Which is heavier, wet or dry sand?

Asked by: Katherine Robertson


Which would you think is heavier? If wet sand is simply dry sand with water in it than clearly wet sand is heavier. But wait! We have to define our terms here. This is why this is such a good question! I am impressed that you should wonder about something so obvious!

When doing science one has to be very careful how they speak, or write. Your question sounds like one I used to trick my elders with when I was a child. 'Which,' I would ask, 'is heavier: A ton of bricks or a ton of feathers?' The unthinking adult would answer, to my delight, that the ton of bricks is heavier. I realize your question is not meant to be a trick but it does illustrate how careful one must be.

If you were to ask 'Which takes up more space, a pound of wet sand or a pound of dry sand?' then the answer would be that a pound of dry sand takes up more space since it takes more dry sand to equal a pound. But, you asked which is heavier. Before answering such a question one would have to ask how much space that sand is taking up. In other words you could have asked: Which is heavier, a bucket full of wet sand or a bucket full of dry sand? The answer to that question would be the bucket of wet sand since that bucket is full of sand and water.

At this point your question really gets good. Why? Because the next question would be something like this: How could the bucket of wet sand be heavier? After all, water is not a heavy (or dense) as sand and if the sand in the bucket has to make room for the water it would have to hold less sand and therefore the sand and water combination should weigh less than the sand alone!

Now, there is a good question! I can see why you, or anyone who is a thinking person, would think, logically, that if you have equal volumes of wet and dry sand the wet sand would weigh less. this would have to be true since the water takes up the space the sand would have taken if the water were not there and since water is less dense than sand the combination of water and sand to equal a volume of dry sand would have to weigh less.

There is just one thing: There are spaces between the sand, not big spaces but spaces nevertheless; spaces big enough for water to get into without moving any sand out of the way. In other words, you could take your bucket of dry sand and fill it with water and move not one single grain of sand out of the bucket. So equal volumes of wet and dry sand would not weigh the same; the wet sand would weigh more because it has more mass, the mass of the water in-between the sand and the mass of the sand itself. The dry sand has only the mass of the sand and the air between the grains of sand.

But, you can see how wonderfully this question shows how careful scientists must be when they ask a question. You know, the art of asking a question is a much more valuable skill than the ability to answer them. You are the better scientist for asking the question than I am for answering it! Keep asking questions!
Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School, Texas