Seawater TA Initials: for finished Activity. 1 & 2 Or lose 10% of credit!

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1 Name: Section/ TA: Seawater TA Initials: for finished Activity. 1 & 2 Or lose 10% of credit! Seawater is an unusual substance. It is pure water mixed with various salts, trace elements, and gases. The physical and chemical properties of seawater allow it to store and transport heat, help keep atmospheric concentrations of CO 2 low, and support a remarkable variety of life forms. Activity 1: Seawater Density MUST BE DONE IN CLASS The density of a substance relative to the substance around it control which substance will sink or float. For example, when fudge syrup is added to milk it sinks to the bottom of the glass. This is because the fudge syrup is denser than the milk: its mass to volume ratio is higher than the mass to volume ratio of milk. The separation of the substances of different densities is called stratification. In the ocean we observe the stratification of water masses of different densities similar to the stratification of syrup and milk in a glass. Density in the ocean is controlled primarily by temperature and salinity differences. These properties drive the thermohaline circulation of water in the ocean. Thermo refers to the temperature component of density while haline refers to the salinity component. Temperature effect on density: There is an inverse or negative relationship between temperature and density. If temperature increases, density decreases; if temperature decreases, density increases. Increasing temperatures cause substances to expand and become less dense. This is because as a substance acquires heat, the molecules vibrate more and demand more space. The addition of heat does not change the mass of the substance. An increase in the volume without an increase in mass decreases the mass to volume ratio; thus the density decreases. The maximum density of pure water occurs at 4 C. As pure water cools below 4 C, it becomes less dense until it freezes at 0 C. This is because as liquid water freezes, hydrogen bonds form an open structure of ice. Because the structure of ice is more open than liquid water it occupies a greater volume and is less dense. That is why ice floats on water. Salinity effect on density: There is a direct positive relationship between salinity and density. If salinity increases, density increases; if salinity decreases, density decreases. Before we can understand this relationship, we must first define salinity. Salinity is the total amount of dissolved salts per unit of water. The salinity of seawater is commonly measured in g/kg or (parts per thousand or ppt). These two units are equal because there are 1000g/kg. The typical salinity of seawater is about 35 ppt = 35, but there is a range of salinities seen in the ocean, depending on the water depth, temperature, and other factors. Seawater- 1

2 Water masses in the ocean are composed of volumes of seawater having the same density (mass/volume). Water that is colder or more saline is denser. These two primary controls on density (temperature and salinity) are illustrated in Figure 1. Figure 1: The relationship between temperature, salinity and density of seawater. The lines indicate combinations of temperature and salinity that result in the same density. Note that many combinations of temperature and salinity can lead to the same water density. 1. Using Figure 1, estimate the densities of three seawater masses: A: salinity = 34.5, temperature = 3.5 C, density = B: salinity = 34.5, temperature = 13.5 C, density = C: salinity = 36.5, temperature = 20 C, density = 2. Deep ocean water tends to be very dense. Which of the three water masses would be most likely to form deep ocean water? 3. Imagine that the salinity of water mass B decreases, but the water mass maintains the same density. How must the temperature change to allow this to occur? Look at Figure 1 and explain your reasoning. Seawater- 2

3 4. Consider a water mass with a salinity of 35 and a temperature of 5 C. What is the density of this water mass after it warms to 15 C, if salinity stays constant? 5. Consider a water mass with a salinity of 35 and a temperature of 10 C. If the salinity increases by 1, by how much must the temperature change in order for the density to remain the same? Activity 2: Heat Capacity MUST BE DONE IN SECTION (Get TA Initials on pg 1 or lose 10%) Heat capacity is defined as the amount of heat (calories) required to raise the temperature of 1 gram of a substance by 1 o C. Heat capacity values indicate the extent to which a substance can absorb or release heat energy without a change in temperature. Substances with high heat capacities absorb (or release) more heat before changing temperature than do substances with low heat capacities. The heat capacity of liquid water is 1 cal/g o C, higher than all other common substances. The high heat capacity of water is due to hydrogen bonding between water molecules. The unequal sharing of electrons in the covalent bonds between hydrogen and oxygen atoms result in bipolar water molecules (the bonds forming the water molecule are described as polar covalent). A hydrogen bond forms between the slightly negatively charged oxygen atom of one water molecule and the slightly positively charged hydrogen atom of another water molecule. Each water molecule can form up to four hydrogen bonds. As a result, energy must be expended in breaking these hydrogen bonds before the temperature of the water will rise. This is particularly important as water changes state between solid, liquid, and vapor (gas). When a substance changes state, there may be no increase of temperature at the point where state change occurs even though heat is continuously applied. The heat energy is used entirely to break the bonds necessary to complete the state change. For water, 80 calories of heat are required to convert 1 gram of ice to 1 gram of liquid; this is the latent heat of melting. The amount of heat required to convert liquid water to vapor, the latent heat of vaporization, is 540 calories. 1. How much heat would be required to raise the temperature of 10 grams of pure water from 25 o C to 35 o C? Seawater- 3

4 2. How much heat would be required to raise the temperature of 10 grams of pure water from 95 o C to 110 o C? 3. How much heat is released when reducing the temperature of pure water from 57 o C to 50 o C? You can provide the answer as heat per unit volume, and assume that we are talking about 1 cm How much heat is released when reducing the temperature of pure water from 8 o C to -2 o C? 5. Is heat absorbed or released from/to the atmosphere when it snows? Explain. Seawater- 4

5 Activity 3: World Ocean Salt (can take home!) 1. Calculate the amount of salt in the oceans today (in grams) using the following information and steps: Assume that the oceans of the world can be approximated by a basin with totally vertical sides like a bathtub whose bottom has a constant area. Water density = 1000kg/m 3 Average ocean salinity = 35 g/kg Average depth of the ocean = 3800 m Radius of the earth (r) = 6.37 X 10 6 m Equation for surface area of a sphere (A): A = 4πr 2 Area = side X side, or side 2 Volume = side X side X side, or side 3 a. In order to calculate the total volume of the ocean, you ll need to first determine the surface area of the Earth. Show your work. b. The global ocean covers 70% of earth s surface area, so take 70% of the value you calculated for Earth s surface area to determine the surface area of the ocean. c. The average depth of the ocean is 3800m. You have two components of volume in the value you just calculated for ocean surface so multiply your ocean surface area value by 3800m to get ocean volume. d. Use water density to convert volume to mass so that you have the total mass (in kg) of water in the ocean. e. Use the salinity to calculate the total amount of salt in the ocean. Seawater- 5

6 2. Determine how thick a layer covering the ocean floor this salt would form using the following steps: a. Convert your answer from question 1 part e from grams of salt to volume using the density of salt (2.165 g/cm 3 ). b. Distribute that volume on the ocean floor by dividing the volume of salt by the area of the ocean to get thickness of salt covering ocean floor. Extra Credit: There are about 30 million cubic kilometers of ice on the planet (3x10 7 km 3 ). If all of it melted and ended up in the oceans, what would the new salinity of the oceans be? Don t forget to correct for the change in volume between ice and water! Pure water has a density of 1 gram/ml, ice has a density of 0.92 gram/ml. How much heat would be required to melt all of the ice if we assume that it starts at -2 C and we heated it to 4 C? Seawater- 6