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Khuong Mai - Ideal gases and Their Properties

Khuong Mai - Ideal Gas and Their Properties

Lesson Plan for 10-12 Grade Chemistry

Duration: 55 minutes

Chapter: Ideal Gas and Their Properties

Title of experiment: Can water be boiled using cold water?


Principle(s) Investigated:

This set of demonstrations is designed to help students understand the Gas Law in real-life settings. Students will be able to see and to correlate the relationship between the pressure, temperature, and volume, and quantity of  gas according to the Ideal Gas Law formula P V  =  n R T. Students will observe and record the effect of pressure and gas quantity changes on the temperature.

California Content Standards Addressed:

Chemistry Standard Set #4:

The kinetic molecular theory describes the motion of atoms and molecules and explains the properties of gases. As a basis for understanding this concept:

c. Students know how to apply the gas laws to relations between the pressure, temperature, and volume of any amount of an ideal gas or any mixture of ideal gases.

d. Students know the values and meanings of standard temperature and pressure (STP).

e. Students know how to convert between the Celsius and Kelvin temperature scales.

Materials:

Worksheet "How To Boil Water Using Cold Water," heating plate or Bunsen burner, iron stand with iron ring, 500-mL Flask, rubber stopper, 2-L beaker, one‑gallon plastic bottle, plastic water bath, cloth glove, water, scratch paper, and pen/pencil. [Optional: a thermometer inserted in the stopper]

Procedure:

The teacher announces the procedure and performs the experiment.  Students are asked to predict various effects, and then record their observations on the provided data sheet as the teacher demonstrates.


Part 1: The teacher boils 200 mL of water in a 500-mL Erlenmeyer flask,   cap and invert it on a iron ring stand, then cold water on it. Students answers teacher's questions, and record their observation.


Part 2: The teacher holds the still-warm flask in his palm (to show it's no longer hot) and asks the students how long does it take to boil the water again - a couple minutes is the anticipated answer. The teacher places the capped flask on the hot plate. The water should boil again within 5 seconds. Students record temperature, pressure, etc., and their observation.


Part 3: With the cap still on, the flask is placed on the hot plate for about 30 seconds and the water should boil. The flask is then placed in a cold bath. The water boils again. Students are to explain  How could the same water boil in both hot and in cold conditions?

Student Prior Knowledge:

Students should have learned the Ideal Gas Laws and know the correlation between the pressure, temperature volume, and quantity of gas.

Explanation:

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Applications to Everyday Life:

1. Pressure cooker: In industries, the cooking vessel lid (manhole) is bolted tightly so that the temperature can be raised above the normal boiling point of water of 100°C. This would help shorten the cooking time and thus save energy.

2. Drying: For heat-sensitive material, vaccum (low pressure) and low temperature may be applied to accelerate the drying process. For instance, the extract from ground coffee is cooled to an ice-solid and freeze-dried under high vacuum. This method minimizes the loss of the flavor while effectively removes the water.


Reference/Credit:

Part 1 of this demonstration was taken from Professor Norm Herr's demonstration in class SED 525 at California State University, Northridge, Fall 2009.


Questions and Grading rubric

Answer the following questions (2 points each, total 20 points possible)

Q1:       At Part 1, what is the pressure inside the open flask before and during boiling?

Q2:       At Part 1, what is the "gas" generated in the flask during boiling?

Q3:       At Part 1, when the flask is inverted on the stand, what did you notice?

Q4:       At Part 1, explain why the water boils when cold water is poured onto the flask.

Q5:       At Part 2, estimate the temperature of the water.

Q6:       At Part 2, explain why does it take less than 10 second to boil the water?

Q7:       At Part 3, Why does the water in the same flask boil at both higher and lower temperatures? 

Q8:       Which factor remains constant throughout the experiment?

Q9:       Does the chemical composition of water change before, during and after heating/cooling?

Q10: What conclusions can you draw from the experiment?

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HANDOUT                                                                                             Name:  _________________________    P: ____

IDEAL GAS LAW EXPERIMENTAL PLAN         

Answer the following questions:


Part 1 of experiment:

Q1:       What is the pressure inside the open flask before and during boiling?

A1:       When the flask is open, the pressure is the same as that of atmosphere, which almost equal to 1 atm.

Q2: What "gas" is generated  in the flask during boiling?

A2:       The gas is actually steam.

Q3: When the flask is inverted on the stand, what did you notice?

A3: One should notice that some bubbles "bubble up" from water layer. This is due to cooling effect from the surrounding.

Q4: Explain why the water boils when cold water is poured onto the flask.

A4:       As cold water is poured on to their inverted flask, two effects are noted: (1) The cooling immediately effects the steam, which condenses into (liquid) water, and thus creates a negative pressure; and (2) The cooling also affect the hot water; however, the water is still hot and immediately boils under reduced pressure.  

Q5: Estimate the temperature of the water.

A5:       About 50-90˚C, which is sufficiently hot to boil under reduced pressure. 


Part 2 of experiment:

Q6: Explain why does it take less than 10 second to boil the water?

A6:       In the flask, the water was still relatively hot (50-60˚C); so when placed on the hot plate in about 3-10 seconds, it boils almost immediately. This is due to the reduced pressure inside the flask; the temperature was actually about 60-80˚C, and not 100˚C. 


Part 3 of experiment:

Q7: Why does the water in the same flask boil at both higher and lower temperatures? 

A7:       In each case the water boils for different reasons: When the flask is placed on a hot plate, the water boils for reason as explained in Answer A6. When dipped into cold water, the water boils because of the condensation of steam and the ensued generation of reduced pressure as explained in Answer A4.     

Q8: Which factor remains constant throughout the experiment?

A8:       The volume of the flask.

Q9: Does the chemical composition of water change before, during and after heating/cooling?         

A9:       No, water as gas (steam), liquid (water) or solid (ice), the chemical composition is still the same as H2O.

Q10: What conclusions can you draw from the experiment?

A10: The experiment demonstrates the correlation between the three factors: temperature, pressure and quantity of gas. Since the flask is used the volume remains constant throughout the experiment. The following conclusions are drawn from the experiment:

(1)        The temperature is directly proportional to the pressure. As the pressure is reduced, the boiling point is decreased (water boils when cooling is applied to the hot flask).

(2) The quantity of gas is directly proportional to the pressure. As the steam condenses to water, the amount of gas particles is reduced. This leads to a reduction in pressure resulting in water boiling.

Employing the Gas Laws, one may change the boiling point of any liquid to achieve certain desired effect; e.g., distillation under high vacuum. .  

 

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