Fundamentals of Experimental Chemistry: Measurements and Separation Techniques

Gas Collection and Drying

The method used to collect a gas depends on its solubility in water and its density relative to the surrounding air:

1. Water Displacement: Used for gases that are insoluble or only slightly soluble in water, such as hydrogen or oxygen.
2. Downward Delivery: Used for gases that are denser than air, such as chlorine or sulfur dioxide.
3. Upward Delivery: Used for gases that are less dense than air, such as ammonia.

Sometimes, gases must be "dried" to remove water vapour before they are used in a reaction. This is achieved by passing the gas through a drying agent:

• Concentrated Sulfuric Acid: Effective for most gases (like $HCl$), but cannot be used for ammonia because they react.
• Quicklime (Calcium Oxide): Used for ammonia, but cannot be used for carbon dioxide.
• Fused Calcium Chloride: Used for hydrogen and nitrogen, but not for ammonia.

Separation of Mixtures

A significant portion of experimental chemistry involves separating mixtures into their pure components. The technique chosen depends entirely on the state of matter and the properties of the substances involved.

Separating Solid-Solid Mixtures

When two or more solids are mixed, several methods can be employed:

• Magnetic Attraction: If one component is magnetic, a magnet can pull it away from the non-magnetic solids.
• Sieving: Separates solids based on different particle sizes.
• Sublimation: If one solid can change directly from a solid to a gas (sublime) upon heating, it can be separated from a stable solid. The gas is then collected by cooling it on a surface to form a "sublimate".
• Solvent Extraction: Using a solvent that dissolves only one of the solids. For example, adding water to a sand and salt mixture will dissolve the salt, allowing the sand to be filtered out.

Separating Solid-Liquid Mixtures

To separate a solid from a liquid, we consider whether the solid is dissolved or suspended:

• Filtration: Used to separate an insoluble solid from a liquid.
• Evaporation to Dryness: The solution is heated until the solvent evaporates, leaving the solid solute behind.
• Crystallisation: A gentler method used for saturated solutions to obtain pure crystals, especially if the solute might decompose under intense heat.
• Simple Distillation: Used to separate a pure solvent from a solution by exploiting differences in boiling points.

Separating Liquid-Liquid Mixtures

Liquids can be miscible (forming a single uniform solution) or immiscible (forming distinct layers or phases):

• Separating Funnel: The standard tool for separating immiscible liquids. The denser liquid settles at the bottom and is drained out first.
• Fractional Distillation: Used to separate miscible liquids with different boiling points. A fractionating column filled with glass beads provides a large surface area for repeated condensation and evaporation, allowing for much finer separation than simple distillation. This is used industrially to refine crude oil and separate liquefied air.

Chromatography and the $R_f$ Value

Chromatography is a powerful technique used to separate substances based on their different solubilities in a specific solvent. This is widely used in forensics (DNA analysis) and food safety (detecting pesticides).

To identify substances, we calculate the Retention Factor ($R_f$ value). This is a constant ratio for a specific substance in a specific solvent:

$\text{Rf value} = \frac{\text{distance travelled by the substance}}{\text{distance travelled by the solvent}}$

If a substance is colourless on a chromatogram, scientists use locating agents or ultraviolet (UV) light to make the spots visible.

Identifying Pure Substances

Finally, it is important to distinguish between a mixture and a pure substance. A pure substance is characterized by having a specific, fixed melting point and a specific, fixed boiling point under defined conditions. In contrast, mixtures often melt or boil over a range of temperatures.