Question

Which of the following represents a system that is completely open?

a pressurized canister of helium gas

a pressure cooker

a capped soda bottle

a pot of boiling water

Answer 1

Since the question doesn't say that the pot of water has a lid on it, then that's the one I'd go for. What is open about it is that as the water boils, the very energetic water molecules can escape from the pot and "vanish into thin air", so to speak With the others, the fluid contained in each of them can't escape, not without someone opening a tap, that is.
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Answer 2

An open system is defined as that system, that is free to exchange both energy and matter with its surroundings. Let's examine the options given:

(1): Emphasis on the word "pressurized"; this suggests that the can is sealed tightly and hence, it cannot exchange matter(ie. He molecules) w/ its surroundings. Therefore, this system is not open.

(2): A pressure cooker is sealed at the top. Applying the same argument as above, we infer that this is not an open system.

(3): Emphasis on the word " capped", which implies that the bottle is sealed and again, applying the same argument as above, we infer that it is not an open system.

(4): Like @osprey has noted, it is not mentioned if the pot is closed with a lid. But given that the pot is boiling, assume that it is open and so, along the lines of osprey's answer, this system is open.

I hope this helped. Regards.