Is it possible for two (or more) ingredients, when mixed, to weigh more than they do separately? If so, what and why? When I make porridge in the morning it certainly seems as if this is the case. If so, have I stumbled on a potential dieting gold mine?
鈥� The simple answer is no. One of the fundamental laws of physics is the conservation of mass and energy. Normal chemical processes do nothing to alter matter, they just rearrange atoms in different ways. Nuclear reactions can convert matter to energy, but as long as this energy is not lost from the system, the system will still weigh the same.
There is certainly no way for a system to gain mass unless there is a flow of energy or matter into it from outside. For instance, a rusting nail weighs more than the original because it has reacted with oxygen from the air.
Advertisement
鈥淭here is no way for a system to gain mass unless there is a flow of energy or matter into it from outside鈥�
In the case of porridge, the questioner is seeing a swelling of the oats as they absorb water. The contents of the pot may even swell to exceed their original volume, due to the expansion of gases trapped inside the oat grains during the cooking process. However, the mass of the porridge cannot exceed the original mass of the oats and water before they were mixed together. If anything, there will be a loss of mass due to evaporation of some of the water. Be careful not to confuse volume with mass.
Simon Iveson
Chemical Engineering Discipline, The University of Newcastle, Callaghan, New South Wales, Australia
鈥� Theoretically, because , the mass would increase very slightly if there is an increase in energy. But usually, mixing two substances decreases the energy, otherwise they wouldn鈥檛 mix 鈥� like oil and water do not.
There is one explanation for an apparent increase in the weight, though. When you weigh something, you are actually measuring its true weight minus the buoyancy due to the volume of air it displaces. And weight is different from mass. So if mixing two ingredients results in a smaller volume, then there will be less buoyancy and it will seem heavier. For instance, mixing soda (sodium carbonate) and water will show this effect, but it鈥檚 a very small one.
I suspect that the enquirer is fooled by the density. The mixture of water and, say, oats is denser than the bulk density before mixing because of all the air that was previously between the dry oats, so it looks heavier even though it鈥檚 not.
Eric Kvaalen, Les Essarts-le-Roi, France
鈥� It is possible for two substances to weigh more after they are mixed than they did separately. Alcohol and water love to mix and form hydrogen bonds. A 50:50 mixture of ethanol and water takes up about 96 per cent of the volume of the separate liquids. Thus a litre of alcohol mixed with a litre of water contracts by about 80 millilitres and thereby displaces 80 millilitres less air than the separate liquids do. And 80 millilitres of air weighs about 0.1 grams. So the mixed liquids are heavier by 0.1 grams because less displaced air means less buoyancy from the weight of displaced air.
David Emanuel, Tulsa, Oklahoma, US