02 Experimental Chemistry

Chemistry

Collection of gases

Depends on two factors

• Solubility of the gas in water

• Density of the gas as compared to air

• Displacement of water: gas is insoluble/slightly soluble in water

• Downward delivery: gas is denser than air

• Upward delivery: gas is less dense than air

Collecting a dry sample of gas

Concentrated sulfuric acid

• used to dry acidic gases

• cannot be used to dry alkaline gases as it reacts with them

Quicklime (calcium oxide)

• used to dry alkaline gases

• cannot be used to dry acidic gases as it reacts with them

Fused calcium chloride

• can be used to dry most gases

• cannot be used to dry ammonia

Properties of common gases

• memorise colour, solubility in water and formula

• see notes for bigger table

Definitions of chemistry terms

Atom

• smallest particle of an element that has the chemical properties of the element

Molecule

• two or more atoms covalently bonded together

Element

• pure substances that cannot be split into two or more simpler substances by physical or chemical means

Compound

• substance which contains two or more elements chemically combined in a fixed composition

Mixture

• two or more substances physically mixed in any proportion

Differences between compunds and mixtures

What are the different methods of separation?

1. filtration

2. separating funnel

3. crystallization

4. evaporation

5. sublimation

6. simple distillation

7. fractional distillation

8. paper chromatography

Filtration

• used to separate a mixture of an insoluble solid and a liquid/solution

• residue: solid that remains on filter paper

• filtrate: liquid that passes through the filter paper

Use of a separating funnel

• used to separate immiscible liquids

• mixture is poured into the funnel and the layers are allowed to separate out

• The lower, more dense layer is run off by opening the tap and is collected first

Crystallisation

• used to recover a soluble solid from a liquid

• solution is first heated to make it saturated

• saturated solution is cooled to form crystals

• filter mixture to obtain crystals as residue

• crystals are washed with a small amount of cold, distilled water

• drying of the crystals between filter paper

• employed for substances whose solubility differs appreciably with temperature (solubility must decrease significantly with decreasing temperature)

• used for substances that will decompose on heating

Evaporation to dryness

• used to recover souble solid from a liquid

• recover solutes whose solubility do not change much with temperature

• not used for substances that may decompose on heating

• not used for crystals which lose their water of crystallisation and collapses into powders on heating

Sublimation

• process by which a substance in the solid state changes to the gaseous state without passing through the immediate liquid state

• separate a solid mixture from a sublimable component from a non-sublimable component by heating the mixture