<text><span class="style10">mall Molecules (2 of 3)</span><span class="style7"></span><span class="style10">Hydrogen</span><span class="style7">Hydrogen (H2) is the simplest of all stable molecules, consisting of two protons and two electrons. It is a colorless, odorless gas and is lighter than air. The last of these properties led to its use in lifting airships, but this use was discontinued - because of its explosiveness when ignited - following the </span><span class="style26">Hindenburg</span><span class="style7"> disaster in 1937. Most hydrogen is used on the site where it is produced, but it is also transported as compressed gas in steel cylinders and in liquid form at very low temperature.</span><span class="style10">Water</span><span class="style7">The total amount of water (H2O) on Earth is fixed, and most is recycled and re-used. The largest reservoirs are the oceans and open seas, followed by glaciers, ice caps and ground water. Very little is actually contained within living organisms, al though water is a major constituent of most life forms.Water is one of the most remarkable of all small molecules. On the basis of its molecular weight (18), it should be a gas; its high boiling point (100 deg C / 212 deg F) is due to the interaction of water molecules with each other (hydrogen bonding; see p. 45), which effectively increases its molecular weight. Water is also unusual in that - as ice - it is less dense than the liquid at the same temperature.</span><span class="style10">Carbon dioxide and oxygen</span><span class="style7">Carbon dioxide (CO2) is a colorless gas with a slight odor and an acid taste. It is available as gas, as liquid and as the white solid known as 'dry ice'. Its cycle in nature is tied to that of oxygen, the relative levels of the two gases in the atmosphere (apart from human activity) being regulated by the photosynthetic activity of plants. It is produced on a vast scale, mostly as a by-product of other processes.With the ever-increasing input of carbon dioxide to the atmosphere, due largely to the burning of fossil fuels and forests and the manufacture of cement, the natural 'sinks' for carbon dioxide - chiefly photosynthesis and transfer to the oceans - can no longer keep pace with the total input. If this imbalance continues, it is thought that levels will be reached where the infrared-absorbing properties of carbon dioxide will result in a progressive warming of the Earth's atmosphere, accompanied by melting of the polar ice and flooding of what is now dry land - the so-called </span><span class="style26">greenhouse effect</span><span class="style7">. This is perhaps too extreme and pessimistic a view: in the past there have been many warm interglacial periods due to factors not ascribable to human activity.Oxygen (O2) is a highly reactive colorless, odorless and tasteless gas. At low temperature, it condenses to a pale blue liquid, slightly denser than water. Oxygen supports burning, causes rusting and is vital to both plant and animal respiration.</span><span class="style10">Ozone</span><span class="style7">Ozone (O3) is a highly toxic, unstable, colorless gas. Its primary importance stems from its formation in the stratosphere. In this layer of the atmosphere, temperature in creases with height, principally because of the reaction of high-energy ultraviolet solar radiation with oxygen. This can be expressed in the form of two chemical equations: O2 + sunlight 2 O (atoms)O2 + O (atom) O3Ozone in the stratosphere functions as a very effective filter for high-energy ultraviolet solar radiation. Radiation in this energy range is sufficiently high to break bonds between carbon and other atoms, making it lethal to all forms of life. It is currently thought that the introduction of CFCs (used in sprays and refrigerants) and the related 'halons' (used in fire extinguishers) may contribute to the partial or even total destruction of the ozone layer. These classes of compounds are highly volatile, chemically very stable and essentially insoluble in water, so that they are not washed out of the atmosphere by rain. When, by normal convection, they reach the stratosphere, they react destructively with ozone.</span><span class="style10">Carbon monoxide</span><span class="style7">Carbon monoxide (CO) is a colorless, odorless, toxic gas. The input to the atmosphere due to human activity is about 360 million tons per year, mostly from the incomplete combustion of fossil fuels. The natural input is some 10 times this figure and results from the partial oxidation of biologically produced methane. The background level of 0.1 parts per million (ppm) can rise to 20 ppm at a busy road intersection, and a five-minute cigarette gives an intake of 400 ppm.Since the atmospheric level of carbon monoxide is not rising significantly, there must be effective sink processes, one being its oxidation in air to carbon dioxide. In addition, there are soil microorganisms that utilize carbon monoxide in photosynthesis.</span></text>
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<text>ΓÇó WHAT IS CHEMISTRY?ΓÇó CHEMICAL BONDSΓÇó THE EARTH'S STRUCTURE AND ATMOSPHEREΓÇó THE HYDROLOGICAL CYCLEΓÇó WEATHERΓÇó PLANT PHYSIOLOGYΓÇó THE BIOSPHEREΓÇó THREATS TO THE ENVIRONMENT</text>
