Water Key Terms Polar Electronegativity Hydrogen bonds Solvent

Water

Key Terms • • • Polar Electronegativity Hydrogen bonds Solvent Specific Heat Capacity Latent heat of vaporization

Background Information • All elements have a property called electronegativity • Electronegativity is a measure of how strongly an atom attracts electrons • If an element has a high electronegativity then it attracts electrons very strongly

Table of Electronegativities

Electronegativity and Polar Bonds • As you can see from the previous page, Oxygen has a high electronegativity (3. 5) while hydrogens is quite low (2. 1) • So when oxygen and hydrogen form a covalent bond, the electrons are not shared equally • Oxygen holds the electrons closer to its nucleus which makes is slightly negative • Hydrogen has the electrons further away from its centre which makes it slightly positive

Polar Molecules • The result is a molecule that has a positive side (i. e. the hydrogens) and a negative side ( the oxygen)

Polar Molecules • When a molecule has a positive side and a negative side, it is said to be “Polar” • The positive side of a polar molecule will be attracted to the negative side of another polar molecule, forming what is called a hydrogen bond

Hydrogen Bonds • Hydrogen bonds are not as strong as covalent bonds • They are constantly forming as water molecules move around

Properties of Water • Water has several important properties that make it such a vital part of life. They are: • • • It is cohesive and adhesive It has a very high specific heat capacity It has a very high latent heat of vaporization It is an excellent solvent It has a higher density in its liquid form

Cohesion and Adhesion • Cohesion – Water sticks together. – Ever see a glass of water filled above the rim? Water can do this because it is cohesive – Why is water cohesive? HYDROGEN BONDS!! • They cause the water molecules to stick together!

Cohesion/Adhesion • Adhesion – Water clings to other things. – Ever see water climb up a thin glass? – Or climb up a thin tube? – Again, caused by Hydrogen bonds being attracted to the glass This allows water to move up a tree from the roots through the vascular tissue (tiny tubes)

Specific Heat Capacity • Water can hold a lot of heat! • In other words, it takes a long time and a lot of energy to increase the temperature of water • That’s a good thing! Especially if you’re a fish! You wouldn’t want the oceans to get scalding hot every time it was hot outside • Our lakes and oceans are what modifies the temperature on the planet

Latent heat of Vaporization • It takes a lot of energy to increase the temperature of water but even more to make it boil and turn into a gas • That’s also a good thing if you’re a fish. It makes the oceans less likely to vaporize (change to a gas)

Excellent Solvent • Water can dissolve many solutes. Specifically, if they’re also polar. It can form hydrogen bonds with other polar molecules and pull them apart to help them dissolve • However, water will not dissolve non-polar substances like oil

Water is More Dense in Liquid Form • Hmmm… how can we prove that? ?

Liquid Density • When water freezes, the hydrogen bonds become more rigid and it forms a crystalline structure • See how the molecules are closer together when water is a liquid? That means it’s more dense.

Density • Water is not the only substance that is denser as a liquid but it’s pretty rare • The vast majority are more dense in the solid state • Some people say that this property of water is “proof that God exists”!! • Why would they say that? ? – Think about what the world would be like if ice was more dense than water