1 — experimental techniques

Question 1

Measurement of time

Answer 1

SI: seconds (s)
Apparatus: digital stopwatch is used to measure time.
Record seconds to a whole number.

Question 2

Measurement of temperature

Answer 2

SI: Kelvin K
Commonly measured as degree Celsius in the lab.

Apparatus: alcohol or mercury thermometer

Question 3

Measurement of length

Answer 3

SI:m
1m = 10dm =100cm = 1000nm
Apparatus: Metre rules, measuring tapes are used

Question 4

Measurement of mass

Answer 4

SI: kg
Apparatus: electronic balance

Question 5

Measurement of volume

Answer 5

SI: (m^3)
Apparatus: measures:

Pipette -> accurate fixed volumes(10.0, 25.0cm^3)
Volumetric flask -> accurate fixed volumes (100cm^3, 250cm^3)

Measuring cylinder -> a range of volumes to nearest 0.5cm^3
Burette -> range of volumes to nearest 0.05cm^3

Gas: gas syringe -> up to 100cm^3

1 volume: pipette < volumetric flask
Range of volumes: measuring cylinder > burette
Most accurate: burette

Question 6

How to avoid parallax error

Answer 6

Read from bottom of the meniscus at eye level

Question 7

Collection of gases

Answer 7

1.gas syringe
2. Solubility
- water displacement (H2, O2, CO2, N2)
3. Density
- downward or upward delivery (other than H2, N2, NH3, all downward)

Question 8

Drying of gases

Answer 8

Concentrated Sulfuric acid
- for acidic and neutral gases
- cannot be used to dry alkaline gases as they react w acid to form salt (NH3)

Quicklime (CaO)
- must be freshly heated b4 use as it absorbs moisture n CO2 from air
- suitable for alkaline n neutral gases
- cannot be used to dry acidic gases as they react w CaO to form salt

Fused calcium chloride
- must be freshly heated b4 use as it absorbs moisture from air
- suitable for all gases except NH3

Question 9

Sublimation

Answer 9

Sublimation is the process a substance changes from solid to gaseous state directly.

Question 10

Substances that undergo sublimation
（I Need AirCon)

Answer 10

Iodine, ammonium chloride, naphthalene (used in mothballs)

Question 11

Procedure of sublimation

Answer 11

1. Solid sublimes into a gas on heating and moves away from the mixture.
2. By providing a cool surface for he gas to deposit on, the gas will change state back into a solid.
3. Resulting solid deposit, known as the sublimate, can be collected by scraping them off the cool surface.

Question 12

Filtration

Answer 12

Filtration can be used to separate insoluble solids from liquids.

Question 13

evaporation to dryness

Answer 13

Evaporation to dryness is used to separate a dissolved solid from its solvent by heating the mixture until all the solvent has vaporised (gaseous state)

Procedure:
Heat the solution in an evaporating dish until all the solvent evaporates and only the solute is left.

Question 14

Crystallisation

Answer 14

Crystallisation is used to obtain a pure solid from its saturated solution. A saturated solution is one in which no more solute can be dissolved in the solvent.

Question 15

Procedure of crystallisation

Answer 15

1 gently heat the CUSO4 solution in an evaporating dish to evaporate most of the liquid until a saturated solution is obtained. Stop heating before all the solvent has evaporated.
2. Cool the solution gradually until al the solid CUSO4 crystals appear within the solution
3. Carefully pour the mixture thru a filter runner lined w filter paper to filter n collect the solid crystals (residue)
4. Wash CuSO4 crystals w some cold distilled water to remove impurities n dry the crystals betw a few pieces of filter paper.

Question 16

Simple distillation

Answer 16

Simple distillation is used to separate a pure solvent (liquid) from a solution.

