Hydrogen: Preparation, Properties, and Reactions
Hydrogen is the lightest element, and its molecular gas \(H_2\) combines simple structure with important reactions and strict safety requirements.
1. Molecular hydrogen
At ordinary conditions hydrogen is a colorless, odorless gas made of diatomic \(H_2\) molecules. It has very low density and is only slightly soluble in water. Low density makes escaping hydrogen rise and disperse in open, well-ventilated conditions, but it can accumulate where ventilation is poor. Hydrogen cannot be identified safely by smell.
2. Laboratory preparation
Some metals above hydrogen in the activity series displace hydrogen ions from dilute acids. A common controlled example is zinc with dilute hydrochloric acid:
The reaction is single displacement: zinc forms zinc ions while hydrogen ions form \(H_2\). Because hydrogen is only slightly soluble in water and does not react rapidly with it at ordinary conditions, it can be collected over water in an inverted jar. The first gas leaving the apparatus contains air that was already present, so it must not be used for the flame test.
3. Identification and combustion
A small separate sample of hydrogen gives a characteristic squeaky pop when ignited at the mouth of a test tube. The sound results from rapid reaction with oxygen. The test is performed only after air has been flushed from the preparation apparatus and only under trained supervision.