Properties of hydrogen

Physical properties. Hydrogen gas is colourless, odourless, and tasteless — it gives no sensory clue to its presence, which is exactly why chemists rely on a chemical test (the pop sound, described below) rather than sight or smell to identify it. It is the lightest gas known: a hydrogen molecule, $\text{H}_2$, has a molecular mass of only $2$, compared with about $29$ for the mixture of gases in air. Because it is so much lighter than air, hydrogen can be collected by the downward displacement of air method — a jar held mouth-downward fills with hydrogen from the top as the lighter gas rises into it and pushes the heavier air out below. Hydrogen is also only slightly soluble in water — not completely insoluble, but soluble enough so little that essentially none escapes into solution during a short collection — which is why it can equally well be collected by the downward displacement of water method, bubbling up through an inverted, water-filled jar. Tested with litmus paper, hydrogen gas causes no colour change at all: it is neutral, being neither an acidic gas (like carbon dioxide, which turns litmus red) nor a basic one (like ammonia, which turns litmus blue).

Chemical properties. The signature chemical property of hydrogen is combustibility: it burns readily in air or oxygen with a pale blue flame, combining with oxygen to form water.

where: $\text{H}_2(g)$ is hydrogen gas, $\text{O}_2(g)$ is oxygen gas, and $\text{H}_2\text{O}(l)$ is the liquid water formed; the equation is balanced with two molecules of hydrogen reacting with one molecule of oxygen to give two molecules of water. It is essential to separate two ideas that ICSE examiners routinely test together: hydrogen is combustible (it burns), but it is not a supporter of combustion (it does not help other things burn, the way oxygen does). A splinter burns in oxygen more brightly because oxygen supports the burning; the same splinter, if merely surrounded by hydrogen with no oxygen present, does not burn faster — hydrogen itself has nothing to combine with. Hydrogen also acts as a reducing agent. Passed over certain heated metal oxides, it removes the oxygen from the oxide, reducing the metal oxide to the free metal while hydrogen itself is oxidised to water. The classic classroom example uses black copper(II) oxide:

where: $\text{CuO}(s)$ is solid black copper(II) oxide, $\Delta$ over the arrow marks that heat is supplied, $\text{Cu}(s)$ is the reddish-brown metallic copper produced, and $\text{H}_2\text{O}(l)$ is the water formed as a by-product. The colour change from black $\text{CuO}$ to reddish-brown $\text{Cu}$ is itself visible evidence that reduction has occurred, and a fine film of moisture (the water) is often seen condensing on the cooler part of the test tube.

Every one of hydrogen's properties traces back to the same fact: an $\text{H}_2$ molecule is the smallest, lightest molecule that exists, made of just two protons and two electrons. Because it is so light, a given volume of hydrogen weighs far less than the same volume of air, so hydrogen floats upward relative to air — the basis for the downward displacement of air method and for the familiar sight of a hydrogen balloon rising. Because the molecule is small, non-polar, and forms no strong attraction to water molecules, only a tiny fraction dissolves — hence the low, but not zero, solubility that still permits collection over water. Because hydrogen contains no acidic hydrogen ion in solution and no basic group, it does not affect litmus at all. On the chemical side, the hydrogen–hydrogen bond in $\text{H}_2$ is strong, but once broken (by the heat of a flame or spark) each hydrogen atom combines readily and exothermically with oxygen to form the very stable water molecule — this large release of energy is what makes hydrogen combustible and gives the flame its characteristic pale blue colour and the ignition its sharp pop. The same appetite for combining with oxygen is precisely what makes hydrogen a reducing agent: when hydrogen gas is passed over a heated metal oxide, it is thermodynamically more favourable for the oxygen atoms to leave the metal and join hydrogen (forming water) than to stay bound to the metal, so the metal is left behind in its free, reduced form. In short, hydrogen's small size explains its physical lightness, while its strong pull toward oxygen explains both its combustibility and its reducing power — two sides of the same underlying tendency.

Question: A student sets up the standard laboratory preparation of hydrogen (granulated zinc and dilute sulphuric acid in a flask, gas delivered through a tube into an inverted test tube held over water). After collecting a tube of gas, the student wants to (a) confirm it is hydrogen and (b) then use some of the same gas to show hydrogen's reducing action on heated copper(II) oxide. Describe both steps and the expected observations.

Step 1. To confirm the gas, keeping the collected test tube inverted (mouth down, so no gas escapes upward and is lost), bring a lit splinter close to its open mouth.

Step 2. If the gas is hydrogen, it ignites immediately and burns with a characteristic squeaky "pop" — the rapid combination of hydrogen with the oxygen of the surrounding air happens almost explosively in the confined space at the tube's mouth, producing the pop sound. This test is specific enough to common laboratory gases that a positive pop confirms hydrogen with confidence.

Step 3. To demonstrate the reducing action, a fresh stream of hydrogen gas (generated continuously from the same zinc–acid flask) is passed over black copper(II) oxide powder heated strongly in a hard glass tube, following $\text{CuO}(s) + \text{H}_2(g) \xrightarrow{\Delta} \text{Cu}(s) + \text{H}_2\text{O}(l)$.

Step 4. As the reaction proceeds, the black copper(II) oxide changes colour to reddish-brown, confirming that metallic copper has formed, and droplets of water (the other product) condense on the cooler walls of the tube just beyond the heated zone.

Answer: the pop test confirms hydrogen by combustibility, and the colour change from black CuO to reddish-brown Cu, together with condensing water, confirms hydrogen's reducing action.

• Saying hydrogen "supports combustion" — it burns itself, but does not help other substances burn the way oxygen does; these are opposite roles that examiners deliberately test together.
• Describing the confirmatory test vaguely as "it catches fire" instead of naming the specific squeaky "pop" sound, which is the detail actually being examined.
• Treating hydrogen as completely insoluble in water — it is only slightly soluble, which is exactly why collection over water still works without significant gas loss.
• Confusing the reducing-agent property (hydrogen removing oxygen from CuO to leave copper metal) with an acid–base property — hydrogen gas itself is neutral to litmus and carries no acidic or basic character.

Ask the tutor: Want to try more practice questions on identifying hydrogen's properties or work through another reducing-agent example step by step?