Envision that tremulous moment when little Bobby reached to place the nine-hundred thirty-second domino (with only sixty-eight remaining) just as the telephone rang. He twitched, and the tap from his finger sent the entire chain spilling dramatically in spirals and branches, mere moments before he could call his mother in to watch. Without knowing it (and certainly without being happy about it), Bobby just learned about metastable states.
A metastable state occurs when the energy of a given system is at a localized minimum. It is stable, so long as nothing disturbs it by adding enough energy for it to change (or transition) to another, usually lower, energy state. Consider the case of the domino. When a domino is standing on its end, it has a fair amount of gravitational potential energy (based on the average distance from the ground – half the height of the domino). It would have much less energy if it was laying flat on its side, but it can’t simply fall over. (We’re assuming that you have a level surface and a domino with a flat end, of course.) For the domino to fall over, it first has to tip, and tipping requires that the end of the domino actually rises a little bit further from the table than it was, increasing its potential energy. (If you doubt this, grab a ruler. See how tall a standing domino is, then tip it just enough to make the top edge of the back side lean over the bottom edge of the front side and see how far from the ground that topmost edge is.) That little bit of tipping requires an input of energy from outside. It doesn’t take much – a breath of air, the tap of a finger – but it is still greater than zero. The fact that, if left entirely alone, the domino would remain standing indefinitely describes a metastable state. It is only stable so long as it doesn’t have the necessary energy to fall over into a more stable (lower energy) state.
When scientists describe energy levels and metastable states, they usually draw graphs with different states along the x-axis and the energy of each state along the y-axis. Working with the domino, the plot would look something like the top of the emblem employed by the Arby’s restaurant chain. At the center would be the metastable state, which has a fairly high energy, but not nearly as high as the energy to either side, which represents the slightly tipped – and tallest – position of the domino. Off to the sides, the energy decreases rapidly, representing positions where the domino falls closer to the ground until finally it lays flat and the lowest energy is reached.
The difference in energy between the metastable state and the highest (transition) energy state is sometimes called the “barrier energy” or “activation energy”. This is the amount of energy required for the change from metastable state to lower energy state to commence. The larger the barrier energy, the more stable a meta-stable state is. Consider that a very thin domino is much less stable than a thicker domino would be. The thin domino would have a very small barrier energy compared to the thick domino. (Go ahead and use a ruler to compare the change in height while tipping if you’re unsure about this.)
While dominos make an excellent model for discussing metastable states, they apply widely throughout chemistry and physics in general. One example in chemistry would be the case of supersaturated solutions. A supersaturated solution is one in which more of a solute is dissolved in a solvent than that amount of solvent would normally be able to dissolve under its current conditions. (Most supersaturated solutions are made by cooling a saturated solution.) If left undisturbed, the supersaturated solution remains as it is, but scratch the glass (creating a mechanical vibration – a form of kinetic energy) and the extra solute begins to crystallize, returning the solution to a more stable state.
Many chemical reactions, especially in organic chemistry, have metastable states along the way. Chemists will often try to isolate and observe these states (along with the corresponding transition states when possible) in order to learn more about how particular reactions occur. By lowering the temperature of the materials, for example, they can take energy out of the system, trapping some of the material in the metastable state when it doesn’t have enough heat to overcome the barrier energy needed to continue on to the next stage of the reaction.
One last point to remember is that “metastable” is not a synonym for “unstable”. If something is unstable, it is going to react, degenerate, or do whatever it is prone to do whether or not external energy is supplied. Metastable states require an input of energy before they can be changed.