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org02 Testing for ethyne with bromine water and with silver salt solution

Ethyne (acetylene) molecules can react as an acid  by donating a hydrogen atom without its electron (H⁺, "proton") to a base. As an unsaturated hydrocarbon it is also "hungry" for atoms of other substances like bromine solution.
What you need: A device to develop gas (= injection bottle closed with a stopper which is pierced by the blunted needle of a 10-ml syringe and by a plastic tubing).  Water in the syringe, a small piece of calcium carbide in the bottle. 1 ml diluted silver nitrate solution with a drop of conc. ammonia solution (to test for acidity). 1 ml bromine water (to test for unsaturated property)  made from a potassium bromide solution by bubbling chlorine through it.
Experiment to test for acidity (left photo):  1. Drop water from the syringe on the calcium carbide.
2. Bubble the gas through the solution with silver nitrate and ammonia  (bottle in the middle of the photo).
Observation: A solid substance is formed in the right bottle of the photo. The mixture is transfered on a piece of tissue paper. Middle: A tiny quantity of the dried solid (aside a match) explodes when the paper is lit on a tile.  Explanation:       C₂H₂(g) + 2 ([Ag⁺] OH^-) (aq) ---> Ag₂C₂(s) + 2 H₂O          (Ag₂C₂(s)= silver acetylide).
Experiment with bromine water (Photo 3): 1. Fill a 10-ml syringe with acetylen.
2. Slowly bubble the gas through a small bottle with 1 ml bromine water.    3. Close it and shake.
Observation right photo: The brown colour and the smell of bromine disappear.
Explanation using molecule models:
A colourless compound (C₂H₂Br₂) was formed by bromine and acetylen molecules
C₂H₂ + Br₂ ----addition----------> C₂H₂Br₂  . This reaction is called addition, the compound is 1,2-dibromo ethene. -------->   In the left photo models of acetylene and chlorine molecules made by welding plastic beads can be seen. Right photo: Two different models of 1,2-Dibromo ethene. This substance exists as two isomers: In trans-1,2 dibromo ethene molecules the two bromine atoms are positioned on different sides of the C=C plane, in  cis-1,2 dibromo ethene they are on the same side.