Final answer:
The standard state of matter of chlorine at 25°C is a gas, as it exists as a diatomic gas under standard temperature and pressure conditions.
Step-by-step explanation:
The standard state of matter of chlorine at 25°C is gas. At room temperature, chlorine is found as a diatomic gas, which means it consists of molecules made of two chlorine atoms bonded together. Chlorine, being a halogen, exists in this gaseous form under standard conditions of temperature and pressure, which include 25°C (room temperature) and 1 atmosphere of pressure.
Matter most commonly exists as a solid, liquid, gas, or plasma. At 25°C, chlorine does not exist as a liquid or solid under standard pressure. Plasma is a high-energy state that does not commonly occur on Earth at standard conditions; hence, it is safe to say that chlorine is not in a plasma state at 25°C either.