Solutions Liquid Water and Its Properties

Review What is meant by the term polarity? What is meant by the term polarity? It refers to the net molecular dipole arising out of electronegativity differences between covalently bonded atoms. It refers to the net molecular dipole arising out of electronegativity differences between covalently bonded atoms. This means the overall molecule has a positive side and a negative side. This means the overall molecule has a positive side and a negative side. + -

The Water Molecule Are the O-H bonds in a molecule of water polar? Are the O-H bonds in a molecule of water polar? Yes; the oxygen atoms have a higher electronegativity, so they have a partial negative charge when bonded to hydrogen. Yes; the oxygen atoms have a higher electronegativity, so they have a partial negative charge when bonded to hydrogen. This leaves the hydrogen atoms as partially exposed protons. This leaves the hydrogen atoms as partially exposed protons.

The Water Molecule What is the shape of a water molecule? What is the shape of a water molecule? Bent Bent Is water a dipole? Is water a dipole? Yes, the bent shape allows a positive side and a negative side. Yes, the bent shape allows a positive side and a negative side.

The Water Molecule What are the predominant intermolecular forces in water? What are the predominant intermolecular forces in water? Hydrogen bonds Hydrogen bonds

Surface Properties Surface tension: Surface tension: An inward force that tends to minimize the surface area of a liquid; it causes the surface to behave as if it were a thin skin. An inward force that tends to minimize the surface area of a liquid; it causes the surface to behave as if it were a thin skin. Draw arrows on the picture to indicate the hydrogen bonds. Which molecules have the least connections? Draw arrows on the picture to indicate the hydrogen bonds. Which molecules have the least connections? The outermost molecules are only pulled in one direction, which causes the surface tension. The outermost molecules are only pulled in one direction, which causes the surface tension.

Surface Properties

Surfactant: Surfactant: Any substance with molecules that interfere with hydrogen bonding between water molecules, reducing surface tension Any substance with molecules that interfere with hydrogen bonding between water molecules, reducing surface tension Soaps and detergents Soaps and detergents AKA “wetting agents” – they help the water spread out and cover a surface AKA “wetting agents” – they help the water spread out and cover a surface

Water Vapor and Ice What is the melting point of a substance? What is the melting point of a substance? The temperature at which it changes from a solid to a liquid The temperature at which it changes from a solid to a liquid What is the freezing point of a substance? What is the freezing point of a substance? The temperature at which it changes from a liquid to a solid The temperature at which it changes from a liquid to a solid How is the melting point related to the freezing point? How is the melting point related to the freezing point? They are the same numerical value. They are the same numerical value.

Water Vapor and Ice What is the change from a liquid to gas called? What is the change from a liquid to gas called? Vaporization Vaporization What is the name for vaporization just on the surface of the liquid? What is the name for vaporization just on the surface of the liquid? Evaporation Evaporation What is vaporization throughout a liquid? What is vaporization throughout a liquid? Boiling Boiling

Water Vapor and Ice What is the change from gas to liquid called? What is the change from gas to liquid called? Condensation Condensation What is the name of the temperature at which a gas changes to a liquid? What is the name of the temperature at which a gas changes to a liquid? Boiling point Boiling point

Evaporation and Condensation What examples of evaporation and condensation are found in normal day-to-day situations? What examples of evaporation and condensation are found in normal day-to-day situations?

Ice Are solids typically more or less dense than liquids? For example, is solid lead more or less dense than molten lead at the same temperature? Are solids typically more or less dense than liquids? For example, is solid lead more or less dense than molten lead at the same temperature? More dense – the particles are closer to each other in the solid More dense – the particles are closer to each other in the solid Does water follow this trend? How can you tell? Does water follow this trend? How can you tell? No, ice is less dense than liquid water. This is obvious because ice floats in liquid water. No, ice is less dense than liquid water. This is obvious because ice floats in liquid water.

Ice How does hydrogen bonding affect the structure of ice? How does hydrogen bonding affect the structure of ice? Liquid water has hydrogen bonds, but the molecules also have enough energy to move around quite a bit. As the temperature lowers and the molecules lose energy they slow down. At the freezing point, the hydrogen bonds “lock” the molecules into place within a crystalline structure. This crystal pattern is an open-faced hexagon shape, which spreads the molecules out compared to those in liquid water. Liquid water has hydrogen bonds, but the molecules also have enough energy to move around quite a bit. As the temperature lowers and the molecules lose energy they slow down. At the freezing point, the hydrogen bonds “lock” the molecules into place within a crystalline structure. This crystal pattern is an open-faced hexagon shape, which spreads the molecules out compared to those in liquid water.

Ice

Ice Is the floating nature of ice important for life? Why or why not? Is the floating nature of ice important for life? Why or why not? Yes. It creates an insulating barrier above cold water. It allows deep water (under high pressure) to maintain a temperature of 4 o C. Yes. It creates an insulating barrier above cold water. It allows deep water (under high pressure) to maintain a temperature of 4 o C.