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How can free chlorine be recognized by its colour and smell?

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A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

Free chlorine can be identified by its distinct physical characteristics:

Colour: At room temperature and atmospheric pressure, chlorine exists as a greenish-yellow gas.1
Smell: It possesses a characteristic choking odour.1

It is important to exercise extreme caution, as chlorine is highly toxic. Inhalation can lead to severe health consequences, including constriction of the chest, throat tightness, suffocation, and, in cases of severe exposure, edema of the lungs. Even low concentrations can be dangerous, and exposure to as little as one part per thousand in the air can be fatal within minutes.1