how much actual matter is in a given volume.
consider this:
fill a balloon with air.
you can squeeze the balloon by squishing the air particles closer together. you increase the density (pack it closer together) and decrease the volume (make the air take up less space or "volume")
fill a glass to the to top with water and weigh it.
now empty it and fill it up to the top with dirt. weigh it.
both have the same volume (take up the same amount of space) but one weighs more because it has more density (more atoms crammed in to the volume space)
consider this:
fill a balloon with air.
you can squeeze the balloon by squishing the air particles closer together. you increase the density (pack it closer together) and decrease the volume (make the air take up less space or "volume")
fill a glass to the to top with water and weigh it.
now empty it and fill it up to the top with dirt. weigh it.
both have the same volume (take up the same amount of space) but one weighs more because it has more density (more atoms crammed in to the volume space)
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a measure of mass per unit volume