Chemical Garden

A garden that grows by osmosis

Drop a crystal of cobalt chloride into a jar of diluted water glass (sodium silicate solution) and within minutes a deep blue stalk starts to climb, then another, twisting and branching like coral. Nothing is alive. The garden is built by chemistry and physics working together.

Step 1: a membrane forms

As the crystal starts to dissolve, its metal ions meet silicate ions in the surrounding liquid. The two react at once to form an insoluble metal silicate, a gel-like solid that coats the crystal in a thin skin. This skin is semipermeable: water molecules can pass through it, but most of the dissolved ions cannot.

Step 2: osmosis pumps water in

Inside the skin is a very concentrated solution of the metal salt. Outside, the water glass is less concentrated in dissolved particles. Water moves through a semipermeable membrane from the side with fewer dissolved particles to the side with more, a process called osmosis. So water floods in, and the pressure inside the skin rises.

Step 3: burst, rise, repeat

Eventually the skin ruptures, usually near the top. A jet of metal-salt solution escapes, and because it is less dense than the water glass around it, it rises. Wherever it touches fresh silicate, new membrane forms around it, so the jet builds itself a hollow tube. The tip keeps bursting and resealing, and the tube grows upwards in fits and starts, wandering and sometimes splitting. Growth slows as the crystal is used up.

Why the colours

Each metal gives its silicate a characteristic colour: cobalt gives blue to purple tubes, copper pale blue, iron(III) rusty orange-brown, nickel green and manganese a pale pink. Calcium gives white.

For the curious: osmotic pressure

For dilute solutions the osmotic pressure follows van ’t Hoff’s law, Π = icRT, where c is the concentration, i the number of ions each unit splits into, R the gas constant and T the absolute temperature. For 1 mol/L of cobalt chloride (i = 3) at 20 °C that is about 7 MPa, roughly 70 times atmospheric pressure, in the ideal case. Real concentrated solutions deviate from the ideal law, but the pressures involved are large, which is why the membranes keep bursting. In the simulation each tube tip climbs at a speed that decays as its crystal dissolves, v = v₀e^−t/τ, scaled by a mild temperature factor, with a little random wandering and an occasional branch. The growth speeds are typical rather than measured.

Safety

Water glass is strongly alkaline and irritates skin and eyes. Cobalt and nickel salts are toxic and can cause allergic reactions, and cobalt chloride is classed as a possible carcinogen. Use gloves and goggles, work with an adult and dispose of the solutions properly. Please read our disclaimer.