Physical Science 8th grade (1).pdf

Solubility of gas increases with pressure the

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Solubility of gas increases with pressure The solubility of gases in liquids increases with pressure. Soda is fizzy because the carbon dioxide was dissolved in the liquid at high pressure. When you pop the tab on a can of soda, you release the pressure. The solution immediately becomes supersaturated, causing the CO 2 to bubble out of the water and fizz. Dissolved oxygen Table 9.2 also shows that 0.04 grams of oxygen dissolves in a kilogram of water. Dissolved oxygen keeps fish and other underwater animals alive (Figure 9.5). Just like on land, oxygen is produced by underwater plants as a by-product of photosynthesis. Solubility of gas decreases with temperature When temperature goes up, the solubility of gases in liquid goes down (Figure 9.6). When the water temperature rises, the amount of dissolved oxygen decreases. Less dissolved oxygen means less oxygen for fish. When the weather is warm, fish stay near the bottom of ponds and rivers where there is cooler, more oxygenated water. Solubility of liquids Some liquids, such as alcohol, are soluble in water. Other liquids, such as corn oil, are not soluble in water. Oil and vinegar (water solution) salad dressing separates because oil is not soluble in water. Liquids that are not soluble in water may be soluble in other solvents. For example, vegetable oil is soluble in mineral spirits, a petrolem-based solvent used to thin paints. Figure 9.5: Fish and other aquatic life are sustained by dissolved oxygen in water. Figure 9.6: The solubility of gases in water decreases as temperature increases. Table 9.2: Solubility of gases in water at 21°C and 1 atm. Gas Solubility Oxygen (O 2 ) 0.04 g/kg water Nitrogen (N 2 ) 0.02 g/kg water Carbon dioxide (CO 2 ) 1.74 g/kg water
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186 U NIT 4 M ATTER AND C HANGE Figure 9.7: Water is a polar molecule because it has a negative pole and a positive pole. Figure 9.8: Water dissolves sodium chloride to form a solution of sodium (+) and chlorine (-) ions. polar - describes a molecule that has charge separation, like water. Water as a solvent The universal solvent Water is often called the “universal solvent.” While water doesn’t dissolve everything, it does dissolve many different types of substances such as salts and sugars. Water is a good solvent because of the way the H 2 O molecule is shaped (Figure 9.7). Water is a polar molecule A water molecule has a negative end (pole) and a positive end. This is because electrons are shared unequally; pulled toward the oxygen atom and away from the two hydrogen atoms. The oxygen side of the molecule has a partially negative charge and the hydrogen side of the molecule has a partially positive charge. A molecule (like water) with a charge separation is called a polar molecule. How water dissolves salt The polar molecules of water dissolve many ionic compounds.
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