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Observations
Add bromine water and shake...
Aim: To distinguish between saturated (alkanes) and unsaturated (alkenes) hydrocarbons.
Principle: Bromine water (Br₂ aq) is orange-brown. Alkenes (C=C double bond) react with bromine in an addition reaction, decolourising the bromine water. Alkanes do not react — bromine water stays orange.
Reaction: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane) — colourless product.
Tip: look at each formula. A general alkane fits CₙH₂ₙ₊₂ (all single bonds); an alkene fits CₙH₂ₙ (one C=C double bond).
• Bromine water is toxic and corrosive — its vapour irritates eyes and lungs; use small amounts in a fume cupboard and wear goggles and gloves.
• The hydrocarbons (ethene, propene, ethane, hexane) are highly flammable — keep away from flames and sparks.
• Stopper the tube before shaking and point it away from yourself and others.
• The dibromo products are harmful — collect all mixtures as halogenated organic waste; do not pour down the sink.
• Wash hands after the experiment; treat any bromine splash on skin with plenty of water.
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