Energy & Green Tech

Obtaining hydrogen from methanol: Optimized reformers

Methanol reformers convert easy-to-transport methanol into hydrogen. But conventional reformers still come with a number of drawbacks—catalyst attrition, to name but one example. An innovative methanol reformer for mobile ...

Energy & Green Tech

More research needed to correct the paradox of sustainable batteries

Electric vehicles, powered by renewable energy instead of finite fossil fuels, are the best green choice—but they could be more sustainable. The most common power source in electric vehicles and portable electronics, lithium-ion ...

Energy & Green Tech

Carbon-air battery as a next-generation energy storage system

One of the barriers to generating electricity from wind and solar energy is their intermittent nature. A promising alternative to accommodate the fluctuations in power output during unfavorable environmental conditions are ...

Energy & Green Tech

Improving plastic waste separation with magnetic fields

In 2018, 61.8 million metric tons of plastic waste was produced in the European Union with only 9.4 million metric ton recycled. This constitutes a huge amount of plastic waste, which rapidly needs to be addressed. One solution ...

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Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ (the Greek letter rho). In some cases (for instance, in the United States oil and gas industry), density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight. Different materials usually have different densities, so density is an important concept regarding buoyancy, purity and packaging. Osmium and iridium are the densest known metal elements at standard conditions for temperature and pressure but not the densest materials.

Less dense fluids float on more dense fluids if they do not mix. This concept can be extended, with some care, to less dense solids floating on more dense fluids. If the average density (including any air below the waterline) of an object is less than water (1000 kg/m3) it will float in water and if it is more than water's it will sink in water.

In some cases density is expressed as the dimensionless quantities specific gravity (SG) or relative density (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas. (For example, a specific gravity less than one means that the substance floats in water.)

The mass density of a material varies with temperature and pressure. (The variance is typically small for solids and liquids and much greater for gasses.) Increasing the pressure on an object decreases the volume of the object and therefore increase its density. Increasing the temperature of a substance (with some exceptions) decreases its density by increasing the volume of that substance. In most materials, heating the bottom of a fluid results in convection of the heat from bottom to top of the fluid due to the decrease of the density of the heated fluid. This causes it to rise relative to more dense unheated material.

The reciprocal of the density of a substance is called its specific volume, a representation commonly used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density; rather it increases its mass.

This text uses material from Wikipedia, licensed under CC BY-SA