Covalent bond nonpolar atoms share electrons equally

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covalent bond Nonpolar •atoms share electrons equally Polar •atoms share electrons UNequally •atoms have partial positive or negative charge Water (H 2 O) δ δ + δ + H H O H 2 O Oxygen (O 2 )
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Living cells are mostly water Liquid water is essential for life Cells are 70-95% water Credit NASA/Reuters Streaks from seasonal water flow on Mars Move materials around the “body” Maintain body temperature Medium for chemical reactions Support/volume for cells Chloroplasts in plant cells How do organisms use water?
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Water’s polarity gives it emergent properties Water molecules are polar. Water molecules form hydrogen bonds with each other. Hydrogen bond δ H δ + H O δ + δ + δ + δ δ δ Four emergent properties: •cohesive behavior •moderates temperature •expands when frozen •versatile solvent
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Hydrogen Bonds: Easy come, easy go Hydrogen bonds form when an H atom with partial charge (from covalently bonding an electronegative atom) is attracted to another electronegative atom δ δ + δ + δ δ + δ + δ + Water (H 2 O) Ammonia (NH 3 ) Hydrogen bond Hydrogen usually interacts this way with O or N atoms in living cells Individual H bonds are weak and transient Powerful if many H bonds
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Water is cohesive and adhesive Adhesion Water is attracted to some substances by H bonds Water-conducting cells Adhesion Cohesion 150 µ m Direction of water movement Cohesion H bonds hold water molecules together High surface tension (how hard it is to break the surface)
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Water moderates the environment for organisms Water has a high heat of vaporization – The evaporation of water from a surface makes that surface cooler Water has a high specific heat – Need a lot of energy to change its temperature – Heat is absorbed when H bonds break – Heat is released when H bonds form
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Water moderates the environment for organisms Solid water is less dense than liquid water – Hydrogen bonds in ice are more ordered than in liquid water – Bodies of water freeze from the top down – Floating ice insulates bodies of water Ice Hydrogen bonds are stable Water Hydrogen bonds are transient What would happen if ice sank in water?
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Water is the solvent of life Solution = a liquid that is a homogenous
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