Is it possible to gain more weight than the food you eat weighs? For example if you eat a pound of butter, can you gain more than a pound of weight as a result or is the weight gain always less than or equal to the weight of the food consumed? Based on conservation of mass I understand that the food would need to combine with something else for you to gain more than the weight of the food, but if for example it somehow combines with the air you breath would this be possible or is the actual weight of the food the limit to the weight you could gain by eating it?
If you’re not including the water you need to drink to stay alive, then yes - because your body stores fat in cells that also contain water.
So if you eat a block of lard and drink a pint of water, you could - in theory at least, gain more weight than the block of lard.
Your body will also use some of the energy in the food, but even if you only consider food above what your body needs, there will still be some loss just to digest it. Also, your body doesn’t make additional fat cells, so only fat is stored as additional weight.
As for combining with air, that’s what happens when your body burns food and then eliminates the waste products, so that actually causes weight loss, and water only causes a temporary increase in weight (or, as many dieters find to their consternation, a temporary loss, which can be 10 pounds in a few days, but then they stall out as true weight/fat loss begins, or quickly regain it if they decide to stop).