Environmental Problems, Their Causes, and Sustainability 1

aluminum foil. When solid water (ice) is melted orliquid water is boiled, none of the H 2 O molecules involvedare altered; instead, the molecules are organizedin different spatial (physical) patterns.In a chemical change, or chemical reaction, thereis a change in the chemical composition of the elementsor compounds. Chemists use shorthand chemicalequations to represent what happens in a chemicalreaction. For example, when coal burns completely,the solid carbon (C) it contains combines with oxygengas (O 2 ) from the atmosphere to form the gaseouscompound carbon dioxide (CO 2 ):CReactant(s)+OOblack solid colorless gas colorless gasOProduct(s)carbon + oxygen carbon dioxide +C + O 2 CO 2 +COenergyenergy+ energyEnergy is given off in this reaction, making coal auseful fuel. The reaction also shows how the completeburning of coal (or any of the carbon-containing compoundsin wood, natural gas, oil, and gasoline) givesoff carbon dioxide gas. This is a key gas that helpswarm the lower atmosphere (troposphere).The Law of Conservation of Matter: WhyThere Is No “Away”When a physical or chemical change occurs, no atomsare created or destroyed.We may change various elements and compoundsfrom one physical or chemical form to another, but inno physical and chemical change can we create or destroyany of the atoms involved. All we can do isrearrange them into different spatial patterns (physicalchanges) or different combinations (chemical changes).This statement, based on many thousands of measurements,is known as the law of conservation of matter.The law of conservation of matter means there isno “away” as in “to throw away.” Everything we thinkwe have thrown away is still here somewhere on the planetwith us in one form or another. Dust and soot from thesmokestacks of industrial plants, substances removedfrom polluted water at sewage treatment plants, andbanned chemicals like DDT all have to go somewhereand can come back around to haunt us. For example,selling DDT abroad means it can return to the UnitedStates as residues in imported coffee, fruit, and otherfoods, or as fallout from air masses moved long distancesby winds.How Do Chemists Keep Track of Atoms?Atomic BookkeepingChemical equations are used as an accountingsystem to verify that no atoms are created ordestroyed in a chemical reaction.In keeping with the law of conservation of matter,each side of a chemical equation must have the samenumber of atoms of each element involved. Mindingthis law leads to what chemists call a balanced chemicalequation.The equation for the burning of carbon (C O 2 CO 2 )isbalanced because one atom of carbonand two atoms of oxygen are on both sides of the equation.That is an easy one.Now we try a slightly harder one. When electricityis passed through water (H 2 O), the water molecule canbe broken down into hydrogen (H 2 ) and oxygen (O 2 ).This chemical reaction can be represented by the followingshorthand chemical equation:H 2 O H 2 O 22 H atoms 2 H atoms 2 O atoms1 O atomThis equation is unbalanced because one atom of oxygenis on the left but two atoms are on the right.We cannot change the subscripts of any of the formulasto balance this equation because that wouldmake the substances different from those actuallyinvolved. That is a no-no according to the law of conservationof matter. Instead, we could use differentnumbers of the molecules involved to balance the equation.For example, we could use two water molecules:2 H 2 O H 2 O 24 H atoms 2 H atoms 2 O atoms2 O atomsThis equation is still unbalanced because althoughthe numbers of oxygen atoms on both sides are nowequal, the numbers of hydrogen atoms are not.We can correct this by having the reaction producetwo hydrogen molecules:2 H 2 O 2 H 2 O 24 H atoms 4 H atoms 2 O atoms2 O atomsNow the equation is balanced, and the law of conservationof matter has been observed. We see that forevery two molecules of water through which we passelectricity, two hydrogen molecules and one oxygenmolecule are produced. By the way, this equation maychange your life. It represents how we could use heator electricity to decompose water and produce hydrogengas that someday may replace oil as a fuel tohttp://biology.brookscole.com/miller1447