Are these two sentences correct?
1. The gas rises up.

2. The water starts to come out as a gas.

Question

Are these two sentences correct? 
1. The gas rises up.

2. The water starts to come out as a gas.

Answer

Water vapor emerges (from what it comes out of) and then rises.

Water is a liquid, not a gas.

Answer

1. The gas rises/ goes up.
2. Water starts to evaporate/ is about to turn into gas.

Answer

They’re not wrong, but sound more natural like this :
1.  The gas rises  [no preposition]
2.  The water starts to come out as gas  [no article]   It could be better to say ‘escape’ for ‘come out’  if you are referring to a valve on a pressure vessel, or even emission from the ground.

Answer

However，you should  firstly write the second sentence  and then write the first sentence, that will be better for the sentences' order.

Answer

The second sentence is wrong.｀gas’ means ｀ranqi’.

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