Understanding Concentration

Concentration is defined as the amount of solute in a solvent. Whether a substance is very concentrated or diluted depends on the amount of solute compared to the amount of solvent. For instance, a solution can be very concentrated if there is a large amount of solute, or if there is a small amount of solvent.

Solute

The substance that is being dissolved

Solvent

The substance that dissolves the solute (usually water)

As the standard unit for concentration, molarity is expressed as moles/liter. Molality is another unit used with concentration, however it is expressed in moles/kilogram.

SOUND ON! Use the PHET simulation below to experiment with concentration and molarity

Parts per billion (PPB) is the standard unit for determining the concentration of toxins in a sample of water. PPB is used because the actual molarity of the solution would be incredibly small. Parts per billion signifies the number of units of a contaminant per 1 billion units of water; in this case, grams of lead per one billion grams of water. Since the following is true for water — 1g = 1 ml, PPB can also be expressed using the measurement of micrograms per liter, or μg/L. Parts per million (PPM) is also used when the amount of solute is slightly larger, as it is the units of solute per one million units of solvent.

PPB and PPM are difficult to grasp, so here are some examples to help conceptualize them:

1 second in 32 years is approximately one part per billion
1 large droplet of water in a single olympic-size swimming pool is one part per billion
A single piano key out of 11,363 pianos is one part per million

During the Flint, Michigan water crisis, PPB was primarily used when testing the concentration levels of lead in the water. While the Environmental Protection Agency (EPA) considers 15 PPB to be the maximum concentration of lead for safe drinking water, the lead concentration levels found in Flint highly exceeded this generous limit.

Detroit Water

As shown above, Detroit, with which Flint had shared the same water supply system before making the infamous switch to Flint River water, yielded reasonably low levels of lead concentration, only 2.3 PPB.

Flint Water (2014)

In the summer of 2014, Virginia Tech conducted tests of Flint's water and received some alarming results — 27 PPB in the 90th percentile. However, at over 10 times more concentrated than EPA regulations, 158 PPB was the highest lead concentration reading found in the 271 Flint homes that were tested.

Flint Water (2015)

The following spring, government officials tested the water of Flint resident, LeeAnne Walters, and received a reading of 397 PPB. Suspecting that the officials took measures to minimize the lead readings (leaving water running before testing, collecting samples at a very slow flow rate, etc.), Virginia Tech's team returned to Flint once again to confirm these numbers. What they encountered, a lead concentration reading so high that the water was considered toxic waste by EPA standards.

Works Cited

Washington Post, This is how toxic Flint’s water really is, Christopher Ingraham, Jan. 15, 2016

Ted-Ed, “How to visualize one part per million,” Kim Preshoff & 100 animators of Ted-Ed Community, Aug. 15, 2016

The Edward S. Rogers Department of Electrical and Computer Engineering, University of Toronto, Everyday Examples of Metric

Created by Ben and Bella

January, 2021

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