Question 17

Procedure for simple distillation

Answer 17

The saltwater is heated. Boiling chips are added for smooth boiling. At 100d.c., the water boils. The vapour rises and enters the condenser thru the exit sidearm of the distillation flask. The water vapour cools in the condenser and condense water to liquid water. Pure water is collected in the conical flask as distillate.
2. The water vapour cools in the condenser and condenses water to liquid water, pure water is collected in the conical flask as distillate.
3. As ore water vapour uses, the salt solution becomes more concentrated. Eventually, a solid residue of salt remains in the distillation flask.

Question 18

What does the thermometer measure in simple distillation

Answer 18

The thermometer measures the tempt of the vapour entering the condenser and therefore tells you the boiling point of the liquid. The tempt remains constant until all the liquid has boiled off.

Question 19

Purpose of condenser in simple distillation

Answer 19

Water enters from the bottom and exits from the top, ensuring an efficient cooling system as it prevents the condenser from heating up.

Question 20

Separating funnel

Answer 20

Separating funnel r used to separate immiscible liquids (forms a heterogenous mixture when mixed). When a heterogenous mixture of >/2 immiscible liquids r left undisturbed, they can separate into phases. Least dense liquid on top.

Question 21

Fractional distillation

Answer 21

Is used to separate 2 miscible liquids w diff boiling points.

Difference in boiling points of the liquids r not large enuf for simple distillation to work. Hence fractional distillation is used.

Question 22

Fractionating column in fractional distillation

Answer 22

1. Contains glass beads which provide a larger sa for vapour to condense on
2. Length determines its ability to separate substances
3. Liquids w similar boiling points will require a longer fractionating column for better separation.

Question 23

Procedure of fractional distillation.

Answer 23

1. As the solution is heated, both ethanol and water vapour rise up the column.
2. Water has a higher boiling point than ethanol. The water vapour condenses on the cool surfaces within the fractionating column and the liquid water returns to the flask.
3. Ethanol vapour continues to rise, at its boiling point of 78d.c., it exits the column thru the exit sidearm at the top.
4. The hot ethanol vapour cools and condenses into a liquid as it travels thru the inner tube of the condenser.
5. Liquid ethanol flows into the conical flask.

(Conical flask must be replaced once the thermometer reading rises bp of initial distillate)

Question 24

Applications of fractional distillation

Answer 24

Oil refineries, separating different substances from crude oil
Liquefied air separated to produce N2, O2, and Argon gas for industrial applications.
Ethanol produce by glucose fermentation is extracted in breweries.

Question 25

Chromatography

Answer 25

Is used to separate a mixture of substances which hv diff solubilities in a given solvent.

Question 26

What will happen if the start line is marked using ink soluble in solvent

Answer 26

The ink will dissolve in the solvent and interfere with the results obtained. Hence, start line has to be marked with insoluble in solvent substance such as graphite pencil.

Question 27

Identifying colourless substances

Answer 27

1. Using locating agent
2. Examine chromatogram under ultraviolet light

Question 28

Applications of chromatography

Answer 28

1. Identify unauthorised substances like pesticides n poison in food.
2. Detect small quantities of banned substances
3. Separate components like DNA fragments in forensic investigation

Question 29

Determining method to use when separating 2 substances

Answer 29

Thru physical properties of the components such as particle size, solubility and melting and boiling points.

Question 30

How can components separated in paper chromatographs be identified against known substances

Answer 30

By comparing the Rf value of the components against Rf value of pure substances

Question 31

Determining purity of substances

Answer 31

1. Chromatography
   - pure: only 1 spot
   - impure: more than 1 spot
   - components do not match spots of chromatograms of x -> unidentified impurity
2. Melting and or boiling point
   - pure: fixed
   - impure: over a range of tempt
   - greater amt of impurities, larger the change; increase bp, lowers mp

Question 32

Solubility table

Answer 32

In the notes.

Question 33

A student comments that the ion chromatograms give more information about mixtures than the emission spectrum.

Answer 33

The ion chromatogram provides info about the relative concentrations of the components in the mixtures but the emission spectrum does not.

The ion chromatogram can be used to identify more types of mixtures as it works for all substances that are able to dissolve in water however the emission spectrum is only suitable for elements that produce a coloured flame when burned.