2-2 Properties of Water

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22 Properties of Water Polarity As for most molecules, the and charges in a water molecule are equal so that overall the molecule is neutral. However, note that the molecule is not linear.

1 22 Properties of Water Polarity As for most molecules, the and charges in a water molecule are equal so that overall the molecule is neutral. However, note that the molecule is not linear. The water molecule looks like Mickey Mouse (angular), where the oxygen atom is the face and the hydrogen atoms are the ears. Oxygen has more positively charged protons in its nucleus (8), than the hydrogens (1 each), so oxygen should have a greater affinity (attraction) for electrons. Therefore, the oxygen end of the water molecule should have more negative charge. The hydrogen end of the water molecule should have more positive charge. This uneven distribution of charge across the water molecule means that water molecules are POLAR! This is the basis for all the magical properties of water. Label and regions of the model above. 11

2 22 Properties of Water Hydrogen Bonding The polarity of water molecules produces many unique emergent properties (magical even!). Using and symbols, label each atom in the diagram with its polar charge. As you know, opposite charges attract. The dotted lines in the diagram represent hydrogen bonds which form between negatively charged oxygen in one molecule and a positively charged hydrogen in another molecule. Each water molecule is capable of interacting with four other water molecules in this way. In order of weakest to strongest Van der Waals Forces between MOLECULES, Hydrogen bonds between MOLECULES, Ionic bonds between ATOMS in ionic compounds, Covalent Bonds between ATOMS in molecular compounds. Hydrogen Bond Label and regions and the hydrogen bonds of the model above. 12

Cohesion of water molecules at the surface of a body of water produces surface tension, a property of water that provides a microecosystem known as the surface

3 22 Properties of Water Cohesion Polarity produces hydrogen bonding between water molecules which causes them to stick together. Said another way water sticks to itself, which is the property of cohesion. Water is cohesive. Cohesion of water molecules at the surface of a body of water produces surface tension, a property of water that provides a microecosystem known as the surface film. It also causes water to bead on a hydrophobic (water fearing) surface, like a leaf or freshly waxed car. This water strider is able to walk on the surface film due to the hydrophobic compounds on its feet and the distribution of weight due to its long feet. 13

This meniscus (airwater interface) is u shaped, instead of straight across because the adhesive force of

4 22 Properties of Water Adhesion Due to polarity, water molecules are not only able to hydrogen bond with each other, but other molecules too! This property is called adhesion. Water is adhesive. This meniscus (airwater interface) is u shaped, instead of straight across because the adhesive force of water to glass is stronger than the cohesive force between water molecules. This causes the water in the middle to sag due to gravity. 14

22 Properties of Water Capillarity The very small glass tube in the top photo is known as a capillary tube. The red substance inside it is blood (label them).

Water will move against gravity through or into very fine tubes or mesh. This property is called capillary action, or capillarity.

5 22 Properties of Water Capillarity The very small glass tube in the top photo is known as a capillary tube. The red substance inside it is blood (label them). The primary component of blood is water. It is an aqueous solution/suspension. Plants need to move an aqueous solution up from the soil through their stems to their leave. Water will move against gravity through or into very fine tubes or mesh. This property is called capillary action, or capillarity. This property emerges from the fact that water is both cohesive and adhesive. In the capillary tube, water molecules adhere to the glass while pulling other water molecules behind them due to cohesion. Capillary Tube Blood 15

6 22 Properties of Water Solutions and Suspensions A mixture is two or more substances that are not chemically combined. A solution is a mixture where a solute is disolved (evenly distributed) within a solvent. Water is known as the universal solvent because it can dissolve more compounds than any other substance! Water has this property because it is POLAR! The compounds most soluble in water are other polar compounds ( Like dissolves like. ) and ionic compounds, like NaCl. In the diagram, observe how the water molecules are interacting with the dissolved sodium and chloride ions. This is because the positively charged hydrogen end of the water molecules are attracted to the negatively charged chloride ions and the negatively charged oxygen end of water molecules is attracted to the positively charged sodium inons. Water molecules suround the ions and carry them into solution. Life is in aqueous solution!!!. Cl Cl Na Na Water Water 16

7 22 Properties of Water Solutions and Suspensions The other type of mixture is a suspension. A suspension is a mixture of small particles distributed, but not dissolved, in a fluid, so that if left undisturbed, the particles will settle out, or sink to the bottom. Blood is a solution because it has many ions and polar molecules disolved in it. It is also a suspension because when not flowing, blood cells and platelets will settle out of the mixture. 17

2-2 Properties of Water. Copyright Pearson Prentice Hall

2-2 Properties of Water. Copyright Pearson Prentice Hall 2-2 Properties of Water Water Water is the most important molecule on earth. Because of its unique shape and chemical behavior it easily bonds with other molecules, and itself. Water: Covalent Bond Water