Basic principles of liquid density
Density is one of the characteristics of matter, and it can indicate how dense the matter is. The weight per unit volume of a substance at a specific temperature is called absolute density, which is usually called density. If M and V are used to represent the weight of a substance in physical activity, the density Ï is expressed as:
Ï = M / V
For the measurement of the density of liquid substances, water can be used as the reference liquid as a standard substance for relative density experiments. The measured relative density (also called specific gravity) is the ratio of the weight of the substance and the same volume of water.
Weight of substance (t ℃) Ïx (t ℃)
d (4 ℃ ~ t ℃) = ----------------------------- = ----------- -------
Weight of same volume water (4 ℃) Ïw (4 ℃)
d (4 ℃ ~ t ℃): the specific gravity of the substance to be measured, Ïx (t ℃): density of the substance (t ℃), Ïw (4 ℃): density of water (4 ℃), its value is 1g / cm3
The density or specific gravity of a liquid substance is an important physical constant. In the chemical industry and the food industry, when the composition and concentration of liquid substances such as acids, alkalis, salts, alcohol, petroleum, sugar, etc. change, their density often also follows Change, therefore, frequent measurement is necessary.
For the liquid density of organic compounds, among the many factors that affect it, temperature and pressure are external factors. When the temperature of the system rises or falls, due to the relationship of thermal expansion and contraction, the volume of the liquid will expand or shrink, and the density of the liquid will also decrease or increase. E.g:
Common organic solvents in industry include ethanol, acetone, and isooctane. Taking isooctane as an example, when its temperature is increased from 290k to 300k, the liquid density is reduced by 0.0081g / cm3, a decrease of 1.29%. When the pressure of the system increases or decreases, the volume of the liquid will also decrease or increase slightly, but the magnitude of the change is much smaller than that caused by the temperature change. For example: isooctane, when the pressure is subjected to When 1bar is increased to 20bar, the liquid density increases by 0.0018g / cm3, the increase is only 0.26%, so the change in liquid density caused by pressure is smaller than that caused by temperature.
In addition, internal factors such as the polarity of the liquid and the length of the carbon chain will also affect the liquid density of organic compounds. For non-polar liquids such as n-hexane or cyclopentane, there is only a very weak attraction between the molecules , Known as the London dispersion force, and the attractive force between the two polar molecules, in addition to the dispersion force, there is also the interaction between the permanent dipole or the permanent dipole and the induced dipole, making the arrangement of the molecules closer . Therefore, in general, the liquid density of polar molecules is greater than that of non-polar molecules. For some polar liquids such as alcohols or acids, the attractive force between the molecules is enhanced by the existence of intermolecular hydrogen bonds, so the liquid density of such compounds is usually relatively large. For example, acetic acid and methyl formate are With the same structure, due to the formation of intermolecular hydrogen bonds, acetic acid has a liquid density of 1.0492g / cm3 at 293.5k, which is greater than that of methyl formate at 293.5k and a density of 0.974.2g / cm3.
For the same series of non-polar liquids and liquids with very small dipole torches, the increase in the number of atoms in the molecules makes the gravitational force between the molecules larger and the density of the liquid increases accordingly. For polar liquids with large dipole torches such as alcohols or acids, the growth of the carbon chain actually reduces the density of the liquid. This is because the molecular chain of such compounds will increase the molecular dipole torch. Becomes smaller, and the attractive force between molecules also decreases, so the density also decreases.
In general, the more symmetrical the shape of a compound molecule, the greater the liquid density. This is because the more symmetrical the molecules are, the tighter the molecules are arranged and the more attractive the molecules are. For example: n-pentane and isopentane, the shape of n-pentane is more symmetrical, the liquid density at 293.15k is 0.626g / cm3, which is slightly larger than that of isopentane at 293.15k at 0.620g / cm3. As mentioned above, the internal factors that affect the density of a liquid are closely related to the chemical structure of the liquid molecule. Therefore, when discussing the liquid density of an organic compound, its chemical structure is a very important influencing factor, which must be considered.
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