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KEY CONCEPT Water’s unique properties allow life to exist on Earth.

KEY CONCEPT Water’s unique properties allow life to exist on Earth. _. O. H. H. +. +. Life depends on hydrogen bonds in water. Water is a polar covalent molecule. Polar molecules have slightly charged regions. Nonpolar molecules do not have charged regions.

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KEY CONCEPT Water’s unique properties allow life to exist on Earth.

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  1. KEY CONCEPTWater’s unique properties allow life to exist on Earth.

  2. _ O H H + + Life depends on hydrogen bonds in water. • Water is a polar covalent molecule. • Polar molecules have slightly charged regions. • Nonpolar molecules do not have charged regions.

  3. Life depends on hydrogen bonds in water • Hydrogen bonds form between slightly positive hydrogen atoms and slightly negative atoms.

  4. Hydrogen bonds are responsible for four important properties of water. • universal solvent (adhesion) • high specific heat • surface tension (cohesion/polar) • less dense as a solid Fig. The water's surface (left, dyed red) is curved down because water has greater adhesion than cohesion. The surface of the mercury (right) is curved up because mercury has greater cohesion than adhesion.

  5. Examples of each property of water • Universal solvent (adhesion) – the attraction among molecules of different substances is called adhesion. In other words, water is “sticky” to lots of other substances. transpiration Example: Adhesion helps plants transport water from their roots to their leaves because water sticks to the sides of cellular structures in plants called xylem.

  6. Examples of each property of water • High specific heat – hydrogen bonds give water abnormally high specific heat. This means water resists changes in temperature. • Water also helps maintain homeostasis inside cells by keeping temperatures stable. For instance, our fish in the pond is indeed happy because the heat capacity of the water in his pond above means the temperature of the water will stay relatively the same from day to night. He doesn't have to worry about either turning on his air conditioner or putting on his woolen flipper gloves.

  7. Examples of each property of water • Cohesion is the attraction among molecules of a substance. In other words, water is “sticking” to other water molecules due to hydrogen bonds. Fig. Water’s surface tension comes from hydrogen bond’s that cause water molecules to stick together.

  8. Examples of each property of water • Water is less dense as a solid due to its hydrogen bonds. Compare: How are hydrogen bonds similar to ionic bonds?

  9. solution Many compounds dissolve in water. • A solution is formed when one substance dissolves in another. • A solution is a homogeneous mixture. • Solvents dissolve other substances. • Solutes dissolve in a solvent. Connect: What are the solvent and solutes in a beverage you drink?

  10. Polar solvents dissolve polar solutes. • Nonpolar solvents dissolve nonpolar solutes. • Polar substances and nonpolar substances generally remain separate. • “Like dissolves like.” Polar Ex: The liquid part of your blood, called plasma, is about 95% water. The solvent in plasma is water and all the dissolved substances are the solutes: sugars and proteins etc.. Nonpolar Ex: Fats and oils rarely dissolve in water. Fats and oils are nonpolar so they do not have charged regions so they are not attracted to polar molecules.

  11. stomach acid pH between 1 and 3 more acidic Some compounds form acids or bases. • An acid releases a hydrogen ion when it dissolves in water. • high H+ concentration • pH less than 7

  12. bile pH between 8 and 9 more basic • low H+ concentration • pH greater than 7 • A base removes hydrogen ions from a solution.

  13. pure water pH 7 • A neutral solution has a pH of 7. Apply: Cells have higher H+ concentrations than blood. Which has a higher pH? Why?

  14. Biology Connection and pH • Most organism’s (including humans) need to keep their pH within a very small range (around 7). pH can be regulated by buffers, compounds that can bind to an H+ ion when the H+ concentration increases and can release H+ ions when the H+ concentration decreases. • Exceptions: A few organisms thrive in very acidic environments: azalea’s prefer soil around a 4.5 pH and microorganisms called Picrophilus survive best at extremely acidic pH (0.7).

  15. pH indicators • Indicators are chemicals that can be used to determine the pH of a substance. Ex: natural indicator = cabbage Ex: Litmus papers are indicators

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