Let's look at what we have with tin metal (Sn) dissolved in hydrochloric acid (HCl).
Sn (s) + 2 HCl (aq) → SnCl2 (aq) + H2 (g)
In the above reaction, we have no chlorine gas (Cl2), but we do form hydrogen gas, and stannous chloride in the reaction.
Of coarse you will have other metals in solution also, if you will check your reactivity series of metals chart, you will see most of the base metals are above hydrogen on the chart, so these metals will also act similar, displacing hydrogen from the acid in the reaction.
In the reaction above the HCl acid is reduced to chlorides (by the tin metal), as the tin or other metal are oxidized to ions (by the acid), leaving both the tin and chloride ions in this solution.
The acid contains water even if you did not dilute the acid,32 % HCl is 68% water.
The tin metal forming the tin cation, the acid forming the chloride anion, and these Ions in solution forming a dissolved salt of tin chloride or stannous chloride SnCl2 in solution.
The Hydrogen gas (H2) escapes the solution as vapors
We do not want to seal this vessel, and leave that hydrogen gas with no escape route, which could blow our container under the pressure this gas would form in a closed sealed vessel.
If the reaction was run to completion (no metal left to oxidize, or no free acid left to be reduced, or both) then we could seal the vessel as the stannous chloride would not build pressure in the vessel as the Hydrogen gas would no longer be forming in the tin salt solution.
