Water, a liquid like no other

Authors : Melanie Schultalbers(plus d'infos)
Summary :
Water has amazing properties. Thus, it “takes more space” (it expands) when it freezes, unlike almost all other liquids. This explains why water pipes sometimes burst in winter and that we should not put closed bottles filled with water in the freezer. This is what is called the “anomaly” of water.
Publication : 28 August 2012
Objectives :
This sequence has the goal of familiarizing the children with water’s exceptional property of expanding when it freezes, which they study through experiments and learn to recognize basis of the experiments playful way.
Duration :
7 séances
Material :

Freezing mixture - It is a mixture of ice and salt in a 2:1 ratio. If we mix well ice and salt the ice-water-salt mixture can reach about -20°C. Two processes are at work in a freezing mixture: the thin film of water covering the ice dissolves the salt and the ice melts. Both processes require energy, which is absorbed from the external environment, the mixture cools and cools itself.

Note :
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This sequence is extracted from the activities of the site   Sonnentaler, mirror site in Germany of Sonnentaler, site miroir en Allemagne the website for La main à la pâte. It was created at the Technical University of Braunschweig, under Melanie Schultalbers’ license (bachelor) within Professor Rainer Müller’s didactics of physics group. Access the sequence in German on the site of Sonnentaler. It is of course not a sequence directly designed for the teaching of French programs.. Nevertheless, French teachers will find that the objectives of this sequence are not so far away from their own and they will make the most out of the proposed activities.
A scientific clarification  for the teachers accompanies this sequence. The translation of this sequence was provided by AuroreFargette student at the ÉcolePolytechnique, and an intern at La main à la pâte.

 


Water has amazing properties. Thus, it “takes more space” (it expands) when it freezes, unlike almost all other liquids. This explains why water pipes sometimes burst in winter and that we should not put closed bottles filled with water in the freezer. This is what is called the “anomaly” of water.

This sequence has the goal of familiarizing the children with water’s exceptional property, which they study with experiments and learn to recognize the basis of the experiments in a playful way.

In addition to this sequence a scientific clarification for teachers is also provided. It explains more precisely the phenomenon and its causes.

This sequence was created at the Technical University of Braunschweig, under Melanie Schultalbers’ license (bachelor) within Professor Rainer Müller’s didactics of physics group.


 

Possible course of the sequence

The duration of sessions is 90 minutes; in fact, when conducting experiments requiring the production of a refrigerant mixture, the results of the experiment can only be observed after about 60 minutes, the time required for the solidification process.


 Notes for the teachers 

The advantage of a refrigerant mixture is that the students can see what is going on: that is impossible with a refrigerator. If you are going to buy crushed ice in a beverage shop, you will immediately have a large quantity of ice and the refrigerant mixture is easier to achieve. Make sure not to use de-icing salt, in case the children have the idea of tasting it.

  

Session 1

1. Begin by discussing the story of Captain Cook’s experiment. Formulation of a question: “What happens to the ice in barrels?”
2. “How can we verify it?” Development of a suitable experimental setup (e.g. Captain Cook’s experiment).
3. Implementation of Captain Cook’s experiment and of the “the North Pole in a glass of water” experiment.
4. Transcript of the experimental setup, assumptions, observations, and conclusions in the experiment notebook.

The model of the water molecule is first explained with the help of small snowmen made of pipe cleaners, because they are more “flexible” than the human body when it comes to make an angle of 105° with the legs. In addition, the structure of the ice is easier to represent as well.

Material 
• Pipe cleaners in three different colors


 

 

Session 6

1. Introduction of the model (see the introduction for the teachers). It is important to show the limitations of the model to avoid misrepresentations.
2. Making of little figures with pipe cleaners [that’s right!], layout of water molecules and ice crystals (formation of hexagonal rings in a plane (2D) and in space(3D)).

 

Session 7

1. Introduction of the analogy in the form of a game.
2. Elaboration of the rules of the game (What does a water molecule look like? [tetrahedral structure] How are the molecules arranged? [formation of hydrogen bonds] How long does the intermolecular bonding in liquid and in solid state last? [linking of short-term liquid, durable in solid state])
3. Implementation of the game according to the rules previously agreed
 


 

Session 1

Captain Cook’s experiment

 


The "GorchFock" amid icebergs,   © Bundeswehr / Bednarzik

For sailors, the supply of fresh drinkable water has always been of paramount importance, because you cannot drink seawater. In 1773, a special way to get water was found by the expedition of Captain James Cook, who had entered in the Antarctic during his voyage of exploration around the world. Georg Forster, one of the participants of the expedition, reports:
“On the ninth morning, we saw a great ice island, surrounded by numerous loose fragments, and as we had a tempered wind, we had to resolve ourselves to put a boat in the water to collect as many fragments as possible. These blocks were then thrown onto the deck of the ship, broken into pieces and packed in barrels. After eating, we let the pieces melt in bowls and we poured something warm on what was left in the barrels so that this ice could also melt. The water that we recover from the melted ice was very soft and tasted purer than the one from the Cape that we had in stock [...]
Some people on board who must have not had knowledge of physics were seriously worried, thinking that once the ice melted it would explode in the barrels in which it was enclosed. They had not considered that, as the ice floats on water, it takes up more space than water. To open their eyes, the Captain…”
The teacher will certainly have to help the children in “deciphering” this text, for example: children can then reflect in small groups to try to continue the story; Different objectives can then be presented to the entire class and discussed.
How could we represent in the classroom what happened with the barrels of Captain Cook? All proposals are gathered and reviewed. A possible experiment is described below.

