Boiling water with ice: Effect of pressure on the boiling point of water
Initial Publication Date: September 26, 2007
Summary
In this activity, which is a discrepant event with guided inquiry, the teacher will go through the steps of boiling water with ice. Through the students telling and recording WHAT is happening, the teacher is showing the students what observations are. Only after the students have answered WHAT is going on, can they start to try to answer WHY/HOW the event is occurring. This is showing the students what inferences or hypotheses are. I use this as a first day activity to get the students familiar with some of the crucial components of the scientific method.
CollapseLearning Goals
The learning goals of this activity are to get the students to record their observations in the lab setting, differentiate between observations and inferences, and have the students start to use the scientific method to answer questions they have in their lives.
Concepts—One key concept this activity is trying to address is the effect of pressure on the boiling point of liquids. Another concept is the idea that the scientific method can be used to solve problems both in science and other (everyday) situations.
Vocabulary terms—Observation, inference, hypothesis, scientific method
Concepts—One key concept this activity is trying to address is the effect of pressure on the boiling point of liquids. Another concept is the idea that the scientific method can be used to solve problems both in science and other (everyday) situations.
Vocabulary terms—Observation, inference, hypothesis, scientific method
Context for Use
This activity is great for the first day of physical science or chemistry class. It allows for the students to see a new scientific concept on the first day and they are curious about why it happens. It is appropriate for all age groups in any class setting. It is always good to review the scientific method and scientific inquiry at the beginning of the course. This activity works in both lab settings and in a classroom setting. It takes about 30 minutes to complete the entire activity.
Description and Teaching Materials
Materials- Hot plate, 500 mL Florence flask, rubber stopper, ring stand, iron ring, water, ice, cooler.
This activity can take place in the lab or the classroom, but easy access to a sink is useful.
Procedure-
- Place a 500 mL Florence flask with ~ 200 mL of water in it on a hot plate.
- Ask the students to make observations about what is happening, have them record them in their notebooks.
- When the water starts to boil, have the students record what is happening in their notebooks. Discuss with the students (have them think, pair, share) the temperature that the water is at right now (212 degrees F, 100 C). Why? How do they know? Record the temps on the board.
- When there is steam coming out the top of the flask (having pushed the air out), place a rubber stopper in the top of the flask and seal firmly. IMMEDIATELY (using hot pad holders) remove the flask from the hot plate. Invert the flask (taking care to not point the stopper at anybody) and place it in the iron ring with the stoppered-opening facing down.
- Record observations.
- Allow the water to stop boiling in the flask.
- Talk about what is happening to the temperature of the water as it is sitting in the flask (it is going down). Compare it to a cup of hot cocoa cooling, etc.
- Record observations.
- Take ice from the cooler and rub it generously on the top of the flask (above the water). The water inside the flask will start to boil again. The more ice that is rubbed on the top of the flask, the more violently the water will boil.
- Record observations.
- "Is the ice heating up the water?" The students should know that the ice is NOT heating the water. So what IS causing the water to boil?
- Record possible inferences or hypotheses.
- Discuss the difference between observations (using senses to tell WHAT is happening) and inference (using the information from the observations to try to answer WHY or HOW it is happening).
- Discuss lab safety procedures, including the flask opening never being pointed at anybody.
- Conclusions: Discuss the real reason for the boiling water using ice. The drop in pressure in the flask allows the water to boil at lower temperatures (even room temperature). Lowering the pressure in the flask drops the atmospheric pressure below that of the vapor pressure of the water and the water boils when this happens. The boiling releases water vapor into the flask to increase the pressure once again. You can discuss this with the students... or you can leave them hanging without any answers (sometimes that is more fun). If the learning outcome is to learn about observations and inferences, the explanation is not necessary.
- Assessment: Have the students write a one-page paper about observations and inferences. They will discuss observations they made about the boiling water and they will make inferences about why it worked. They can use their text books, internet, etc. as sources of information.
This activity can take place in the lab or the classroom, but easy access to a sink is useful.
Procedure-
- Place a 500 mL Florence flask with ~ 200 mL of water in it on a hot plate.
- Ask the students to make observations about what is happening, have them record them in their notebooks.
