This document summarizes key concepts about density and buoyancy from a chapter on properties of matter. It explains that density is a measure of mass per unit volume, while weight is the force of gravity on an object's mass. Buoyancy is the upward force a fluid exerts on an object submerged in it. Whether an object sinks or floats depends on if its average density is greater or less than the density of the fluid. Steel boats can float because their average density is lowered by the air pockets incorporated into their structure.