This deep dive covers Testing for Alkenes: The Bromine Water Test within Cracking (HT) for GCSE Chemistry. Revise Cracking (HT) in Organic Chemistry for GCSE Chemistry with 24 exam-style questions and 15 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 7 of 14 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
🧪 Testing for Alkenes: The Bromine Water Test
Since cracking produces alkenes, we need to identify them. The bromine water test is the classic method:
• Reagent: Bromine water (orange/yellow solution)
• Test: Add the hydrocarbon to bromine water and shake
• Positive result (alkene): Orange bromine water turns colourless
• Negative result (alkane): Orange colour remains — no reaction
Br₂ + C₂H₄ → C₂H₄Br₂
bromine + ethene → dibromoethane
• The C=C double bond reacts with bromine in an addition reaction
• Bromine adds across the double bond, removing it from solution
Safety Note: Bromine water is toxic and should be handled with care. Always use in a well-ventilated area and wear eye protection.
Test yourself: the bromine water test
Use the test to identify unknowns and to spot what it cannot prove.
Gas A turns bromine water from orange to colourless. Gas B causes no colour change. Identify which gas is unsaturated and explain in terms of bonding.3 marks
Gas A is unsaturated — an alkene containing a C=C double bond. The double bond reacts with bromine in an addition reaction, decolourising the bromine water. Gas B is saturated (an alkane, single bonds only) so it has no C=C bond to react, and the orange colour stays.
Full marks need the bonding reasoning for BOTH gases, not just naming them.
State the type of reaction between an alkene and bromine water, and name the type of product formed.2 marks
An addition reaction; the product is a dibromoalkane, as bromine atoms add across the broken double bond.
Students often say 'displacement' or 'substitution'. It is addition because atoms are added across a double bond, not swapped.
A gas turns bromine water colourless. A student concludes it must be ethene. Explain why this conclusion is not fully justified.2 marks
The test only shows the gas contains a C=C double bond, so it is an alkene — it cannot show which alkene. Propene or butene would give the same result, so further tests are needed to identify the exact compound.
Tests what a positive result can and cannot prove. Treating it as proof of one specific compound rather than a class is the misconception being examined.
Practice questions for Cracking (HT)
What is cracking in chemistry?
Describe the conditions used in thermal cracking and state the types of product formed.