Acids, Alkalis & Hydrogen
Displacing hydrogen from dilute acids and hot alkalis.
What this lesson covers
Why it matters
Zinc is a metal, yet it fizzes with both acid and alkali, releasing the same flammable gas. How can one element behave like two different types of matter?
The idea in plain words
Analyze the equation: Zn + 2NaOH → Na₂ZnO₂ + H₂. Identify the salt formed and confirm hydrogen is the gas.
Predict first
Why does zinc react with sodium hydroxide, unlike most other metals?
Zinc is amphoteric: it reacts with acids to form salts and with strong alkalis to form complex salts and hydrogen.
- Zinc is more reactive than sodium
- Zinc is amphoteric and dissolves in strong alkalis — correct
- The heat from the reaction melts the zinc
- Zinc contains hidden acid inside it
What you do
First, add zinc to dilute HCl. Note the effervescence. Then, add zinc to concentrated NaOH and heat. Compare the gas produced in both.
Check yourself
Which observation confirms the gas evolved from Zn + dilute HCl is hydrogen?
Hydrogen burns with a characteristic 'pop' sound when tested with a burning splinter.
What is the product formed when zinc reacts with sodium hydroxide?
The reaction is Zn + 2NaOH → Na₂ZnO₂ + H₂. The salt Na₂ZnO₂ is sodium zincate.
Why does copper not react with dilute hydrochloric acid?
Only metals above hydrogen in the reactivity series can displace hydrogen from dilute acids. Copper is below hydrogen.
- It turns lime water milky
- A burning splinter pops with a squeaky sound — correct
- It relights a glowing splinter
- It turns blue litmus red
- Zinc hydroxide and oxygen
- Sodium zincate and hydrogen — correct
- Zinc oxide and water
- Sodium chloride and hydrogen
- Copper is below hydrogen in the reactivity series — correct
- Copper is a non-metal
- Copper reacts only with hot acids
- Copper forms a protective oxide layer instantly