Measurement

Measuring Volume

The following are apparatus for measuring volume.

1. Beaker: To measure volumes of liquids approximately according to the graduated marks on the apparatus.

2. Volumetric flask: To accurately measure fixed volumes of liquids when solutions of flask particular concentrations need to be prepared.

3. Pipette: To accurately measure volumes of liquids when a fixed volume of solution is needed for an experiment.

4. Burette: To accurately measure (nearest 0.1 cm3) volumes of liquids which are used up in an experiment.

5. Measuring cylinder: To measure volumes of liquids with some accuracy (nearest 0.1 cm3) according to the graduated marks on the apparatus.

Syringe: To measure small volumes of liquids with some accuracy according to the graduated marks on the apparatus.

6. Gas syringe: To accurately measure volumes of gases produced in experiments according to the graduated marks on the apparatus.

Collecting Gases Produced

1. Displacement of water: Used to collect gases which are not very soluble in water, such as oxygen and hydrogen.

   

2. Downward delivery: Used to collect gases which are denser than air, such as carbon dioxide, hydrogen chloride and chlorine.

   

3. Upward delivery: Used to collect gases which are less dense than air, such as ammonia and hydrogen.

   

Drying Gases Produced

When gases produced need to be obtained dry, the moisture content has to be removed using appropriate drying agents.

1. Fused calcium chloride: This is calcium chloride which has been heated. This can be used to dry gas which does not react with     calcium chloride.
2. Concentrated sulfuric acid: This is a common drying agent but it cannot be used to dry gases which are basic.
3. Quick lime: This is a drying agent used to dry basic gases such as ammonia.

Methods of purification

Filtration

Used to separate a solid from a liquid

• Mixture goes through a funnel with filter paper, into a flask.
• Insoluble residue remains in the funnel
• Filtrate goes through and collects in flask

Crystallization

Used to separate dissolved solid from solution

• Solution is heated to increase concentration (solvent evaporates)
• A drop of solution is placed on a slide to check for crystal formation
• Solution is left to cool and crystallise.
• Crystals are filtered from solution; washed with distilled water

Simple Distillation

Used to separate a solvent from a solution

• Impure liquid is heated in a round bottom flask
• When it boils, the steam rises into the attached condenser
• Condenser cools the steams to a pure liquid and it drops into the beaker

Fractional Distillation

Used to separate miscible liquids

• Mixture is heated
• Substances, due to their different boiling points, rise in different fractions
• A mixture of gases condense on the beads in the fractional column.
• The beads are heated to the boiling point of the lowest substance, so that substance being removed cannot condense on the beads.
• The other substances continue to condense and will drip back into the flask.
• The beaker can be changed after every fraction

Separation of Two Solids mixture

• Can be done by dissolving one in an appropriate solvent
• Then filter one and extract other from solution by evaporation
• If one solid is magnetic, can use a magnet e.g. sand and iron fillings

Paper chromatography

• paper chromatography is used to separate dye mixture based on solubility and adsorption.
• Solubility is the capacity of a substance to dissolve in a solvent.
• Adsorption is the capacity of a substance to stick to surfaces.
• There are two types of paper chromatography: ascending and descending paper chromatography.

Method used to separate substances in a solvent with different solubilities

• Drop substance onto the start line (pencil) drawn on chromatography paper
• Paper is placed in beaker with solvent; the paper must touch the surface of the solvent while the line must be above the liquid
• Solvent travels up the paper by capillary action
• Different solubilities lead to different travel rates [ high solubility-> high travel rate]
  • Stationary phase is material on which separation takes place
  • Mobile phase consists of the mixture you want to separate, dissolved in a solvent.

• Interpreting simple chromatograms:

Chromatograms are the visual outputs on the chromatography paper

• Number of rings/dots = number of substances
• If two dots travel the same distance up the paper they are the same substance.

Retention factor

• is the ratio of the distance moved by a spot from the start line to the
  distance moved by the solvent.

Rf = distance moved by spot / distance moved by solvent

Used to make colorless chromatograms visible

• Dry paper in oven
• Spray it with locating agent
• Heat it for 10 minutes in oven