Laboratory preparation of hydrogen

A handful of zinc granules, a splash of dilute sulphuric acid, and within seconds a steady stream of colourless bubbles is racing up a delivery tube — this is the classic laboratory preparation of hydrogen, one of the most reliably examined practicals in ICSE Class 9 Chemistry Chapter 6, "Study of the First Element – Hydrogen." The syllabus deliberately asks for a method other than electrolysis (that is reserved for Class 10), which means the whole story has to be told through a metal-acid displacement reaction. This article builds that story from the ground up: why zinc and not magnesium or copper, why dilute sulphuric or hydrochloric acid and never dilute nitric acid, how the gas is washed and dried to purity, how it is collected by downward displacement of water, and how the trusted "pop" test confirms it. Every number and equation here matches the Pass-1 groundwork exactly; this version simply gives you the reasoning, a fully worked practical answer, and exam-ready drill.

The core insight

Hydrogen is prepared by reacting a moderately reactive metal (zinc) with a dilute, non-oxidising acid (dilute $H_2SO_4$ or dilute $HCl$ ), then washing, drying, and collecting the gas by downward displacement of water, confirmed by the characteristic "pop" test.

The core idea

1. The reaction is a metal-acid displacement. Zinc is more reactive than hydrogen (it sits above hydrogen in the activity series), so it displaces hydrogen from a dilute acid:

Zn + H_2SO_4 (\text{dilute}) \rightarrow ZnSO_4 + H_2\uparrow

Zn + 2HCl (\text{dilute}) \rightarrow ZnCl_2 + H_2\uparrow

where: $Zn$ is granulated zinc metal, the dilute acid supplies $H^+$ ions in solution, and $H_2\uparrow$ is the hydrogen gas released as bubbles. The metal sulphate or chloride formed stays dissolved in solution.

2. Zinc is the deliberate choice, not an arbitrary one. A more reactive metal such as magnesium reacts far too vigorously and uncontrollably with dilute acid to be practical in a school laboratory, while a less reactive metal such as copper lies below hydrogen in the activity series and simply does not react with dilute acids at all. Zinc gives a moderate, steady, controllable rate of bubbling — fast enough to collect gas in reasonable time, slow enough to manage safely.

3. Purification removes two kinds of impurity. Commercial zinc granules carry trace impurities (lead, arsenic and similar metals) that also react with the acid, releasing small amounts of foul-smelling gases alongside the hydrogen. The evolved gas is therefore passed first through a wash bottle of water (removing acid spray carried over in the bubbling) and then through a drying agent — anhydrous calcium chloride or concentrated sulphuric acid — which removes water vapour, leaving pure, dry hydrogen.

4. Collection exploits two physical properties of hydrogen. Hydrogen is only very slightly soluble in water and is far lighter than air. Both properties make downward displacement of water the preferred collection method: a gas jar filled with water is inverted over a pneumatic trough, and the delivery tube's mouth is placed beneath it so rising gas bubbles push the water out, leaving a jar of essentially pure hydrogen. Upward displacement of air is also possible (since hydrogen is lighter than air and will rise to the top of an upright jar), but it does not double as a purity check the way water displacement does, so water displacement is the standard classroom method.

Figure

A labelled diagram of zinc reacting with dilute acid in a flask releasing hydrogen through a wash bottle and a drying tube into a gas jar collected by downward displacement of water over a trough

Why it works

There is no numerical formula to derive here — this is qualitative, descriptive laboratory chemistry — but every design choice in the apparatus traces back to one chemical fact: dilute acids are non-oxidising sources of $H^+$ that zinc can safely displace, while everything else is chosen to keep the resulting gas moving and pure.

Why the acid must be non-oxidising. Dilute nitric acid ( $HNO_3$ ) is a strong oxidising agent even in dilute form. Although zinc does react with it, the nascent hydrogen produced is immediately oxidised by the nitric acid itself to form water, rather than escaping as $H_2$ gas — instead, the nitrogen of $HNO_3$ is reduced to nitrogen oxides ( $NO$ or $N_2O$ ), which bubble off in place of hydrogen. This is exactly why dilute nitric acid is excluded from the syllabus method: only dilute $H_2SO_4$ or dilute $HCl$ , which do not oxidise the hydrogen they liberate, will do.

Why the thistle funnel's stem dips below the acid. If the acid is added through a thistle funnel whose lower end sits above the acid surface, gas generated in the flask finds the funnel's open path the route of least resistance and escapes back up through it instead of travelling along the delivery tube to the purification train. Keeping the stem submerged forces all the gas through the intended path.

Why washing comes before drying. The wash bottle removes acid spray carried along with the gas (droplets, not vapour), while the drying tube afterwards removes water vapour specifically. Skipping either leaves the gas contaminated.

