Chemistry · Chemistry of the elements · Grade 7

Water: composition and properties

When we split water with a current, why do we get twice as much hydrogen as oxygen?

The situation

Every day we brew tea, wash dishes and drink — water seems the simplest of substances. Yet in 1800 the English scientists Nicholson and Carlisle passed an electric current through water and it broke up into two different gases. In winter the surface of a lake freezes over, while the fish at the bottom stay alive; if water is poured on quicklime, it gets hot. What elements is water made of, why does ice float on water, and how does water react with other substances?

Topic

Water: composition, properties and reactions. Splitting water with current and re-synthesising it: H₂ : O₂ = 2 : 1; water with sodium, quicklime and oxides

Key ideas

Composition of water, H₂O
The 4 °C anomaly
Water and oxides

Formulas

2 H_{2} O \overset{\overset{}{electric current}}{\to} 2 H_{2} ↑ + O_{2} ↑

Decomposition of water (electrolysis): under the same conditions $V (H_{2}) : V (O_{2}) = 2 : 1$ , and the ratio of masses is $1 : 8$ .

2 H_{2} + O_{2} \overset{}{\to} 2 H_{2} O

Synthesis: hydrogen burns in oxygen and forms water again (H. Cavendish, 1781).

ω (H) = \frac{2 A_{r} (H)}{M_{r} (H_{2} O)} = \frac{2}{18} \approx 11.1 %

The mass fraction of hydrogen in water (with school values; the exact value is 11.19 %), and of oxygen — 88.9 %.

n (H_{2}) = \frac{I t}{2 F}, n (O_{2}) = \frac{I t}{4 F}

In electrolysis one H₂ molecule needs 2 electrons and one O₂ needs 4 — so there is twice as much hydrogen ( $F$ = 96 485 C/mol).

CaO + H_{2} O \overset{}{\to} Ca (OH)_{2}

Basic oxide + water → base (alkali); acidic oxide + water → acid: $SO_{3} + H_{2} O \overset{}{\to} H_{2} SO_{4}$ .

Theory

The composition of water: decomposition and synthesis

Pure water conducts current poorly, so a little sodium sulfate is added to the water in the Hofmann apparatus. At the cathode (−) water molecules gain electrons and hydrogen is released: $4 H_{2} O + 4 e^{-} \overset{}{\to} 2 H_{2} ↑ + 4 OH^{-}$ ; at the anode (+) they give up electrons and oxygen is released: $2 H_{2} O - 4 e^{-} \overset{}{\to} O_{2} ↑ + 4 H^{+}$ . Under the same conditions equal volumes of gases contain equal numbers of molecules (Avogadro's law), so $V (H_{2}) : V (O_{2}) = 2 : 1$ shows that a water molecule has 2 H atoms and 1 O atom — the formula is $H_{2} O$ . The ratio of masses is 1 : 8. The reverse process is synthesis: in 1781 H. Cavendish exploded hydrogen and oxygen in the ratio 2 : 1 and obtained only water; in 1783–1784 A. Lavoisier proved that water is a compound. If a spark is passed through a mixture of gases in a eudiometer, in the ratio 2 : 1 no gas is left over.

Physical properties of water (practical)

Pure water is colourless, odourless and tasteless; at normal pressure (101.3 kPa) it freezes at 0 °C and boils at 100 °C, and its density is ≈ 1 g/mL. Its specific heat capacity is very large — 4.18 kJ/(kg·K): seas and lakes moderate the climate, and in heating systems water carries heat. The water molecule is bent (104.5°) and polar: the O atom is partially negative and the H atoms partially positive. So the molecules form hydrogen bonds with one another. Hydrogen sulfide ( $H_{2} S$ , $M$ = 34 g/mol), similar in structure but almost twice as heavy, boils at −60 °C, whereas water ( $M$ = 18 g/mol) boils only at 100 °C, thanks to hydrogen bonds. Water dissolves many substances well — it is the most important solvent.

