science939

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Identify processes for measuring the quantity of different substances in the environment and for monitoring air and water quality

Water, as you know, is the most abundant substance on Earth's surface. Unfortunately there is more than enough water in some places, but not enough in others. As well, because water is used and re-used, and because it is such a powerful solvent, it is easily polluted.

Canada, with less than 1% of the world's population, has 22% of its fresh water. So, in most regions of our country, the quality of water is usually of greater concern than the quantity available.

The Federal Government assesses the health risks of specific substances in drinking water. Guidelines for acceptable concentrations of each substance are then established in partnership with provincial and teritorial governments. These governments required water suppliers to meet the guidelines. Corrective action may be taken when guidelines are violated.

In many cases, concentration of pollutants is measured in parts per million or parts per billion. Parts per million (ppm) can be defined as the number of 'parts' by weight of a substance per million parts of water. This unit is commonly used to represent pollutant concentrations. Parts per billion (ppb) can be defined as the number of 'parts' by weight of a substance per billion parts of water. Used to measure extremely small concentrations. For example, the government has determined that it is safe to have a concentration of 5 ppb arsenic (a deadly poison) in drinking water.

Scientists use the measure of parts per million (ppm) to work out how much of a chemical is in the environment. That is, they use this measure to figure out how concentrated a chemical is within a substance.

How can we apply what we have reviewed about percentage to understanding parts per million (ppm)? Imagine that an exam had 100 questions and you answered 85 of those questions correctly. Your score would be 85%. If you were given an exam with 1 million questions and you answered 850 000 of those questions correctly, that would be the equivalent of 850 000 parts per million.

Fortunately, exams do not have 1 million questions! But for scientists, this is a useful measure. Because the particles that they are working with are so small, they need to measure them in millions of parts. As well, it allows scientists to compare different solutions in different amounts and see how they are similar or different from one another. A higher level of concentration of a substance within a solution means that there are more parts per million. Try the slider below in order to see how this works.

As you can see, the idea of parts per million (ppm) works well when we need to determine the strength or concentration of a solution. But ppm can also work well with solids and gases.

Pure water does not exist in nature. All water naturally contains dissolved substances or ions. For example, rain water is naturally acidic. Water may also contain magnesium, iron, and sulfate ions. However, these substances pose little or no threat. In contrast, human activities released a wide range of chemical pollutants into water systems.

Point Sources: A manufacturing or processing plant that discharges untreated or insufficiently treated waste water into a river or lake is a point source of pollution. Point sources include wrecked tankers that leak oil and factories that discharge metallic ions, organic compounds, acids, and bases. Water itself can be a point source of thermal pollution, when thermal power plants discharge warm water into a lake from their cooling towers. Point sources of pollution can spread pollution over huge areas.

Diffuse Sources: Pollution that comes from a wide range of sources, not from a single source, is said to come from a diffuse source. A heavy downpour can cause run-off from farm fields to enter rivers and lakes. This run-off often carries undesirable pollutants, including feces (human or animal), pesticides, and fertilizer compounds, such as nitrates and phosphates.

Indirect Sources: Air, water, and soil can become polluted from a variety of indirect sources. Motor vehicles and factory smokestacks release gases that can indirectly cause many different types of pollution. The acidic gases that are produced (sulfur dioxide and nitrogen oxides) dissolve and contribute to the formation of acid deposition. Acidic water can also cause an increase in the leaching of heavy metals such as aluminum into rivers and lakes.

The term LD50 means a lethal dose of 50, or the amount of a substance that would be required to kill 50% of a population. Scientists use LD50 to compare toxins, or poisons, for their ability to cause death. Scientists usually test rats or mice to determine LD50 of certain substances that humans might come into contact with. For example, nicotine from cigarette smoke has an LD50 rating of 0.86 ppm (parts per million). At that strength, half of the people that come into contact with the nicotine would die.

1. Read pages 213 to 221 in Science Focus 9 textbook then complete Topic 4 Review Questions on page 221 #1-6.

2. Complete an internet search on toxic substances and find out more about how toxic substances affect us. You can go to http://www.mcgrawhill.ca/links/sciencefocus9 and find out more about how toxic substances affect us at this site. Click on the Continue and then click on 'expand all'. Click on Unit 3: Environmental Chemistry Topic 4: How Much Is Too Much? INTERNETCONNECT for information. Choose three substances and describe their effects in your email.