It is one of the distinct sounds of summer: the noise of people tapping the tops of their cans of fizzy drink before opening them. But does this widespread ritual really stop a can of beer or pop from gushing over?
When you open a can of fizzy drink, the refreshing “hiss” is the result of gas bubbles escaping from the liquid as a result of a change in the solubility of the carbon dioxide (CO2) in it. This change occurs due to the pressure inside the can decreasing from ~3 bar (can closed) to 1 bar at atmospheric pressure (can open). The solubility of CO2 in water reduces from ~4.5g in one litre of water at ~3 bar, to ~1.5g at atmospheric pressure, something that is described by Henry’s Law.
Before the can is opened, microscopic gas bubbles attach to the inside of it (nucleation). When the can is opened, these bubbles increase in size, due to the decrease in the solubility of CO2. When these bubbles reach a certain size they detach from the inside of the can and rise up to the top of the can due to buoyancy and displace liquid in their path (as shown in Figure 1).
Images captured specifically for this article
So what part could tapping the top of the can play in this process? Whether or not this technique actually works is the subject of some debate but there is a theory explaining why it may work. As described earlier, the bubbles in an unopened can nucleate at the walls (Figure 2a) so tapping the can before opening could dislodge some of the bubbles, enabling them to float to the top of the liquid.
When a can is opened, the bubbles expand (Figure 2b) with those deeper within the liquid travelling further than those near the surface, displacing more of the drink and possibly resulting in greater amounts of ejected liquid. A “tapped” can will have fewer of these “deep” bubbles and so less liquid will be dislodged – and possibly sprayed out – than an “untapped” can (Figure 2c).