Chlorine - Cl2
Almost 100% pure chlorine gas in a 300 ml glass bottle, prepared by adding some dilute HCl to solid calcium hypochlorite. The gas should not be stored, it should be prepared when needed. When the gas is stored in a bottle, then it slowly eats the cap of the bottle and the escaping chlorine gas will corrode everything stored nearby.
Below is a picture of a more durable display of chlorine gas. It is a small quartz ampoule, which contains a tiny amount of liquid chlorine, with the gas above it. The gas is at high pressure inside the quartz ampoule. The liquid is bright yellow, and the gas is green. This ampoule only has a length of 3 cm, and even this thin layer of gas already is clearly visible, indicating that there is quite some pressure inside.
Another nice sample of chlorine is in a glass sphere of approximately 6 cm diameter. The gas inside is pure chlorine at approximately normal pressure.
The picture below shows a glass ampoule which is filled with cold and heavily dried chlorine gas in order to obtain a clearly visible display of the gas. The gas was blown into the ampoule and then quickly sealed. It is at slight overpressure at normal temperature.