Warm Up – take out a sheet of paper and answer the following What is the difference between solute and solvent? Why do you think water is called a universal solvent? Define specific heat in your own terms. What is the formula for density?

Properties of Water Part 1

Objective Students will understand water’s unique properties including: Universal Solvent Cohesion Adhesion Polarity Density Buoyancy Specific Heat

Water Water, which covers nearly 70% OF Earth’s surface, it is one of the most abundant substances on Earth. Water is also one of the most important substances to organisms. Water has many unique properties that allow it to support life on Earth. In fact, scientists look for water on other planets to determine if the planet can sustain life.

Water Molecules and Polarity Water is a simple molecule made up of 2 Hydrogen (H) atoms attached to one Oxygen (O) atom. Each H atom shares 1 electron with the O atom. However, the atoms do not share electrons equally. O attracts electrons more strongly than does H, so O tends to pull the shared electrons closer to its nucleus. This arrangement gives a water molecule a slightly negative charge near the O atom and a slightly positive charge near each H atom This uneven distribution of charges across a molecule is called polarity.

Polarity The polar nature of water molecules causes the molecules to attract each other. Like weak magnets, the positive H on each water molecule attracts the negative O on a neighbor water molecule. This attraction causes water molecules to form hydrogen bonds. Polarity and hydrogen bonding of water causes many unique properties

Review Which best explains why water has its unique properties? The number of neutrons in the H and O atoms The division of protons and neutrons in the H and O atoms The chemical interaction of protons between H and O atoms The unequal sharing of electrons between the H and O atoms.

Key Properties of Water The properties of water include: Adhesion Polarity Cohesion Surface tension Universal solvent High specific heat Buoyancy Density.

1. Adhesion Water molecules are not attracted to only each other Adhesion is the tendency of water to stick to other substances Adhesion is at work as water creeps up at the sides of a cup or graduated cylinder, forming a curved surface called a meniscus.

Review How does making a surface water repellant effect its adhesion? It decreases the forces of adhesion, making the water droplets assume a globular form.

Ex. of Adhesion - Capillary Action Adhesion is being practiced when water molecules stick to the sides of a tube (or straw as shown). Notice the water level inside the straws in the figure, are higher then the water level outside the straws. Capillary Action is the result of adhesion

Why does water rise in a tube? Adhesion of water to the walls of a straw will cause an upward force on the water at the edges and results in the water rising. Capillary action occurs when the adhesion to the walls of the straw is stronger than the forces between the water molecules. Capillary action allows water to move through materials with pores inside.

Review Why is capillary action important to vegetation (plants and trees)? Capillary action is responsible for “pulling” the water up the root against gravity, from the ground. Which is the best example of capillary action of water? Steel sinking in water Ice floating in liquid water Water moving up a paper towel Water pushing up against a boat

2. Cohesion The tendency for water molecules to form weak bonds and stick together is called cohesion. The hydrogen bonding between water molecules allows for cohesion Because of cohesion, water molecules remain joined together as they move within or between the cells of organisms.

3. Surface Tension An example of cohesion is surface tension. Surface Tension is a force that acts on the particles at the surface of a liquid Cohesion between the polar water molecules causes tightness across the surface of water. At the surface of the water, the attractive forces of other water molecules pull only downwards and sideways. This force causes molecules at the surface to held more tightly together. This tightness of the waters’ surface keeps a water strider from sinking

Review What determines the shape of this drop on a twig? Cohesion and adhesion determine the shape of this drop on a twig. Which best illustrates the property of surface tension? Oxygen dissolving in water A branch floating in a stream Water sticking to a branch A copper penny sinking in a beaker of water

Properties of Water Part 2

Objective Students will understand water’s unique properties including: Universal Solvent Cohesion Adhesion Polarity Density Buoyancy Specific Heat

4. Universal Solvent Solvent is a substance in which another substance dissolves. Solute is what gets dissolved Ex) salt water - the water is the solvent because it dissolved the salt, the solute. Water is a universal solvent because it can dissolve more substances than any other known substance. The main property of water that makes it such a good solvent is its polarity

Universal Solvent Many substances dissolve in water because water is polar. The charged ends of the water molecule attract the molecules of other polar substances. Water can dissolve other liquids, gases, and solids Substances that have molecules with no charged regions are called nonpolar substances Nonpolar substances do NOT dissolve well in water.

Review When a solute “disappears” into a solvent, can the original components ever be separated again? Ex) salt water - can the solute salt ever be removed from the new solution, salt water? Why is water called a universal solvent? Water’s polarity allows its molecules to be attracted to other polar molecules and substances.

5. Specific Heat Specific Heat is the amount of heat needed to increase the temperature of 1 Kg of a substance by 1˚C The unit of specific heat is joule per kilogram per degree of Celsius. The specific heat of water is very high Therefore water takes a long time to heat up or cool down. Water’s specific heat is due to the strong attraction among water molecules. This property of water allows lakes, streams, and ocean ecosystems to maintain stable temperatures, even if air temperatures change dramatically. The high specific heat of water also helps your body to maintain a constant internal environment.

Review At the beach why is the sand hotter then the ocean? The attraction between the molecules of the sand is weaker then the attraction of the water molecules, thus allowing the sand to heat up more quickly and hotter.

6. Buoyancy Buoyancy is the ability of a fluid to exert an upward force on an object that is immersed in a fluid. Buoyancy is the property that allows fish, whales, and other organisms to float in water

Buoyant Force All liquids exert an upward force, called buoyant force, on objects. If the upward, buoyant force on an object is greater than the downward gravitational force, the object will float.

7. Density Is a measure of the mass of a substance per unit of volume The density of an object changes with temperature For example cold water is denser than warmer water If you pour cold water into a container of warm water, the cold water will sink to the bottom. Density = Mass Volume

Density An object that is more dense than the fluid in which it is immersed sinks. An object that is less dense than the fluid in which it is immersed floats to the surface. If the density of an object is equal to the density of the fluid in which it is immersed, the object neither rises nor sinks in the fluid. Instead, it floats at a constant depth.

Density and floating Any substance with a lower density than liquid water will float in water. Wood, oil, and wax are examples that float A metal boat is able to float because its shape provides areas that can fill with air, which is less dense than water.

Apply When would the block of oak sink? If you could keep increasing the density of the block, it would sink lower and lower into the water. Which property of water allows a metal boat to float in water? Adhesion Buoyancy Cohesion Density

Density Density changes as the physical state, or phase of a substance, changes. What are the 3 phases of water? Most substances become denser as they freeze because their particles pack more closely together. Water on the other hand is reverse. Water expands as it freezes. Expansion occurs because molecules organize into a 6 sided arrangement as water becomes ice. This arrangement causes ice to have a greater volume and a lower density than liquid water. Because it has a lower density, ice floats in liquid water.

Review Why can fish and other organisms remain alive in the liquid water beneath the ice? Because ice is denser then water the ice floats to the surface, while liquid water remains underneath.

Review Which form of water has the highest density? Ice Warm water Cold water Water vapor Which of the following describes the tendency of a water molecule to stick to other water molecules? Cohesion Density Adhesion Buoyancy

Review Why does a block of wood made of oak, sit deeper in water then a block of wood made of pine? The reason is that the oak is heavier for its size, or denser – This means the molecules that make up the oak block are more closely packed together than the molecules that make up the pine.