Student Exploration: Colligative Properties (ANSWER KEY)
Student Exploration: Colligative Properties
Vocabulary: boiling point, colligative property, concentration, dissociate, freezing point, manometer, osmosis, osmotic pressure, solute, solution, solvent, vapor pressure
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
1. When the forecast calls for freezing rain, salt is often spread on the roads. Why is this done?
2.Antifreeze is added to car radiators to prevent the water in the radiators from both freezing and boiling. How do you think antifreeze affects the temperatures at which water freezes (the freezing point) and boils (the boiling point)?
Gizmo Warm-up
Adding salt or other substances to water can affect the temperature at which it freezes or boils. These effects and others, known as colligative properties, are explored in theColligative Properties Gizmo™.
Check that No solute is selected and the Air temp is 25 °C.
1. Describe the motion of the water molecules in the beaker.
2.Change the Air temp to –10 °C. What do you see as the water freezes
3.Now change the Air temp to 110 °C. What do you see as the water boils
Introduction: At the surface of a body of water, water molecules are constantly moving from the liquid phase to the gas phase, and vice versa. The molecules that evaporate collide against the walls of the beaker, creating vapor pressure.
Vapor pressure can be measured by a manometer, a liquid-filled tube that is connected to the top of the beaker. Increasing the vapor pressure pushes the water in the tube, causing the water in the right arm of the tube to rise to a higher level than the water in the left arm of the tube.
Question: How does salt affect the properties of water?
1.Observe: Gradually increase the Air temp to 95 °C. As you do this, observe the water in the manometer. What do you notice, and what does this indicate about the vapor pressure?
2.Record: Click the Record button below the table to see the vapor pressure in kilopascals (kPa). Use the Gizmo to find the vapor pressure at 5 °C, 50 °C, and 95 °C.
3.Analyze: How does the temperature of the liquid relate to the vapor pressure
4.Measure: To measure the boiling point, increase the Air temp to 110 °C. When you see bubbles, click Record. The water temperature at this time is equal to the boiling point.
A.What is the boiling point of pure water?
B.What is the current vapor pressure
C.Atmospheric pressure at sea level is equal to 101.32 kilopascals (kPa). Why do you think water boils when the vapor pressure is equal to atmospheric pressure?
5.Compare: Click Clear. Set the Air temp. to 25 °C, and click Record. Compare the vapor pressure with No solute to the vapor pressure with Sodium chloride selected.
A.How does the presence of sodium chloride affect the vapor pressure of water?
B.Based on this effect, how do you expect the presence of sodium chloride to affect the boiling point of water
6.Predict: How do you expect the addition of sodium chloride (table salt) to affect the freezing point of water
7.Gather data: Use the Gizmo to find the vapor pressure at 25 °C, the boiling point, and the freezing point of pure water and the sodium chloride solution. To find the freezing point, lower the Air temp to –10 °C, wait for the molecules to start freezing, and click Record. The water temperature at this time is equal to the freezing point. Complete the table below.
8.Analyze: How does adding sodium chloride affect the boiling and freezing points of water?
9.Apply: Based on what you have learned, why do people spread salt on the roads in winter?
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