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

Collection of gases

Answer 6

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 7

Drying of gases

Answer 7

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 8

Sublimation

Answer 8

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

Question 9

Substances that undergo sublimation
（I Need AirCon)

Answer 9

Iodine, ammonium chloride, naphthalene (used in mothballs)

Question 10

Process of sublimation

Answer 10

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

Step 1: In an evaporating dish, heat gently to sublime ammonium chloride.
Step 2: Place an inverted filter funnel over the evaporating dish to provide a cool surface for the NH4Cl to cool and condense the gaseous ammonium chloride to form white solid.

Question 11

evaporation to dryness

Answer 11

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 12

Crystallisation

Answer 12

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 13

Process of crystallisation

Answer 13

1. Gently heat the copper (II) surface solution in an evaporating dish to evaporate most of the liquid until a hot saturated solution is obtained, when mass of the solution becomes constant.
2. Cool the solution gradually until all the solid copper (II) sulfate crystals appear within the solution
3. Filter the mixture via filter funnel lined with filter paper to collect solid residue crystals.
4. Wash copper (II) sulfate crystals with cold distilled water (to remove impurities) and dry the crystals between a few pieces of filter paper.

Question 14

Process of simple distillation

Answer 14

1. The saltwater is heated. Boiling chips are added for smooth boiling. At 100dgC, the water boils. The vapour rises and enters the condenser through the exit sidearm of the distillation flask
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 more water vapour condenses, the salt solution becomes more concentrated. Eventually, a solid residue of salt remains in the distillation flask.

Question 15

What does the thermometer measure in simple distillation

Answer 15

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 16

Purpose of condenser in simple distillation

Answer 16

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

Question 17

Separating funnel

Answer 17

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 18

Fractionating column in fractional distillation

Answer 18

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 19

Procedure of fractional distillation.

Answer 19

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 20

Applications of fractional distillation

Answer 20

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 21

Chromatography

Answer 21

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

Question 22

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

Answer 22

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 23

Identifying colourless substances

Answer 23

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

Question 24

Applications of chromatography

Answer 24

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 25

Determining method to use when separating 2 substances

Answer 25

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

Question 26

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

Answer 26

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

Step 1: On a chromatography paper, draw the starting line (approximately 0.5 – 1 cm from the
bottom) using a pencil.
Step 2: Apply the orange colouring, E110 and E129 to the chromatography paper.
Step 3: Place the chromatography paper into a solvent. Ensure that the start line is above the
solvent level.
Step 4: Remove the chromatography paper when the solvent front is near the top of the
chromatography paper. Draw in the solvent front using a pencil. Add a locating agent.
Step 5: Calculate the Rf values of the substances by taking distance travelled by substance divided by distance travelled by solvent.
Step 6: Compare the Rf values.

Question 27

Determining purity of substances

Answer 27

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 28

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

Answer 28

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.

Question 29

Describe and explain the process of fractional distillation of liquid air separates liquid air into nitrogen and oxygen. [3]

Answer 29

Liquid air is heated and vapourised. [1]
Vapour cools and condenses at different temperatures as the temperature gets progressively cooler higher up the column. [1]
Nitrogen will be collected at the top of the fractionating column as it has a lower boiling point but oxygen will be collected lower down the fractionating column as it has a higher boiling point. [1]

Question 30

Describe fully the steps to obtain solid CaCl2 from its amours reaction mixture. [2]

Answer 30

Filter to collect the filtrate and heat to evaporate most of the of the water until a hot saturated solution is obtained.
COOL to room temperature TO FORM CRYSTALS.
Filter to obtain crystals as solid residue, wash with cold distilled water and dry between pieces of filter paper.
Note: COLD distilled water because CaCl2 is a soluble salt. Soluble salt can be obtained using crystallisation. FWD still applies but for soluble salts, COLD dw is very important so that it does not become aqueous. Also, if H2O is evolved along with other gases, such as collecting H2 gas or CO2 gas, use a DRYING AGENT.

Question 31

Which salts can be made by precipitation reactions?

Answer 31

X y and X. They are salts that are insoluble in water which can be prepared by 2 aqueous salts and collected as residue by filtration.

Question 32

Outline a method for process 1. [2]

Answer 32

Evaporation. [1]
Dilute aqueous ammonium sulfate will be heated in an evaporating dish until most of the water has vapourised. [1]