Experimental Techniques & Chemical Analysis

Question 1

Apparatus for Measurement (7)

• Time
• Temperature
• Mass
• Volume

Answer 1

Time: Stopwatches

Temperature: Thermometers

Mass: Balances

Volume: Burettes, Volumetric pipettes, Measuring cylinders, Gas syringes

Question 2

Definition of solvent, solute, solution, saturated solution, residue and filtrate

Answer 2

Solvent: Dissolves a solute

Solute: Dissolved in a solvent

Solution: Mixture of solute dissolved in a solvent

Saturated Solution: Maximum concentration of solute at specified temperature

Residue: Remains after evaporation, distillation, filtration

Filtrate: Liquid that passes through a filter

Question 3

Acid-Base Titration
- Apparatus Used (3)
- End-Point Identification (3)

Answer 3

Apparatus Used:
(a) Burette: Used to deliver the titrant solution (acid/base) into the analyte solution.
(b) Volumetric Pipette: Used to measure precise volumes of solutions.
(c) Suitable Indicator: Changes color at the endpoint of the titration, indicating when the reaction is complete.

The end-point is identified by the indicator’s color change.

(a) Add the indicator to the analyte solution.

(b) Titrate with the titrant solution.

(c) The indicator will change color at the end-point, indicating the completion of the reaction.

Question 4

Chromatography

• Separating coloured compounds
• Separating colorless compounds
• Interpreting Chromatograms
• Calculating Rf

Answer 4

Paper Chromatography:
Used to separate mixtures of soluble colored substances.

Involves placing a spot of the mixture on chromatography paper and allowing it to move up the paper in a suitable solvent.
Different substances travel at different rates, resulting in separation based on their affinity for the solvent and paper.

Interpreting Chromatograms:
(a) Unknown substances can be identified by comparing their migration distances with those of known substances.
(b) Pure substances appear as single spots, while impure substances show multiple spots due to the presence of different components.

Separating Colorless Substances: Paper chromatography can also separate mixtures of soluble colorless substances using a suitable solvent and locating agent.

Calculation of Rf (Retention Factor): Rf = distance traveled by substance / distance traveled by solvent

Rf value helps identify substances based on their migration distances.

Question 5

• Separation and Purification Methods (5)

Answer 5

Methods of Separation and Purification:

(a) Suitable Solvent: Used in techniques like extraction to dissolve specific components from a mixture.

(b) Filtration: Separates solid particles from a liquid by passing through a porous material.

(c) Crystallization: Allows dissolved substances to form crystals as the solvent evaporates, separating them from the solution.

(d) Simple Distillation: Separates components based on differences in boiling points, suitable for purifying liquids with significantly different boiling points.

(e) Fractional Distillation: Separates components with closer boiling points by multiple distillations, utilizing a fractionating column.

Question 6

Choosing suitable separation & purification techniques

Accessing purity

Answer 6

Suitable Techniques for Separation and Purification:

Based on substance properties like solubility, volatility, and boiling points.
Examples include using suitable solvents for extraction, filtration for solid-liquid separation, crystallization for purifying solids, and distillation for liquid purification.

Identifying Substances and Assessing Purity:

Melting point and boiling point information used to identify substances and assess purity.
Pure substances have characteristic melting and boiling points, while impurities can alter these values.

Question 7

Tests To Identify The Anions
- CO3 2-,
- Cl-, Br-, I-
- NO3 -
- SO4 2-
- SO3 2–,

Answer 7

(a) Carbonate (CO3^2–):
Reacts with dilute acid to produce carbon dioxide gas (CO2), which turns limewater milky upon bubbling through it.

(b) Chloride (Cl–), Bromide (Br–), and Iodide (I–):
Acidification with dilute nitric acid followed by addition of aqueous silver nitrate (AgNO3) results in the formation of white precipitates for silver chloride (AgCl), cream precipitate for silver bromide (AgBr), and yellow precipitate for silver iodide (AgI).

(c) Nitrate (NO3–):
Reduction with aluminum foil and aqueous sodium hydroxide (NaOH) leads to the detection of ammonia gas (NH3), which can be detected by its distinctive odor or using damp red litmus paper, turning it blue.

(d) Sulfate (SO4^2–):
Acidification with dilute nitric acid followed by addition of aqueous barium nitrate (Ba(NO3)2) causes the formation of a white precipitate of barium sulfate (BaSO4).

(e) Sulfite (SO3^2–):
Reacts with acidified aqueous potassium manganate(VII) (KMnO4), leading to the decolorization of the purple color of potassium manganate(VII), indicating reduction to colorless manganese(II) ions (Mn^2+).

Question 8

Tests To Identify The Aqueous Cations (8)

• Al 3+
• NH4 +
• Ca 2+
• Cr 3+
• Cu 2+
• Iron (II) 2+
• Iron (III) 3+
• Zn 2+

Answer 8

(a) Aluminium Al 3+:

With aqueous sodium hydroxide: Forms white gelatinous precipitate of aluminium hydroxide (Al(OH)3).

