Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
General reaction
A reactive metal plus a dilute acid produces a salt and hydrogen: metal + acid -> salt + hydrogen. Hydrochloric acid forms chlorides and sulfuric acid forms sulfates. Nitric acid is not normally used for the simple hydrogen pattern because it behaves as an oxidising acid in many reactions.
Example equation
Magnesium reacts with hydrochloric acid: Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g). The equation conserves atoms and shows the gas product. Ionic reasoning is Mg -> Mg2+ + 2e- and 2H+ + 2e- -> H2.
Observations
Typical observations are effervescence, metal disappearing and a temperature change. Hydrogen is confirmed by applying a lighted splint to a small collected sample; a squeaky pop is heard. Safety requires small quantities and no large accumulation of hydrogen.
Reactivity limitation
Metals below hydrogen in the reactivity series, such as copper, silver and gold, do not normally produce hydrogen with dilute hydrochloric acid. Reactivity-series detail is developed in Volume 6, but acid reaction evidence can already compare metals.
Practical or data skill
React equal masses of magnesium with equal acid volumes and measure gas volume against time. Keep particle size and temperature constant.
Examination tip
Name the salt from the acid: hydrochloric acid gives a chloride; sulfuric acid gives a sulfate.
Review questions and suggested answers
Question 1
Name the gas from magnesium and dilute HCl.
Suggested answer
Hydrogen.
Question 2
Write the equation for magnesium with HCl.
Suggested answer
Mg + 2HCl -> MgCl2 + H2.
Question 3
What is the hydrogen test?
Suggested answer
A lighted splint gives a squeaky pop.
