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

Why is water so awesome? Looking back at Earth from space, an astronaut called it “the blue planet,” referring to the oceans of water that cover nearly three fourths of Earth’s surface. The very presence of liquid water tells a scientist that life may also be present on such a planet. Why should life itself be connected so strongly to something so ordinary that we often take it for granted? There is something very special about water and the role it plays in living things.

The Water Molecule How does the structure of water contribute to its unique properties?

Polarity Because of the angles of its chemical bonds, the oxygen atom is on one end of the molecule and the hydrogen atoms are on the other. With 8 protons in its nucleus, an oxygen atom has a much stronger attraction for electrons than does a hydrogen atom with its single proton.

O Water has 2 hydrogens and 1 oxygen Electrons are more attracted to the oxygen the oxygen end of the molecule has a slight negative charge and the hydrogen end of the molecule has a slight positive charge. Water is a polar molecule. Polarity : when a molecule has a slightly positive region and a slightly negative region O H _ +

Opposites attract Hydrogen Bonding the partial positive and negative charges can attract each other. hydrogen bond - attraction between a hydrogen atom on one water molecule and the oxygen atom on another

Hydrogen Bonding Water is able to form multiple hydrogen bonds, which account for many of its special properties. Hydrogen bonds are not as strong as covalent or ionic bonds, and they can form in other compounds besides water.

Hydrogen bonds are responsible for important properties of water. universal solvent (not really) high specific heat cohesion adhesion expansion upon freezing

Solutions Water is not always pure; it is often found as part of a mixture. A mixture is a material composed of two or more elements or compounds that are physically mixed together but not chemically combined. Living things are in part composed of mixtures involving water. Two types of mixtures that can be made with water are solutions and suspensions.

Solutions If a crystal of table salt is placed in water, sodium and chloride ions on the surface of the crystal are attracted to the polar water molecules.

Solutions Ions break away from the crystal and are surrounded by water molecules. The ions gradually become dispersed in the water, forming a type of mixture called a solution.

Water can not dissolve nonpolar substances such as lipids (fats) Solutions Water’s polarity gives it the ability to dissolve other polar molecules. Water easily dissolves salts, sugars, minerals, gases, and other polar molecules. **water dissolves so many substances it is called the universal solvent even though it is not entirely accurate Water can not dissolve nonpolar substances such as lipids (fats)

Solutions Solution - mixture of two or more substances ex. salt water. solute—the substance that is dissolved. - ex. the salt solvent—the substance in which the solute dissolves. - ex. the water

Think of a hot day at the beach. Which would be hotter… Think of a hot day at the beach. Which would be hotter….the sand or the water??? Why? The sand is hotter because it takes more energy to change the temperature of the water. The sand, having a lower heat capacity, would therefore heat up faster than the water.

Examples: Heating a pot of water on stove: pot gets much hotter than the water Hot summer day: the sand is much hotter than the water High Specific heat Specific heat- amount of heat energy required to increase the temperature of a substance Water has a high specific heat Water resists temperature changes moderates earth’s temperatures, does not vary very widely

Most substances get progressively more dense as they are cooled Most substances get progressively more dense as they are cooled. Water becomes more dense as it cools to 4° C, then becomes less dense. This means that ice is less dense than water and this is why ice floats. Ice fishing anyone??

Density Most substances get more dense as they are cooled. Water becomes more dense as it cools to 4° C, then becomes less dense as it freezes ice is less dense than water and ice floats. important to life in the water in cold climates Lakes and rivers freeze at the surface only allowing life to survive below

Cohesion Cohesion – the tendency of similar molecules to stick together. Water sticks to water causes surface tension: when water molecules will stick to one another at the surface of the water, making a net or film .

Adhesion – attraction between molecules of different substances. Water sticks to other substances causes capillary action : water rises in a narrow tube against the force of gravity. plants use this property to draw water up the roots and through the entire plant

4. What would be cohesion and adhesion in these pictures? Water drop Cohesion- the water is sticking to itself. Spider web Adhesion- the water is sticking to the web.

Identify two facts you discovered about water. Could be adhesion, cohesion, polarity, pH, high heat capacity…. Explain why it is beneficial that ice is less dense than water? It is beneficial because only the surface freezes and it acts like an insulator for organisms to live. What is the most common chemical compound in cells? H2O How do you think water helps the body function? (Hint: there is more than one answer) Water help maintain homeostasis, helps transport ions, helps transport waste……

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