- When the water starts to boil, have the students record what is happening in their notebooks. Discuss with the students (have them think, pair, share) the temperature that the water is at right now (212 degrees F, 100 C). Why? How do they know? Record the temps on the board.
- When there is steam coming out the top of the flask (having pushed the air out), place a rubber stopper in the top of the flask and seal firmly. IMMEDIATELY (using hot pad holders) remove the flask from the hot plate. Invert the flask (taking care to not point the stopper at anybody) and place it in the iron ring with the stoppered-opening facing down.
- Record observations.
- Allow the water to stop boiling in the flask.
- Talk about what is happening to the temperature of the water as it is sitting in the flask (it is going down). Compare it to a cup of hot cocoa cooling, etc.
- Record observations.
- Take ice from the cooler and rub it generously on the top of the flask (above the water). The water inside the flask will start to boil again. The more ice that is rubbed on the top of the flask, the more violently the water will boil.
- Record observations.
- "Is the ice heating up the water?" The students should know that the ice is NOT heating the water. So what IS causing the water to boil?
- Record possible inferences or hypotheses.
- Discuss the difference between observations (using senses to tell WHAT is happening) and inference (using the information from the observations to try to answer WHY or HOW it is happening).
- Discuss lab safety procedures, including the flask opening never being pointed at anybody.
- Conclusions: Discuss the real reason for the boiling water using ice. The drop in pressure in the flask allows the water to boil at lower temperatures (even room temperature). Lowering the pressure in the flask drops the atmospheric pressure below that of the vapor pressure of the water and the water boils when this happens. The boiling releases water vapor into the flask to increase the pressure once again. You can discuss this with the students... or you can leave them hanging without any answers (sometimes that is more fun). If the learning outcome is to learn about observations and inferences, the explanation is not necessary.
- Assessment: Have the students write a one-page paper about observations and inferences. They will discuss observations they made about the boiling water and they will make inferences about why it worked. They can use their text books, internet, etc. as sources of information.
Teaching Notes and Tips
Here are some helpful hints for doing this activity. If you use a Bunsen burner instead of a hot plate, use the same iron ring for the heating and the cooling of the flask. Be careful that the stopper is pushed in enough that it does not come out. Also, make sure that the stopper is big enough that you can pry it out once the activity is over. I have had a rubber stopper almost get stuck in the flask. Try this activity by yourself to see the methods of transferring the flask to the iron ring (use a lot of caution as the flask can be slippery and you do not want to break it with boiling water inside).
There are a few differences between the way I am planning to do this lab and how I have done it in the past. I never had the student physically write their observations in their notebook. I always wrote them on the board. I might even have the students come up and write it on the board themselves when we get to the discussion portion of the activity. I also never went into great depth with why the water actually boils with the use of ice on the top of the flask. By going into this aspect deeper, it allows us to have conversations that the students can appreciate because they have cooked with boiling water and some have been to areas of high altitude before and may have had to cook there too. I am also going to have the students write a short paper about their observations and possible explanations about why the flask boiled when the ice was used on it.
There are a few differences between the way I am planning to do this lab and how I have done it in the past. I never had the student physically write their observations in their notebook. I always wrote them on the board. I might even have the students come up and write it on the board themselves when we get to the discussion portion of the activity. I also never went into great depth with why the water actually boils with the use of ice on the top of the flask. By going into this aspect deeper, it allows us to have conversations that the students can appreciate because they have cooked with boiling water and some have been to areas of high altitude before and may have had to cook there too. I am also going to have the students write a short paper about their observations and possible explanations about why the flask boiled when the ice was used on it.
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Assessment
I am going to have the papers tell me if the students understand what the concepts of observation and inference really mean to them and see how well they can pick through information to answer the question: "Why does the water boil when ice is rubbed on the flask?" I will have the students share their work with each other right before handing it in and together they/the class will come up with the consensus of why the activity worked.
The students will be assessed on their grasp of the terms and concepts of observation and inference in other situations. The test will ask for their understanding of these terms.
The students will be assessed on their grasp of the terms and concepts of observation and inference in other situations. The test will ask for their understanding of these terms.
Standards
I.B.2. - observation vs. inference
I.B.1 - Scientific method
I.B.1 - Scientific method