Worked example

Question: Describe, step by step, how hydrogen gas is prepared in the laboratory using zinc granules and dilute sulphuric acid, including apparatus and precautions.

Step 1. Take granulated zinc in a flat-bottomed flask (or a Kipp's apparatus) fitted with a thistle funnel and a delivery tube.

Step 2. Add dilute sulphuric acid slowly through the thistle funnel, keeping its lower end dipped below the acid level inside the flask, so that evolving gas cannot escape back up the funnel.

Step 3. The reaction begins at once, $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$ , seen as brisk effervescence — a steady stream of bubbles — around the zinc granules.

Step 4. The evolved gas is led through a wash bottle of water to remove acid spray, then through a drying tube of anhydrous calcium chloride to remove moisture, leaving pure, dry hydrogen.

Step 5. The dried gas is collected in a gas jar by downward displacement of water: the jar is filled completely with water, inverted over the pneumatic trough, and the delivery tube's mouth is positioned beneath its rim so that rising bubbles displace the water out of the jar.

Step 6. Once collection is complete, the jar is covered with a glass plate while still underwater, then lifted out and stored mouth-down (since hydrogen is lighter than air and would otherwise escape upward if stored mouth-up).

Step 7. To confirm the gas, a lit splint is brought near the jar's mouth as the glass plate is briefly slid aside — a sharp "pop" sound confirms hydrogen.

Answer: the reaction $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$ produces hydrogen gas, which is washed, dried, collected by downward displacement of water, and confirmed by the "pop" test.

Competitive stretch

Question: A student uses dilute nitric acid instead of dilute sulphuric acid with zinc and obtains almost no hydrogen gas. Explain why.

Step 1. Dilute nitric acid is a strong oxidising agent even in dilute form.

Step 2. Zinc does react with it, but the nascent hydrogen formed is oxidised by the $HNO_3$ itself to water, rather than being released as $H_2$ gas.

Step 3. In its place, the nitrogen of $HNO_3$ is reduced to nitrogen oxides ( $NO$ or $N_2O$ ), which are evolved instead of hydrogen.

Answer: almost no hydrogen appears because the nascent hydrogen is consumed by the oxidising acid, not because zinc fails to react — dilute $HNO_3$ must never be used for this preparation.

Common traps

Believing any dilute acid works — dilute nitric acid must never be used, since it oxidises the liberated hydrogen instead of releasing it as gas.
Forgetting why the thistle funnel's stem must dip below the acid surface — without this, gas escapes upward through the funnel instead of reaching the delivery tube.
Mixing up collection methods — hydrogen is collected by downward displacement of water (or, less commonly, upward displacement of air), never by upward displacement of water, since it is lighter than air and barely soluble in it.
Forgetting the purification step — unpurified hydrogen from commercial zinc can carry traces of foul-smelling gases from metal impurities, so both washing and drying matter, not just the main reaction.

Ask the tutor: Want a labelled diagram walk-through of the apparatus or a chance to test yourself further on this preparation? Ask the AI tutor.

Why can't copper be used instead of zinc to prepare hydrogen from dilute acid?

Copper lies below hydrogen in the activity series, meaning it is less reactive than hydrogen itself and cannot displace it from an acid — there is no driving force for copper atoms to lose electrons to the acid's $H^+$ ions under these conditions. So copper simply does not react with dilute sulphuric or hydrochloric acid at all, and no gas is produced. Only metals above hydrogen in the activity series, such as zinc, magnesium or iron, can displace hydrogen this way, and among these zinc is chosen for its safe, moderate reaction rate.

What happens if I forget to dry the hydrogen gas before collecting it?

The collected gas would still burn and give the "pop" test, since the moisture does not stop hydrogen from being combustible, but the sample would no longer count as pure, dry hydrogen for the purposes of the practical. Water vapour carried along with the gas can also condense inside the delivery tube or apparatus, and in some follow-up experiments (such as reducing a metal oxide with hydrogen) residual moisture can interfere with the reaction being studied. This is why the anhydrous calcium chloride or concentrated sulphuric acid drying step is treated as a required part of the preparation, not an optional extra.

Why is downward displacement of water preferred over upward displacement of air for collecting hydrogen?

Both methods work in principle, since hydrogen is lighter than air, but downward displacement of water is preferred in the school laboratory because it doubles as a rough purity indicator — you can see the gas bubbling cleanly through water and filling the inverted jar, with no evidence of any accompanying froth or contamination. Upward displacement of air, by contrast, gives no such visual check, and it also risks some gas escaping around the mouth of an upright jar since hydrogen is so much lighter than air. Downward displacement of water is therefore the standard method taught and examined.