The mass density or density of a substance is defined as the mass per unit volume of that substance. The symbol commonly used for density is ρ (Greek letter rho). In some cases (for example in the US oil and gas industry) density is also defined as the weight per unit volume of the material. Therefore, density is an important factor in considering the concepts of buoyancy, purification and packing. Osmium is the most dense substance under standard conditions of temperature and pressure.
Considering two immiscible fluids, the less dense fluid floats on top of the more dense fluid. By further expanding this concept, it can be said that the less dense solid matter floats on the more dense fluid. An object floats in water if its average density (including the air under water) is less than the density of water (1.0 gmL-1) and sinks if its density is greater than that of water. Absolute gravity or relative density are quantities used to express density as a unitless quantity. Here, absolute gravity is expressed as a multiple of the density of a standard substance such as water or air/gas. (For example, a specific gravity less than 1 means that the substance floats in water.)
The mass density of a substance varies with temperature and pressure. (The variation is usually smaller for solids and liquids and greater for gases.) The greater the pressure acting on an object, the less its volume and consequently the greater its density. As the temperature of a substance increases, its density decreases due to the increase in volume. Because of the low density of a heated fluid, heat convection occurs from the bottom up when most substances are heated beneath the fluid. A commonly used term in thermodynamics, ideal volume refers to the inverse of the density of a substance. Density is an exponential property. That is, even if the mass of the substance increases, its density does not change.
🦼 History 🦼
According to a well-known but possibly false legend, Archimedes was tasked with investigating whether the goldsmith of King Hiero of Syracuse had committed gold fraud by mixing a cheaper alloy into the gold crown that was to be offered to the gods. Archimedes knew that the irregularly shaped crown, crushed into a cube, could easily calculate the volume and compare it to the mass. But the king did not allow it. Then one day when Archimedes was drowning in a bathing boat, he saw the water level rise while sinking in the boat and observed that the volume of the golden crown can be found by the volume of water displaced. Delighted by this discovery, he jumped out of the bathing boat naked and Eureka! Eureka! (Εύρηκα! Greek for "found!") and ran down the street. As a result, the word eureka has since been used in common parlance to denote a moment of realization.
This story was first published in written form in the architectural books of Marcus Vitruvius Pollio. That was two centuries after this happened. However, some scholars doubt the accuracy of this story because it must have been difficult to get the very accurate measurements required for this method at that time. The density of a point is equal to the ratio of its total mass to its total volume. Mass can be measured using a suitable balance. Volume can be found directly (from the geometry of the object) or by displacement of a fluid. If the given object is inhomogeneous, the density of a point on that object is a function of the position of that point. In such a case the density at a given point can be found by calculating the density of a small volume around that point. Accordingly, the density of a very small volume of an inhomogeneous object is given by this equation. ρ(r)=dm/dV, where dV is the initial volume at position r. The mass of the object can be expressed as:
The density of a granular material may vary from case to case. It varies depending on how the volume of the substance is defined. It can also lead to measurement errors. The most common example is sand. Sand, if gently poured into a pot, is less dense. If the same volume of sand is compacted, it exhibits greater density due to its reduced volume. The reason for this is that in sand, like all other particulate materials, there is a lot of air trapped between the particles.
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