 

 

Captain Cook’s experiment

 

Materials:
• A glass containing frozen water
• (A pan of water)
• (1 hotplate) 

 Development and execution of  the experiment
 

The glass of frozen water is placed in the pan of water. The pan of water is then carefully heated
Excerpt from an experiment notebook


Observation 
Not only the water has not overflowed the glass, but the volume of water is also less than the initial volume of the ice.

 
Explanation

The density of the ice is lower than that of water, which is to say that the ice occupies a greater volume than water at a given mass. If the ice melts, the mass remains the same, but the density increases. This is why the volume decreases.

This is exactly Captain Cook what did. Georg Forster’s report continues:
“To open their eyes, Captain Cook carried a container filled with small pieces of ice to a warm cabin, where the ice melted little by little and took up less space than before… 


 

Captain Cook’s experiment: before and after 


 

It can impressively be shown that ice has a lower density than water through another experiment. The children most likely by themselves will not think of the “The North Pole in a glass of water” experiment; in this case, the teacher can describe its experimental setup and implementation. The children may nevertheless perform the following steps (formulation of a hypothesis, carrying out the experiment, discussion in small groups, written records in the experiment notebook) independently.



 

The North Pole in a glass of water


 Materials
• 1 glass
• hot water
• 1 ice cube
• coins (optional)

Development and execution of  the experiment
We fill the glass with hot water almost to the brim. The ice cube is then carefully placed in the water. Finally, the glass is filled to the brim, either by adding water with a spoon, or by gently adding coins.


 
Excerpt from an experiments notebook


 
Observation 
The ice cube melts and the water filled up to the brim of the glass does not overflow


Explanation
Due to the lower density of ice compared to water, ice occupies a greater volume for the same mass. When the ice below the surface of the water melts, it occupies a smaller volume, and the water level decreases. The ice which is located above the surface of the water melts as well, so that the glass is filled to the brim with water.

 
The North Pole in a glass of water




Session 2

 

Ice in a bottle

 What would have happened if there had been liquid water in the barrels, and that it then froze? After a discussion with the whole classroom, the children reflect on how to verify their assumptions. Possible experiments are presented below.

Materials 
• 1 small glass bottle (100-250 ml) with a fixed screw cap 
• Water
• A small bucket
• Freezing mixture (mixture of ice and salt in a 2:1 ratio)

Develoing and carrying out of the experiment
The glass bottle is filled to the brim with cold water and then closed with the screw cap. The bottle is then placed upside down in the bucket  and completely surrounded by the freezing mixture.

 
Excerpt from an experiment notebook

 Observation 
The bottle explodes after about an hour.

Explanation 
Water occupies a larger volume in solid state than in liquid state. As the glass bottle is closed, it explodes during the solidification process.



 

Ice in a bottle 

Ice needs space

Materials 
• A surprise-egg with plastic egg inside (similar to “kinder egg”, N. of T.)
• 1 disposable syringe

• Water
• A bucket
• Freezer (compartment) or freezing mixture (mixture of ice and salt in a 2:1 ratio)

Development and execution of the experiment 
Fill the entire plastic egg with water by plunging the two halves into a bucket filled with water and closing the egg underwater. Fill the syringe with water. The plastic egg and syringe are then placed in the freezer or the freezing mixture.Matériel
 

Observation 
The two halves of the plastic egg separated, and you can see the ice in the middle. One can read an increase in volume on the graduation of the syringe.

Explanation 
Water occupies a larger volume in solid state than in liquid state. Since the plastic egg is completely filled with water, it opens up during the solidification process.


 


Surprise egg, before and after freezing



Disposable syringe, before and after freezing


Links with everyday life
Who has not already seen cracks on a paved road? Water gets under the pavement through cracks. In winter, the water freezes, taking more space and “popping” the layer of asphalt. 


Session 3

Do all “liquids” take up more space in the solid state than in  the liquid state? Which liquids - apart from water and liquids composed largely of water (milk, lemonade, etc.) do children know well? Have children already observed what happens with liquid wax around the wick after blowing out a candle? How can we check what happens in the case of these “other” liquids?

The teacher writes suggestions on the chalkboard. The proposals are then examined one by one in the classroom. A possible experiment is presented below.

