Density
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What determines the density of a material? What are density and specific gravity? Does air occupy space? What is specific gravity? Density is defined as an objects mass per unit volume. In chemistry, the density of many substances is compared to the density of water. Does an object float on water or sink in the water? If an object such as a piece of wood floats on water it is less dense than water vs. if a rock sinks, it is more dense than water. Density = Mass / Volume Specific gravity is the density of a substance divided by the density of water. Since (at standard temperature and pressure) water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is usually very close to the same value as density (but without any units). Relative Density (Specific Gravity) Relative density of a substance is the ratio of the substance to the density of water, i.e. Example - Use the Density to Identify the Material: An unknown liquid substance has a mass of 18.5 g and occupies a volume of 23.4 ml. (milliliter). The density can be calculated as ρ = [(18.5 g) / (1000 g/kg)] / [(23.4 ml) / (1000 ml/l) (1000 l/m3) ] = (18.5 10-3 kg) / (23.4 10-6 m3) = 790 kg/m3 If we look up densities of some common substances, we can find that ethyl alcohol, or ethanol, has a density of 790 kg/m3. The liquid may be ethyl alcohol! Example - Use Density to Calculate the Mass of a Volume The density of titanium is 4507 kg/m3. Calculate the mass of 0.17 m3 titanium! m = (0.17 m3) (4507 kg/m3) = 766.2 kg
Yes, air does occupy space (volume). Air is made up of matter. The molecules in air, however, are much farther apart than one finds in liquids and solids. People express this by saying that air, a gas, is less dense than solids or liquids. Density really has to do with how much mass there is in a fixed volume. If you filled a box with air (which an "empty" box naturally is), it would have less mass, and weight, than a box filled with, say, water or iron. One way to help you see that air occupies space is to consider a balloon. When you breathe in, air fills you lungs. If you breath out into the balloon, you blow it up. You can see that whatever is in the balloon (air) takes up space. What determines the density of a material? Factors affecting the density of a material include the mass of its individual molecules, the energy of those molecules and interactions between the molecules. Materials consist of atoms which form molecules. The mass of a unit volume depends on the number of molecules in it and the mass of the molecules. The more molecules per unit volume and the heavier they are - the denser is the material. The molecules in solids tend to be packed into very well organised structures, which help to give the solid its strength and rigidity. In liquids, the molecules are typically packed into about the same space, but the bonds between them are much weaker, giving liquids their 'sloppy' nature. For many materials, the solid state is denser than the liquid state. A notable exception is water, which is heavier than ice. In gases, the molecules have very weak bonds and can move apart to take up more space, making the gas much less dense. There are typically about 1000 times more molecules in a volume of solid or liquid, compared to the same volume of gas, so generally the densities of gases are about 1000 times smaller. Substances containing elements with high atomic number (many protons and neutrons) tend to be denser. This is strictly only true for gases and one mole (a standard number of particles) of a gas always uses the same volume (22.4 litres) at normal conditions. Organic substances with a large number of oxygen atoms (e.g. sugars) tend to be slightly denser than those mainly containing carbon and hydrogen (e.g. fats and oils). Many organic liquids, like hydrocarbons, are less dense than water, despite having heavier molecules. Some examples: Water Density: 1 g/cm3. Mass of molecule (H20): 18 units. Number of molecules in 1 cm3 = 3.3e+22 Mercury Density: 13.5 g/cm3. Mass of atom (Hg): 200 units. Number of molecules in 1 cm3 = 4.1e+22 Sulphuric acid Density 1.83 g/cm3. Mass of molecule (H2SO4): 98 units. Number of molecules in 1 cm3: 1.1e+22 Kerosene Density 0.8 g/cm3. Mass of molecule (C4-C8): around 50 - 100 units (5 times heavier than H20). Number of molecules in 1 cm3 (C5): 8e+21. What is specific gravity? Specific gravity is the density of a substance relative to some reference substance: Specific gravity = density of substance density of reference substance Specific gravity was defined because it's convenient to have a unitless measure of density. The reference substance for solids and liquids is usually water at 4°C. At that temperature, water has a density of 1.0000 g/mL. So if you know the density of a substance in g/mL, for most practical purposes that's numerically equal to the specific gravity. Specific gravity is a special case of relative density. In this case the density of a substance is divided by the density of water at 4 F (0 C). Since water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is the same number as density but without any units (adimensional). Relative density is another adimensional number which measures density of any substance in relation to another. A relative density greater than one means that the substance in the numerator has a bigger density than the one in the denominator and vice versa. Applying the same concept to specific gravity: if the substance is not miscible with water (do not mix or dissolve) it will float for specific density over one and will sink for specific gravity under one. Density is the ratio between mass and volume. It reveals interesting features of a substance and is essentially related with the way the atoms and molecules in the substance are arranged. It will depend on the temperature among other things. Specific Gravity DeterminationLiquids: The specific gravity of a liquid can be determined with a hydrometer. The depth to which the hydrometer sinks is inversely proportional to the specific gravity of the liquid. A hydrometer is a hollow, sealed, calibrated glass tube. Solids: Some electronic scales can measure specific gravity of solids. This is particularly useful for purity determination in gems. Gases: The specific gravity of gases is measured by specific gravity transducers. They are particularly useful for power, oil, gas, aerospace and process industries. Practical uses of Specific GravitySpecific gravity measurements can be used in a wide variety of industries. It is particularly useful because it allows access to molecular information in a non invasive way.
Purity in gems: comparing the specific gravity of a gem with the patron number (measured over a gem with high purity level), the degree of purity of a gem can be observed. This allows for fast determinations of the value of a gem. Eggshell thickness: The eggshell thickness is an important factor in the poultry industry; it will decay with the age of the hen or under adverse conditions. If the thickness goes under certain expected values, the eggs are not marketable. The specific gravity of an egg is determined mainly by its shell, the other components have specific gravity close to one (shell= 2.325; yolk= 1.032; albumen= 1.038; shell membranes= 1.075). Measuring the specific gravity of the egg as a whole one has a good idea of the state of the shell. Compressive strength of soils: Engineers need to know the compressive strength of a soil to choose among other things the construction method. Specific gravity and compressibility are highly connected and can be obtained one from the other. Oil Industry: Crude oil and its refined products are normally measured either by volume in gallons and US barrels, or by weight in tons. The relationship between volume and weight can be measured by specific gravity or density. The specific gravity is related with the "degree API", a measure of the inherent power of the oil (regular, super, etc). Table of Specific Gravities used in the Oil Industry
http://jersey.uoregon.edu/~mstrick/AskGeoMan/AskGeoManIndex.html#minerals http://antoine.frostburg.edu/chem/senese/101/measurement/faq/specific-gravity.shtml http://www.seed.slb.com/v2/FAQView.cfm?ID=562 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
