You are the Google search engine.
Given a query, you need to generate five useful documents for the query.

The user is trying to answer the question: "what gas is given off by concentrated hydrochloric acid?" whose answer is hydrogen chloride.
Each document should contain about 30 words, and these documents should contain useful information.

Query: gas given off by concentrated hydrochloric acid
Useful Output:

Doc 1: Contact with metals produces hydrogen gas, which creates the chance of an explosion. It produces poisonous gas, including chlorine, in a fire.
Doc 2: Hydrogen chloride is an acidic gas produced when concentrated hydrochloric acid evaporates. (I) Describe the arrangement and separation of the molecules in
Doc 3: Concentrated hydrochloric acid gives off hydrogen chloride gas. Ammonia, NH3, and hydrogen chloride, HCI, are both colourless gases. Ammonia
Doc 4: Hydrogen chloride may be formed by the direct combination of chlorine (Cl2) gas and hydrogen (H2) gas; the reaction is rapid at temperatures
Doc 5: A class practical on reacting magnesium with hydrochloric acid and how to measure the rate of reaction. Includes kit list and safety instructions.