With aqueous ammonia: Precipitate of aluminium hydroxide initially forms, then re-dissolves in excess ammonia to form colorless complex ion, [Al(OH)4]^−.

(b) Ammonium, NH4^+:
with dilute sodium hydroxide: gently heating it, NH4+ will be converted to ammonia gas if present. Ammonia has a distinct odor. It also turns red litmus to blue.

(c) Calcium, Ca^2+:
With aqueous sodium hydroxide: Forms white precipitate of calcium hydroxide (Ca(OH)2), soluble in excess NaOH.

With aqueous ammonia: Forms white precipitate of calcium hydroxide

(d) Chromium(III), Cr^3+:
With aqueous sodium hydroxide: Forms green precipitate of chromium(III) hydroxide (Cr(OH)3).

With aqueous ammonia: Forms green precipitate of chromium(III) hydroxide

(e) Copper(II), Cu^2+:
With aqueous sodium hydroxide: Initially forms pale blue precipitate of copper(II) hydroxide (Cu(OH)2), then oxidizes to form dark blue precipitate of copper(II) oxide (CuO).

With aqueous ammonia: Forms deep blue precipitate of tetraamminecopper(II) hydroxide complex, [Cu(NH3)4(OH)2].

(f) Iron(II), Fe^2+:
With aqueous sodium hydroxide: Forms green precipitate of iron(II) hydroxide (Fe(OH)2), rapidly oxidizes to form brown precipitate of iron(III) hydroxide (Fe(OH)3).

With aqueous ammonia: Initially forms green precipitate of iron(II) hydroxide, then oxidizes to form brown precipitate of iron(III) hydroxide, similar to reaction with NaOH.

(g) Iron(III), Fe^3+:
With aqueous sodium hydroxide: Forms brown precipitate of iron(III) hydroxide (Fe(OH)3).

With aqueous ammonia: Forms brown precipitate of iron(III) hydroxide, similar to reaction with NaOH.

(h) Zinc, Zn^2+:
With aqueous sodium hydroxide: Initially forms white precipitate of zinc hydroxide (Zn(OH)2), then dissolves in excess NaOH due to formation of soluble zincate ion, [Zn(OH)4]^2−.

With aqueous ammonia: Forms white precipitate of zinc hydroxide, similar to reaction with NaOH.

Question 9

Tests To Identify The Gases
- Ammonia
- Carbon Dioxide
- Chlorine
- Hydrogen
- Oxygen
- Sulfur Dioxide

Answer 9

(a) Ammonia, NH3, using damp red litmus paper:

Ammonia turns damp red litmus paper blue, Ammonia is Basic

(b) Carbon dioxide, CO2, using limewater:

Carbon dioxide reacts with limewater (aqueous calcium hydroxide, Ca(OH)2), forming a white precipitate of calcium carbonate (CaCO3).
This reaction causes the limewater to turn milky.

(c) Chlorine, Cl2, using damp litmus paper:

Chlorine gas does not directly react with litmus paper.
However, it bleaches the paper, turning red litmus paper white and destroying the color of blue litmus paper.

(d) Hydrogen, H2, using a lighted splint:

Hydrogen gas ignites with a ‘pop’ sound when ignited with a lighted splint. This is due to its highly flammable nature, rapidly undergoing combustion in the presence of oxygen to form water (H2O).

(e) Oxygen, O2, using a glowing splint:

Oxygen supports combustion.
When introduced to a glowing splint, oxygen reignites it with a bright flame, enhancing combustion.

(f) Sulfur dioxide, SO2, using acidified aqueous potassium permanganate(VII) (KMnO4):

Sulfur dioxide gas reduces acidified aqueous potassium permanganate(VII) (KMnO4) to manganese(II) ions (Mn^2+).

This reaction results in the purple color of the solution turning colorless or pale pink, indicating the reduction of manganese(VII) to manganese(II).

Question 10

Flame Test To Identify The Cations

• Li +
• Na +
• K +
• Ca 2+
• Ba 2+
• Cu 2+

Answer 10

(a) Lithium, Li +:

• it gives crimson or red color to the flame. This color is often faint and may require careful observation.

(b) Sodium, Na^+:

• it gives a bright yellow flame when introduced into a flame. The yellow color is intense and easily visible.

(c) Potassium, K^+:

• it gives a lilac or violet color to the flame. This color is often quite distinct and noticeable.

(d) Calcium, Ca^2+:

• Calcium ions produce an orange-red flame when introduced into a flame. it tends to be more intense than lithium.

(e) Barium, Ba^2+:

• it gives a pale green flame when introduced into a flame. The color can be somewhat faint, so careful observation is necessary.

(f) Copper(II), Cu^2+:

• it produces a blue-green flame when introduced into a flame. This color is often quite striking and distinct from the colors produced by other cations.