Question
When zinc metal is added to dilute hydrochloric acid, what happens? Write the chemical equation and describe how you would test for the gas produced.
Solution — Step by Step
Zinc reacts with hydrochloric acid to form zinc chloride (the salt) and hydrogen gas. The pattern is: metal + acid → salt + hydrogen gas. Always identify your metal and acid before writing the formula equation.
We need 2 HCl because zinc forms Zn²⁺ — it needs two chloride ions to make zinc chloride. One HCl can only donate one Cl⁻, so we take two.
When you drop zinc granules into dilute HCl, you’ll see bubbles rising rapidly from the metal surface. The zinc slowly dissolves. The colourless gas collecting above is hydrogen.
Bring a burning splinter (or lit matchstick) near the mouth of the test tube. Hydrogen gas burns with a pop sound. This is the standard test taught for Class 7 and Class 10 board practicals.
The gas produced is hydrogen (H₂). It is confirmed by the pop test. The solution left behind contains zinc chloride, ZnCl₂.
Why This Works
Acids contain hydrogen ions (H⁺). When a reactive metal like zinc meets these ions, it displaces the hydrogen — zinc is higher in the reactivity series than hydrogen, so it “kicks hydrogen out” of the acid.
The hydrogen atoms pair up as H₂ molecules and escape as gas (shown by the ↑ arrow). This is why you see bubbling — it is not boiling, it is hydrogen gas being released.
Zinc chloride stays dissolved in the water, making the solution. If you evaporated the water, you’d get zinc chloride crystals — the salt.
The pattern metal + acid → salt + hydrogen works for most reactive metals (Zn, Fe, Mg, Al) with dilute HCl or H₂SO₄. Memorise the pattern, not each individual reaction. For boards, this pattern alone covers 4-5 marks worth of questions.
Alternative Method — Using Magnesium
Try the same experiment with magnesium ribbon instead of zinc:
Magnesium reacts much faster than zinc — the bubbling is vigorous and the test tube gets warm. This shows that different metals have different reactivities, but the product (hydrogen gas) is always the same. The pop test confirms H₂ in this case too.
This comparison is a favourite in Class 10 — examiners ask why magnesium reacts faster (it is higher in the reactivity series than zinc).
Common Mistake
Students write the equation as Zn + HCl → ZnCl + H₂ — using only one HCl. This is wrong. Zinc has a valency of 2, so it forms ZnCl₂, not ZnCl. Always check the valency of the metal when writing the salt formula. Zn²⁺ needs two Cl⁻ ions, so the correct salt is ZnCl₂ and you need 2HCl on the left side to balance.
Also, do not confuse the pop test with the glowing splinter test. The glowing splinter relights in oxygen. The burning splinter pops in hydrogen. These two tests appear together in practicals and many students swap them under exam pressure.