 Hills of ice and valleys of wax 
Materials
• 2 metal screw caps for a bottle (Perrier type)
• Melted wax
• Water
• Freezer or freezing mixture (mixture of ice and salt in a 2:1 ratio)

Development and execution of  the experiment
Fill one of the metal caps to the brim with liquid wax, and the other with water. While observing the cap containing liquid wax at room temperature, place the cap containing water in the freezer or in a freezing mixture. Be careful not to put the cap on a material that is a good thermal insulator, but on the bottom of the freezer, or on something that is already frozen and contains a lot of water (high water content).
• 
Excerpt from an experiment notebook

Observation 
While the solidified wax has formed a hollow (a valley), the ice has formed a little bump (a hill).

Explanation 
When the wax solidifies, it takes a smaller volume than in liquid state, which means that the level of liquid wax dropped during solidification and a hollow appears. In contrast, water occupies a greater volume in the solid state, which means that the water level rises in the process of solidification, and a bump appears.

 

Hills of ice and valleys of wax




Sequence  4

“The candle that sinks, the ice cube that floats" is another experiment to show that water behaves differently from other liquids. Perhaps this experiment has already been proposed by the children at the beginning of session 3. If this is not the case, the teacher can guide the children again, by pointing out the ice floating in water (session 1). The children will probably think by themselves to watch what would happen if you put solid wax in liquid wax (or oil “cubes” in oil).

If you want to perform the experiment with oil, it is recommended to pour oil on a dish and then place it in the freezer. The consistency of the frozen oil approaches that of the ice cream (at least at the temperatures prevailing in a freezer), so the easiest method is to take small “portions of oil” from the frozen mass using a small spoon. The experience must also be of short duration because the oil jelly melts very quickly 

The candle that sinks, the ice cube that floats


Materials
• 2 glasses
• Water
• Ice cubes
• Liquid wax
• Leftovers of candles

Development and execution of the experiment
Fill one of the glasses with water and the other with liquid wax. Put an ice cube in the glass of water and a piece of candle in a glass filled with liquid wax.


 

Excerpt from an experiment notebook

Observation 
While the ice cube floats in water, pieces of the candle wax sink in liquid wax..

Explanation 
Ice, having a lower density than water, floats. Solid wax, having a greater density than liquid wax, sinks.

 

«

The candle that sinks, the ice cube that float»




Session 5

The teacher asks a “difficult” question to the children: “Do you have an idea how to sink the ice cubes?” The teacher writes once again the children’s ideas on the board and lets them talk about it. The following experiment could be one of the proposals made by the children.

Ice cubes that sink

 
Materials 
• Water
• Oil
• 3 ice cubes
• 3 glasses

Development and execution of the experiment
Two of the glasses are filled (to one third of their height approximately) with water and oil respectively. In the third glass, first put water and then oil. Finally, the ice cubes are placed in the glasses carefully.


 

Excerpt from an experiment notebook

Observation 
In the glass filled with water, the ice cube floats. In the glass filled with oil, the ice cube sinks. In the glass filled with water and oil, the ice cube goes through the oil and stops at the border between oil and water.

Explanation 
Ice has a lower density than liquid water, this is why it floats (on  water). Likewise oil has a lower density than water because it floats (when put into water). Comparing the densities of the three substances (ice, liquid water, liquid oil) shows that water, with 0.998206g/cm³ at 20°C has the highest density. Ice has a density of 0.918g/cm³ at 0°C and oil density of 0.914g/cm³.

Since water has a lower density than ice, the ice cube sinks in the oil and plunges to the border of oil and water in the third glass.


 

Ice cubes that sink




Session 6

This session aims to help the children realize why water behaves “abnormally.” Of course, it is still possible that children, in small groups, try to know more about the water molecule by searching in books and on the Internet. If the teacher lacks time, he can lead this session and the following as described below.

Modeling with pipe cleaners 
We use the human body to model the structure of a water molecule. Hands are both hydrogen atoms, feet are pairs of free oxygen electrons. Placing the arms and legs so as to establish an angle of 90° to the hip, one obtains a tetrahedral configuration. To represent the hydrogen bonds, each hand can catch a foot from another “molecule.”


 
Sketch of “water molecules” holding its hands and feet  Philip Ball)


The model of the water molecule is first explained with the help of small snowmen made of pipe cleaners, because they are more “flexible” than the human body when it comes to make an angle of 105° with the legs. In addition, the structure of the ice is easier to represent as well.

Material 
• Pipe cleaners in three different colors

 

Model using pipe cleaners


 

Representation of water (disordered arrangement of molecules) 


 

Representation of the ice (formation of hexagonal rings in a plane (2D) and in space (3D

Session 7

Analogy in the form of a game
The analogy in the form of game creates a link with the students’ “world.” In addition, the active participation of children makes it very motivating. However, when one uses such an analogy, it is important to discuss its limitations and problems, in order to not initiate any misrepresentations

 

Analogy in the form of a game (representation of ice – formation of hexagonal rings in a plane) 


 

Tip(s):

Do not use de-icing salt for the freezing mixture.







 

 

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