Saturation Test

The test relies on the high electron density of the $\pi$ bond in the $ext{C}= ext{C}$ double bond, which makes alkenes susceptible to attack by electrophiles.

In the case of bromine water, the bromine molecules undergo an electrophilic addition reaction across the double bond, breaking the $\pi$ bond and forming a saturated dibromoalkane.

Because the orange-brown $ext{Br}_2$ molecules are consumed to form a colorless organic product, the disappearance of color serves as a visual indicator that the reaction has occurred.

Bromine Water Test

• Procedure: Add a few drops of orange/yellow bromine water to the unknown organic liquid and shake the mixture thoroughly.
• Positive Result: The solution decolourises immediately, changing from orange to colourless, indicating the presence of an alkene.
• Negative Result: The solution remains orange, indicating the compound is saturated (like an alkane).

Potassium Manganate(VII) Test

• Procedure: Shake the unknown substance with cold, dilute acidified $ext{KMnO}_4$ solution.
• Positive Result: The pale purple solution decolourises to form a colourless solution, and the alkene is converted into a diol (a molecule with two hydroxyl groups).
• Significance: This provides an alternative method to distinguish alkanes from alkenes, as alkanes do not possess the double bond required for this oxidation reaction.

It is vital to distinguish between the behavior of saturated and unsaturated hydrocarbons during these tests to ensure accurate identification.

• Precise Terminology: Always use the word 'colourless' rather than 'clear' or 'white' when describing a positive result. A solution can be clear but still have color (like the original bromine water).

• Observation vs. Inference: In exam questions, if asked for an 'observation', state the color change (e.g., 'orange to colourless'). If asked for an 'inference', state the functional group identified (e.g., 'contains a $ext{C}= ext{C}$ bond').

• Conditions Matter: Note that the $ext{KMnO}_4$ test must be performed under cold, dilute conditions to produce a diol; harsher conditions might break the carbon chain entirely.

• The 'Shaking' Step: Always mention that the mixture must be shaken to ensure the two phases (organic and aqueous) interact sufficiently for the reaction to occur.

• Confusing Decolourisation with Dilution: Students sometimes mistake the lightening of color due to mixing for a chemical reaction. A true positive result is a complete loss of the reagent's characteristic color.

• Ignoring Alkanes in UV: While alkanes don't react with bromine water in the dark, they can react via substitution in the presence of UV light. The 'Saturation Test' specifically refers to the rapid addition reaction that occurs without needing UV light.

• Product Identification: Remember that the product of the bromine test is a dibromoalkane, while the product of the cold $ext{KMnO}_4$ test is a diol.