Main Difference –Concentration vs. Density
Concentration and density are both indicators of an “amount of substance”. The main difference between concentration and density is that concentration refers to how much of a substance is present in a mixture, whereas density refers to the mass of a substance per unit volume.
What is Concentration
Concentration refers to how much of a substance is present in a mixture. Typically, this is expressed in terms of the volume of the mixture. For instance,
Mass concentration gives to how much mass a substance accounts for, in a unit volume of the mixture. This can be obtained by dividing the mass of the substance by the total volume of the mixture. For instance, if you dissolve 5.00 g of pure table salt (NaCl) in water to get a salt solution of 100 cm3, then the concentration of salt in the solution is 5o.0 kg m-3. Typically, in chemistry experiments, concentrations are expressed in g cm-3.
Molar concentration gives the number of moles of the substance per unit volume in the mixture. For an example, let’s take the salt solution described above. A mole of NaCl has a mass of 58.5 g. This means, for the above solution, there should be 855 moles per 1 m3 of the solution. Therefore, the concentration can be described as 855 mol m-3. Typically, chemists employ units of mol dm-3 especially when discussing concentrations of gases. Moles per litre is often given with a symbol M. A 2 M solution of HCl has 2 moles of HCl dissolved in 1 L of the solution.
Volume concentration gives how much of the solution’s volume is occupied by the substance. This could be expressed as a percentage. Percentages are also used to express how much of the mass of a solution is accounted for by the substance. For instance, a 1% glucose solution has 1 g of glucose dissolved in 99 g of water.
Often, concentrations are also given as parts per million (ppm) or parts per billion (ppb). These types of measurements indicate how many molecules of a substance are present per one million/one billion molecules of the mixture. For instance, the graph below shows how the concentration of carbon dioxide in our atmosphere has increased over the past few decades:
The variation of carbon dioxide in the atmosphere, with the concentration in parts per million shown in the y-axis and the year shown in the x-axis
The words concentrated and dilute are often used to indicate the relative concentrations of a solution. Here, concentrated refers to a higher concentration while dilute refers to a lower concentration.
What is Density
Children often pose each other the riddle: “What is heavier? a pound of iron or a pound of feathers?” They both weigh the same, of course! However, intuitively, we may feel that iron should be heavier. This is because we often picture and compare similar volumes of iron and feathers in our minds. Since iron is denser, we feel as though it should weigh more.
The density of a substance refers to its mass per unit volume. A cubic metre of iron clearly has more mass than the a cubic metre of feathers. Iron has a density of about 7900 kg m-3 at room temperature while water has a density of about 1000 kg m-3. Feathers have a density of about 2.5 kg m-3 [1]. Density is an important concept for flotation. A material will sink in a fluid if it has a density greater than that of the fluid. The water in the Dead Sea is so dense, that a person can simply float on it.
A man floating on the Dead Sea
Difference Between Concentration and Density
What it Measures
Concentration measures how much of a substance is present in a mixture.
Density measures the mass of a material per unit volume.
Different Types of Interactions
Concentration of reactants is an important factor in determining the rates of chemical reactions. However, concentration is a less important factor when substances interact physically, without undergoing chemical reactions.
The density of reactants (which does not vary if the temperature and pressure are the same) are less important in determining rates of reactions. However, density is an important factor in physical interactions where chemical bonds are not broken.
Presence of other Compounds
It makes no sense to speak of concentration when there are no mixtures. For instance, it does not make sense to talk about the concentration of an element.
Density is a property that may be used to characterise any substance.
References
- Martha Marie Day, Ed.D., Anthony Carpi, Ph.D. “Density” Visionlearning Vol. SCI-1 (4), 2002.
Image Courtesy
“ A graph depicting CO2 levels from 1958 to 2009 as measured at Mauna Loa, Hawaii….” by Hanno (Graph drawn by Hanno using data published on the web by P. Tans (2007) as “Monthly mean atmospheric carbon dioxide at Mauna Loa Observatory, Hawaii “. Global Monitoring Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, U.S.A.) [Public Domain], via Wikimedia Commons
“A man reading a newspaper in the Dead Seat”, author unknown (Uploaded to en: by Pete, on 14 May 2005) [CC BY-SA 3.0], via Wikimedia Commons
