....

Cola contains a lot of sugar. Yeast can live in sugar solution even if no air is available.
* Left: Threequarter fill an infusion bottle with stale Cola * Add a spatula of dry yeast, shake vigorously.
* Close by an empty balloon in order to stop air supply.
Obervation Photo 2 + 3: A foam is built and a gas can be collected in the balloon.
* Photo 4: Suck the gas from the baloon into a 100-mL syringe, blow it into lime water.
Observation 2, explanation  right : The gas makes lime water  milky because yeast transforms sugar of Cola into carbon dioxide.