The 4 °C anomaly and Hope's experiment

Most liquids become denser when cooled. Water, however, is densest at 3.98 °C (999.97 kg/m³), becomes lighter again below that, and when it freezes its volume increases by about 9 %: the density of ice is 0.917 g/cm³ — ice floats. The reason is that in ice the hydrogen bonds arrange the molecules into an “open” hexagonal lattice. In 1805 the Scottish chemist T. Hope cooled the middle of the water in a tall cylinder with a mixture of ice and salt: the lower thermometer fell to 4 °C and stopped, while the upper one then cooled to 0 °C, and the water froze from the top. A lake is the same: at the bottom the water is ≈ 4 °C, and the ice on the top layer shields the water beneath from the cold.

Chemical properties of water (practical)

Active metals (Na, K, Ca) decompose water at room temperature: an alkali and hydrogen form — $2 Na + 2 H_{2} O \overset{}{\to} 2 NaOH + H_{2} ↑$ . Basic oxides (CaO, Na₂O) give an alkali with water: $CaO + H_{2} O \overset{}{\to} Ca (OH)_{2}$ — when lime is slaked, ≈ 65 kJ/mol of heat is released. Acidic oxides (SO₃, P₂O₅, CO₂) give an acid with water: $P_{2} O_{5} + 3 H_{2} O \overset{}{\to} 2 H_{3} PO_{4}$ ; carbonic acid is weak and unstable: $CO_{2} + H_{2} O \overset{}{⇌} H_{2} CO_{3}$ . Copper(II) oxide, iron oxides and sand ( $SiO_{2}$ ) do not react with water. We test the resulting solution with an indicator: alkali — phenolphthalein raspberry, universal indicator blue-violet; acid — universal indicator red.

The water cycle in nature

In the heat of the Sun water evaporates from seas, lakes and soil, and plants also give off vapour through their leaves (transpiration). The vapour cools higher up and condenses into tiny droplets — a cloud appears. The droplets grow larger and fall as precipitation (rain, snow); in the mountains it collects as snow and glaciers, melts and runs into rivers, and some soaks into the soil and passes into groundwater, and in the end returns to the sea. About 97 % of the water on Earth is salty (mainly the oceans); fresh water is only ≈ 2.5 %, and most of that is in glaciers and underground. In calculations: $n = m / M$ , $V = n \cdot V_{m}$ ( $V_{m}$ = 22.4 L/mol, STP), and the coefficients in a reaction equation are the ratio of moles.

Safety and the model

Safety: sodium is stored under kerosene and handled with tweezers; use only a piece smaller than a pea, with safety goggles and behind a screen; P₂O₅ and SO₃ burn the skin and react violently with water — only in a fume cupboard. A mixture of hydrogen and oxygen (“explosive gas”) explodes — the test is carried out with small volumes in a thick-walled eudiometer. If alkali gets in the eye, rinse with plenty of water. Never taste anything. The model: gases are ideal, at 20 °C and 101.3 kPa (the pressure of water vapour and of the liquid column is ignored), and all the current goes to electrolysis; in the Hofmann apparatus time is speeded up 30 times, in Hope's experiment 300 times, the column is one-dimensional, convection is instantaneous, the walls conduct no heat, and the colour of the water shows temperature (false colour). Solutions are ideal (25 °C): the real pH of limewater is ≈ 12.4 (model 12.7); the heat of reaction is assumed to pass entirely to the water.

Did you know?

Water was first decomposed by electric current in 1800 by the English scientists William Nicholson and Anthony Carlisle — a few weeks after Alessandro Volta announced his “pile” (the first chemical source of current).
Ice takes up about 9 % more space than liquid water — which is why in winter a glass bottle or a water pipe filled with water can crack when it freezes.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Air: composition and pollution — Grade 7
Oxygen and ozone — Grade 7
Hydrogen — the lightest gas — Grade 7
Mineral resources — Grade 7
Lime and classes of compounds — Grade 8
Non-metals and halogens — Grade 8
Chlorine and hydrochloric acid — Grade 8
The oxygen group and sulfur — Grade 8
Sulfuric acid and the contact process — Grade 8
Nitrogen, ammonia and nitric acid — Grade 8