Density
Take 1 L of molten lead and 1 L of water and weigh them and you will find the litre of molten lead weighs much heavier.
Take 1 mL of honey and 1 mL of oil, pour them into a bottle of water, and you'll find honey sinks while oil floats. Why? Don't they have the same volume? Here's a common misconception: volume does not equal mass.
Volume is how much space something takes up.
Mass is how much matter something has.
A way to picture this is if we have a bag of M&Ms, volume is how large the bag is, while mass is how many beans are there inside the packet. So why do things of the same volume do not have the same mass? Think about this: we can have a huge bag with only a few M&Ms inside, but also a super-packed bag that's quite small but is stuffed full of M&Ms. In the same way, substances may have the same volume, but their mass is different, for they may have more spaces between each particle, or each particle may be larger. (What's this?)
In Science, there is actually a measurement for this volume-mass relationship: it is Density.
What's Density?
Density = Mass
Volume
Density is the measurement of how much mass something has for a given volume. For example, lead has a density of 11g/cm^3 while water has a density of 1g/cm^3.
Measuring Density
Find the mass with a scale, then find the volume with a ruler or with displacement.
Finding the Mass or Volume
Sometimes you'll be asked to find the mass or volume of something given the density - this is common in questions involving the pressure of fluids or hydraulics. This is not hard - you just have to rearrange the equation. Density*Volume = Mass and Mass/Density = Volume
(Cover the one you need to find and you'll get the formula!)
Floating and Sinking
Sinks: If the object has a higher density than the fluid
If the fluid has a lower density than the object
Floats:If the object has a lower density than the fluid
If the fluid has a higher density than the object An example would be an iron nail in water - since the iron nail has a higher density than water it will sink. A helium balloon rises and floats because helium is less dense than air.
