Why are liquids not mostly measured in weight or mass?
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Because the density of liquids is not commonly known and not the same from liquid to liquid. Density is the mass or weight per a given volume.
More simply, 1 gallon of water does not weigh the same as a gallon of oil, nor the same as a gallon of gas. Their mass or weight would all be different. This is why it is simpler to use Volume, which is always constant no matter what liquid is used.
If I ask for a gallon of water, or a gallon of oil, or a gallon of gas they all have the same volume but each of their weights would all be different. But you would still be able to bring each one of those in a 1 gallon jug.
If I ask for 1 pound of water or 1 pound of oil or 1 pound of gas, there would be no easy way to figure out how large of a jug would be needed to transport those, since they will all be different amounts. They would all need different size jugs to transport.
Liquids can, by their basic definition, fit near perfectly into a space — a tube or a jug, for instance. Things like water, milk, blood, hydrogen peroxide, etc. We can and do thus measure them in terms of volume. A milliliter, for example, which is the amount of space occupied by a cubic centimeter (about the size of a playing die). You can still weigh these things. Everything has a weight/mass. But volume is more useful.
Unlike fluids, things like couches, televisions, bodies, rocks, and cars do not fit perfectly into containers. Their weight is also not evenly distributed, unlike a liquid. Measuring these in terms of volume doesn’t tell us as much as it relates to quantity. If you fill a liter jug with rocks, the weight will probably be different if you filled it with pebbles than if you filled it with stones. Only liquids (and gases) make sense to measure by volume because the space they occupy is uniform.
With gases though you have to go the extra step of defining the pressure, which can vary if you confine it to an area.
Edit: It’s true that you can measure a room in terms of cubic meters and thus liters. But that’s not really useful other than knowing how much space you have. My presumption with this question is that you’re asking why we measure some things in terms of volume and not others.
It is just for convenience. If you are going to weigh out a liquid you still need a container to put it in that you place on the scales. And this container usually have a scale on it anyway. So it is often easier to just use the container to measure its volume rather then taking out the scales.
It’s difficult to make a 10 gram measuring cup that works with more than one liquid, isn’t it? It is much *much* easier to divvy liquids based on volume.
To me it just seems to make sense for it to be measured in volume. But I can concede that may be due to being the norm. Maybe you can explain why you think it should be measured in weight.
Also, regarding water, the metric system easily allows for both as 1 gram is equal to 1 milliliter.
Liquids have very different densities. It’s easier to make a standard sized volume container than it is to make a standard weight container. Going back to more primitive times, weighing things was far harder than saying “Fill up this bucket”. Even things like grain and vegetables were done like that–the bushel and peck are volume units. Nowadays it does let us standardize containers, but it’s mostly cultural inertia. Kind of like how people in metric will say their weight in kilograms, or as a more extreme example how we say sunset and sunrise as though it’s the Earth that’s moving.
One exception to measuring liquids in volume happens on cargo ships. We tend to measure our liquids (fuel, water, ballast, etc.) in tonnes.
In order to measure things by mass, you need a scale, which involves complex moving parts and needs to rest on a flat surface. Depending on the scale, they may also require calibration or counter-weights (especially if you were talking about things in the past).
In order to measure liquids by volume, you need a glass cup with some markings drawn on it, and you can easily hold it in your hand.
I have a food scale in my house that I use for chopped vegetables and meat, and even with the scale set up and waiting to be used, it’s still much easier to use a measure cup or spoon for liquids or spices.
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