Chapter 3 · Question 5
With the help of Activity 3.11, describe how metals react with dilute acids. Explain why hydrogen gas is not evolved when metals (except Mg and Mn) react with dilute nitric acid. What is aqua regia and what makes it special?
Q5
With the help of Activity 3.11, describe how metals react with dilute acids. Explain why hydrogen gas is not evolved when metals (except Mg and Mn) react with dilute nitric acid. What is aqua regia and what makes it special?
Answer Revealed
Direct Answer:
Metals react with dilute acids to produce a salt and hydrogen gas: Metal + Dilute acid → Salt + H₂↑. Activity 3.11 shows that Mg, Al, Zn, and Fe react with dilute HCl with decreasing vigour (Mg > Al > Zn > Fe), with Mg giving the highest temperature rise. Copper does not react at all. However, nitric acid (HNO₃) is a strong oxidising agent — it oxidises the H₂ produced to H₂O and is itself reduced to nitrogen oxides (NO, NO₂, N₂O), so H₂ is not evolved. Only Mg and Mn produce H₂ with very dilute HNO₃. Aqua regia is a freshly prepared 3:1 mixture of concentrated HCl and concentrated HNO₃. It is one of the few reagents capable of dissolving gold and platinum.
Simple Explanation
When you drop a metal into acid, it fizzes — that is hydrogen gas escaping as the metal takes the place of hydrogen in the acid. Magnesium reacts the fastest and gets the hottest. But there is one acid that breaks the rules: nitric acid. It is so good at grabbing oxygen atoms that it turns the hydrogen into water before it can escape as a gas. Only magnesium and manganese can still release H₂ with very dilute HNO₃. As for aqua regia (Latin for 'royal water'), it is a 3:1 cocktail of concentrated HCl and HNO₃ — so powerful it can dissolve gold, which neither acid can do alone.
Exam-Ready Structure
The reaction of metals with acids forms a central part of NCERT Chapter 3 and the reactivity series. 1. General equation: Metal + Dilute acid → Salt + Hydrogen gas. 2. Activity 3.11 (Reaction with dilute HCl): Metals tested are Mg, Al, Zn, Fe, and Cu. (a) Rate of bubble formation (H₂ gas) is fastest with Mg, which also shows the greatest temperature rise — the reaction is highly exothermic. (b) The decreasing order of reactivity with HCl: Mg > Al > Zn > Fe. (c) Copper does not react — no bubbles, no temperature change. (d) Equations: ; ; ; . 3. The special case of HNO₃: Dilute nitric acid is a strong oxidising agent. Instead of the expected reaction producing H₂ gas, HNO₃ oxidises the hydrogen to water ( → ) and gets itself reduced to oxides of nitrogen (, NO, or NO₂). Hence, H₂ gas is not evolved with HNO₃. Exceptions: Magnesium and manganese react with very dilute HNO₃ to evolve H₂. 4. Aqua regia (Latin: royal water): (a) It is a freshly prepared mixture of concentrated HCl and concentrated HNO₃ in a 3:1 ratio. (b) It is a highly corrosive, fuming liquid. (c) Neither HCl nor HNO₃ alone can dissolve gold (Au) or platinum (Pt), but aqua regia can — this is its signature property. (d) The NCERT references this in the context of a real-world puzzle where a goldsmith uses aqua regia to 'clean' gold ornaments, actually dissolving a layer of gold and reducing their weight.
Key Points
- Metal + Dilute acid → Salt + H₂↑ (general reaction)
- Activity 3.11: decreasing reactivity with HCl — Mg > Al > Zn > Fe; Cu does not react
- HNO₃ is a strong oxidising agent: it converts H₂ to H₂O and produces nitrogen oxides instead
- Only Mg and Mn evolve H₂ with very dilute HNO₃
- Aqua regia = conc. HCl : conc. HNO₃ in 3:1 ratio; dissolves gold and platinum
Related Questions
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Describe how different metals react with water. Classify the metals into groups based on their reactivity with cold water, hot water, and steam. Write chemical equations for sodium, calcium, and iron reacting with water/steam.
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