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Pre-Assessment • Balance the following equation: H2 + O2 H2O • Sketch a molecule of water. Show the covalent bonding that exists in the molecule. • Why is the existence of water important in the search for extraterrestrial life? Enduring Understanding: Water’s polarity and hydrogen bonds give it unique, life supporting properties Section 4.4 I. Unequal electron (e-) sharing creates polar molecules • An atom’s attraction for shared electrons is called its _electronegativity_____ • _oxygen_____ is highly electronegative • Oxygen in water “tugs” on shared e- more strongly than hydrogen, therefore e- spend more time near oxygen atom • Unequal sharing produces polar covalent bond • A polar molecule is one that has an unequal distribution of charge II. Hydrogen bonds are weak bonds between an electronegative atom and a hydrogen atom with a positive region. • Each water molecule can hydrogen bond to as many as _4__ partners. • During which phase of water (solid, liquid, gas) do the most H bonds exist? _solid_ The least? _gas__ III. Hydrogen bonds account for water’s cohesive property • Cohesion is the tendency for _molecules of the same kind to stick to each other_____ • Adhesion is the tendency for _unlike molecules to attract__________ • In what ways does this property of water support life? IV. Hydrogen bonds give water an unusually high surface tension • Surface tension- a measure of how difficult it is to stretch or break the surface of a liquid What effect does soap have on surface tension? _________________________ Soap has a water loving end and a water hating end. When it is added to water, it separates the water molecules and reduces surface tension. V. Due to hydrogen bonding water has a better ability to resist temperature change than most other substances. • Temperature measures the intensity of heat-the average speed of molecules. • When two substances differ in temperature, thermal energy in the form of _heat___ will transfer from _the warmer substance to the cooler one______. • Heat must be _absorbed_ to break H bonds. Thus water requires lg amount of heat to warm up few degrees • When water is cooled, more H bonds form, releasing _heat___ and slowing the cooling process. • How does this unique property support life? VI. Ice is less dense than liquid water due to hydrogen bonding. How does this unique property of water support life? ______________________ VII. Water’s versatility as a solvent results from the polarity of its molecules. • • • • • • Solvent- __________________________ Solute- ___________________________ Aqueous solution- __________________ Dissolving capability due to ___________ Dissolves _____ and ______ compounds What role does this unique property have in supporting life? ___________________ __________________________________ Why does salt dissolve in water? The negative ends of water surround the positive sodium ions. The positive ends of the water molecule surround the negative chlorine atoms. Why doesn’t oil dissolve in water? Oil is non polar, so does not interact with water. It forms a single layer above it, sticking its water fearing ends out.