available which are more precise than traditional thermometers
Traditional thermometers rely upon the uniform expansion and contraction of a liquid substance with temperature; they tend to be more accurate than digital thermometers which often need to be re-calibrated

Question 4

how is mass measured

Answer 4

Mass is measured using a digital balance

Question 5

what is the standard unit of mass

Answer 5

The standard unit of mass is kilograms (kg) but in chemistry grams (g) are most often used
1 kilogram = 1000 grams

Question 6

how is volume of a liquid measured

Answer 6

1. For approximate volumes where accuracy isn´t an important factor, measuring cylinders are used
2. Pipettes are the most accurate way of measuring a fixed volume of liquid
3. Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm3 and 50 cm3

Question 7

how is the volume of gases found

Answer 7

1. A gas syringe is usually the apparatus used
2. A graduated cylinder inverted in water may also be used, provided the gas isn’t water-soluble
   If the gas happens to be heavier than air and is colored, the cylinder can be used upright

Question 8

why does a mizture have a range of melting and boiling points

Answer 8

hey consist of different substances that melt or boil at different temperatures

Question 9

what are retention factor (Rf) values

Answer 9

These values are used to identify the components of mixtures

The Rf value of a particular compound is always the same

Question 10

what are the units of rf

Answer 10

The Rf value is a ratio and therefore has no units

Question 11

what are locating agents

Answer 11

Locating agents are substances which react with the sample and produce a coloured product which is then visible

Question 12

how do you separte a mixture of solids

Answer 12

For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities

Question 13

what is filtration used to seperate

Answer 13

Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water).

Question 14

describe the filtration process

Answer 14

1. Filter paper is placed in a filter funnel above another beaker
2. Mixture of insoluble solid and liquid is poured into the filter funnel
3. Filter paper will only allow small liquid particles to pass through as the filtrate
4. Solid particles are too large to pass through the filter paper so will stay behind as a residue

Question 15

what is crystalasiation used to separate

Answer 15

Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold

Question 16

describe the precess of crystalisation

Answer 16

1. The solution is heated, allowing the solvent to evaporate to leave a saturated solution behind
2. Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution. If the solution is saturated, crystals will form on the glass rod
3. The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow
4. Crystals are collected by filtering the solution
5. They are then washed with cold, distilled water to remove impurities and allowed to dry

Question 17

what is simple distillation used to separate

Answer 17

Used to separate a liquid and soluble solid from a solution (e.g. water from a solution of saltwater) or a pure liquid from a mixture of liquids

Question 18

describe the process of simple distilation

Answer 18

1. The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask
2. The vapour passes through the condenser, where it cools and condenses, turning into pure liquid H2O which is collected in a beaker
3. After all the water is evaporated from the solution, only the solid solute will be left behind

Question 19

what is fractional ditilation used to separate

Answer 19

Used to separate two or more liquids with one another

Question 20

describe the process of fractional distilation

Answer 20

1. The solution is heated to the temperature of the substance with the lowest boiling point
2. This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker
3. All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture
4. For water and ethanol: ethanol has a boiling point of 78 ºC and water of 100 ºC. The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distils out of the mixture and condenses into the beaker
5. When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated