What Is an Acid, and How Do Acids Cause Damage? Harmful Effects of Air Pollution Are Far-Flung, a Study Finds

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1 What Is an Acid, and How Do Acids Cause Damage? Harmful Effects of Air Pollution Are Far-Flung, a Study Finds NEW YORK, NEW YORK Air pollution in the Northeast is not just about lakes without fish. It is also about forests losing their trees and soils that store up acid before releasing it back out to pollute local waters all over again, according to a scientific study of upstate New York and New England. have declined, decades of acid buildup in the forest soils is still being washed into waterways by erosion and spring runoff, the water that comes from melting snow and runs into lakes and other bodies of water. The two-year study concluded that red spruce and sugar maple trees in the Adirondacks have been hurt in different ways by acid soils. Many tree deaths that had been attributed to things like insect infestation or drought, the study said, had in fact been accelerated by acidic soil that made the trees more vulnerable. spring runoff: the water that comes from melting snow and runs into lakes and other bodies of water. Many lakes in the Adirondacks are dead. Acid rain has killed the fish and other organisms that used to live in the lakes. In short, the acid-rain problem is more connected to the ecosystems it affects than scientists had previously believed, the report says and thus harder to fight than it had appeared. It s a lot more complicated, said Charles T. Driscoll, a professor of environmental engineering at Syracuse University and the lead author of the study, which was published in the journal BioScience. The study confirmed that although new deposits of airborne acid compounds, such as sulfur dioxide and nitrogen oxides, Red spruce trees (top) and sugar maple trees (bottom) are especially affected by acid rain. AQ 203

2 Learning Set 4 How Does Air Pollution Affect Other Regions? Acid rain can contain a mixture of acids formed from sulfur trioxide (SO 3 ) and nitrogen dioxide (NO 2 ). You have learned that acid rain forms when these gases react with water in the atmosphere. But what exactly are acids, and how do they harm the environment? What Is an Acid? More than 2000 years ago, the ancient Greeks began to sort substances into categories. One criterion people used was how substances tasted. They recognized that some substances taste sweet, others taste salty, some are sour, and others are bitter. Another criterion was how substances felt. People noticed that some substances stung the skin, especially in open wounds. Based on these observations, the Greeks classified sour substances that sting the skin as oxein, which became the Latin word acidus, or acid. Some sour foods you know about are lemon juice and vinegar. The Greeks knew that these sour substances were very useful. Lemon juice is a source of acid. ion: an atom, or group of atoms, that has more electrons than protons and an overall negative charge; or fewer electrons than protons and an overall positive charge. Recall that atoms are more unstable if they do not have a complete set of outer electrons. Some atoms form a complete set of outer electrons by sharing electrons with other atoms. This was true for the molecules you modeled with your atomic-model kit. In some cases, atoms do not share electrons. Instead, atoms take electrons from other atoms or give electrons away. An atom, or group of atoms, that has gained or lost electrons is called an ion. Project-Based Inquiry Science AQ 204

3 You might also remember that electrons and protons carry electric charges. Protons carry a positive charge and electrons carry a negative charge. An atom does not have an overall charge because the number of protons (positive charges) is equal to the number of electrons (negative charges). As a result, the charges cancel out. If an atom gains electrons, it has more negative charges (electrons) than positive charges (protons), so it has an overall negative charge. In other words, an atom that gains electrons becomes a negative ion. If an atom loses electrons, it has more positive charges (protons) than negative charges (electrons), so it has an overall positive charge. An atom that loses electrons becomes a positive ion. As an example, consider a hydrogen atom. It has one electron. If it loses that electron, it becomes a positive hydrogen ion. In the equation below, the positive hydrogen ion is represented as H +. The plus sign is written upward to the right of the chemical symbol as a superscript. The superscript gives the charge of the particle. The electron that the hydrogen atom lost is written as e-. In this case, the minus sign indicates the charge of the electron. A hydrogen atom can lose its electron. This yields a positively charged hydrogen ion plus a negatively charged electron. H H + + e negative ion: an atom, or group of atoms, that has gained electrons. positive ion: an atom, or group of atoms, that has lost electrons. What do ions have to do with acids? Scientists define an acid as a chemical compound that breaks apart in water to form hydrogen ions plus negative ions. That means that if you put an acidic compound in water, it will produce positive hydrogen ions and an equal number of negative ions. AQ 205

4 Learning Set 4 How Does Air Pollution Affect Other Regions? acidic: solutions that have more hydrogen ions than pure water. acidity: the concentration of hydrogen ions in a solution. A solution is described as acidic if it has more hydrogen ions than pure water. The concentration of hydrogen ions in a solution is called the solution s acidity. Solutions that are more acidic have a greater concentration of hydrogen ions than solutions that are less acidic. Since you know that acid rain is harmful to plants, animals, and structures, you can probably guess that the more acidic a solution is, the more damage it can do. Small amounts of acid may not be harmful and, in some instances, may be beneficial. But very few life forms can survive a strongly acidic environment. Just as matter cannot be created or destroyed by ordinary means, unbalanced charges cannot be created or destroyed. When an acid is dissolved in water, it becomes ionized. This means that it forms a hydrogen ion, which is positive, and a second ion, which is negative. Nitric acid, HNO 3, provides a good example. Nitric acid, in water, yields hydrogen ions and nitrate ions. HNO (in water) H + NO This reaction happens because water molecules surround the positive and negative ions, making them more stable. Stop and Think 1. How is a positive ion different from a negative ion? 2. What happens when acidic compounds are mixed with water? 3. Now that you know a little about how acid rain forms, how do you think acid rain causes damage? Project-Based Inquiry Science AQ 206

5 How Does Acid Rain Cause Damage? To understand what acid rain can do, you should first recognize that acid rain is a broad term. It applies to more than just rain. The term acid rain describes any of several ways that acid falls out of the atmosphere. The term acid deposition is a more precise term. Deposition is the process through which materials are dropped in a new location. Acid deposition can be wet or dry. Wet deposition applies to acid rain, snow, or fog. This acidic water can flow over the ground, seep into the soil, or flow into lakes, ponds, and rivers. Dry deposition applies to acidic gases and acidic particulate matter. Acidic gases and particles can be blown by winds onto trees, buildings, cars, and other surfaces. It can also be washed off those surfaces by acidic rain, making the rain even more acidic. Acid rain can affect the environment in many ways. One way is by changing the natural acidity of soils and water in lakes and ponds. Living things can survive only within a certain range of acidity. If acid rain increases the acidity, many organisms are not able to survive. Two ways acids enter the cells of living organisms are osmosis and diffusion. You will learn more about these processes in another Unit. In addition, acid rain dissolves some building materials by reacting with substances in the materials. Acid deposition can be present in snow or fog, as well as in rain. acid rain: any of several ways that acid falls out of the atmosphere. What s the Point? Acid rain forms when pollutants formed through combustion combine with water in the atmosphere. Acids are substances that taste sour and sting the skin. Acids are compounds that, added to water, produce solutions that have more hydrogen ions than pure water alone. A hydrogen ion is a hydrogen atom that has lost an electron and has therefore become positively charged. Acid rain describes several ways that acids fall from the atmosphere to Earth s surface. Wet acidic deposition is acidic rain, snow, or fog. Dry acidic deposition includes acidic gases and particulate matter. Acids can change the acidity of water and soils on which living things depend. They can also react with substances in building materials. AQ 207