Identify the reagent class before predicting products. A suitable metal with dilute hydrochloric or sulfuric acid can form a salt and hydrogen. A metal oxide or hydroxide forms salt and water. A carbonate also produces carbon dioxide.
The acid supplies the salt’s negative ion: hydrochloric acid gives a chloride; sulfuric acid gives a sulfate. The metal supplies the positive ion. In the reactions used here, iron forms an iron(II) salt with these dilute acids.
Copper metal does not react with dilute hydrochloric acid under the conditions used here. Copper(II) oxide can react with an acid: the metal and its oxide are different substances. These questions do not cover nitric acid or every possible metal and acid combination.
Gas bubbles show that a gas may be forming, but they do not identify it. Hydrogen and carbon dioxide require different tests. The equation, predicted gas and observation must agree.
Worked example
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The carbonate provides calcium for the chloride salt; the remaining products include carbon dioxide.
MgO + 2HCl → MgCl₂ + H₂O. Replacing the carbonate by an oxide removes the carbon dioxide product.
Specification references
AQA 4.4.2.1–4.4.2.2; Edexcel 3.11–3.14. These examples cover part of the course. Other reactions still need practice.
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Explain why copper metal and copper(II) oxide should not be given the same prediction when each is added separately to dilute hydrochloric acid.
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Distinguish the substances.
Use the specified dilute-acid context.
Connect the oxide to neutralisation.
Example answer
Copper metal does not displace hydrogen from dilute hydrochloric acid under the stated conditions. Copper(II) oxide is a base that reacts with the acid to form copper(II) chloride and water. Neither the shared element name nor a balanced-looking equation makes the two reactions equivalent.
An answer to improve
Both contain copper, so both produce copper chloride and hydrogen.
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Replace an unsupported generalisation with the named substances, conditions and evidence. Add the causal link that your first explanation omitted.
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