Teacher Portal:
Heat and Heat Transfer
Investigation 2 – Concept Day
ZERO-IN
The italicized font represents information to be shared orally or physically completed with the students at this time.
The non-italicized font represents additional information included supporting the teacher’s understanding of the content being introduced within the CELL.
ASK WHY
Remind students that heat, thermal energy, and temperature greatly influence the way we live. We monitor our body temperature, dress for the hot and cold, build homes, schools, and cars with heating and cooling systems, and passionately search for alternative fuels as the threat of global warming takes hold. To live safely and productively, it is important to understand the thermal behaviors that govern our bodies and world.
BRANCH OUT
Explain to students that environmental engineers design the structure and irrigation for rooftop gardens, helping reduce building heating and cooling costs. They also help refineries reduce their toxic gas emissions, which contribute to acid rain and global warming.
PRINT IT
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NAVIGATE IT
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SHARE IT
SLIDE HEAT-2-1
- Remind students that
- In the previous Investigation, they explored the freezing and boiling points of water as a means of calibrating their own thermometers.
- When they made an ice bath and caused the liquid water in the test tubes to freeze solid, they added salt to the bath.
- The reason we had them add salt to the water bath was so that the ice/water mix in the bath would drop below 0oC and could, therefore, cause the water in the test to reach the freezing point of zero and freeze.
- Inform students that in this Investigation, they will more specifically study the effect of adding dissolved solute (salt, NaCl in this case) to a solution of water to lower its freezing point.
- Inform students that they will also examine the effect of salt on the boiling point of water.
Note: While it would be simple to tell students that adding salt to water lowers its freezing point and raises its boiling point, our mission in this Investigation is to get students to think in terms of molecules and energy as a means of really understanding this important physical effect.
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SLIDE HEAT-2-2
- Tell students that in the first few slides, we will look at what happens to water molecules at the freezing (0oC) and boiling point (100oC) and at the liquid phase in between. We start, in this slide, with liquid water.
- Explain to students that in solution (liquid phase), there is plenty of kinetic energy and therefore the water molecules are in constant, rapid motion. Thus, the molecules are somewhat in unorganized disarray, leading to the physical properties of a fluid.
- Explain to students that the relative flexibility of the water-water molecular interactions permits water to easily assume the shape of any container it is placed in. This is something that the physical organization of water in ice (as we will see later) would never permit.
- Direct students’ attention to the large water molecule in the upper left.
- Explain that the water molecule consists of one atom of oxygen and two atoms of hydrogen.
- Explain that even though they are combined into a water molecule, the two hydrogens have a weak positive (+) charge and the oxygen atom has a weak negative (-) charge.
- This is said to give the water molecule polarity.
- That is, the positive and negative charges are separated from one another, giving the whole molecule a positive face and a negative face.
- Inform students that, as in many other examples in science, like charges repel each other and opposite charges attract each other.
- Explain that, as is common with many particles with charges, this has an effect on how the molecules interact with each other.
Note: We will discuss charges later when we talk about the salt NaCl in water.
- Explain that in the case of water in solution (liquid phase), there is a weak interaction between a hydrogen atom on one water molecule and the oxygen atom of another water molecule. This interaction is called a hydrogen bond. It is a fairly weak bond that can be broken quite easily.
- Nonetheless, the hydrogen bond between water molecules exerts important effects on the solution, such as surface tension (as shown in the next slide).
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SLIDE HEAT-2-3
- Tell students that this simple slide shows some examples of the surface tension of water.
- From upper left clockwise, we see a water bug using surface tension to move across the water’s surface.
- Next is a simple demonstration of surface tension in which a small paper clip is floated on the surface of the water.
- Finally, the meniscus we see in the lab is caused by surface tension and the “beads” of water that form on certain surfaces are also common effects of the surface tension of water, cause mainly by the hydrogen bonds that form between water molecules, holding them loosely together
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SLIDE HEAT-2-4
- Inform students that, in this slide, we return to the interactions between water molecules, this time in ice.
- Explain that under the conditions in which water solidifies into ice, there is very little kinetic energy due to the low temperature and therefore the molecular motion of water molecules slows down to where they may barely move. Such lack of movement permits the molecules to contract and be packed into much more regular geometry that is also reflected in the regular and beautifully symmetric shapes of individual snowflakes.
- Explain that the tight packing of water molecules at freezing is also stabilized by hydrogen bonds so that movement of the molecules relative to one another is restricted. Liquid water, when poured into a beaker, takes the shape of the beaker. An ice cube, on the contrary, maintains its cube geometry unless it melts.
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SLIDE HEAT-2-5
- Tell students that this slide depicts molecules at the boiling point.
- Explain that under conditions of very high kinetic energy, individual water molecules rapidly and constantly move over considerable distances from each other.
- Explain that Hydrogen bonds formed in the liquid phase are easily broken by this movement and no interactions between molecules are favored.
- Explain that in the gas phase of water (call steam or water vapor), the rapid movement resulting from elevated kinetic energy forces the molecules to expand and apply pressure on the walls of any container holding them. Thus, as water boils in a teapot, the pressure of the steam (water molecules in the gas phase) builds up until it escapes through the whistle.
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SLIDE HEAT-2-6
- Tell students that this next slide shows a phase diagram or a phase transition diagram.
Note: It is very important to take the time to be confident that students are able to read and understand the meaning of this graphic.
- Explain that the phase diagram simple depicts the three phases of water described in previous slides in terms of relative temperature (on the left axis) and kinetic energy (on the right axis).
- Explain that we see the decreasing organization of water molecules as we go from solid to liquid to gas phases. This, of course, is the result of increasing kinetic energy and temperature as we move from left to right across the diagram.
- Direct students’ attention to the bottom of the graph.
- As noted beneath the graph between the solid and liquid phase, this phase transition is referred to as melting going from left to right (from lower to higher temperature) and freezing, going from right to left (from higher to lower temperature).
- The transition from liquid to gas, on the other hand, is referred to as boiling.
Note: The “reverse” of boiling would be called condensation.
- Remind students that from their experiments in Investigation 1, they found that the freezing point of water was 0oC and its boiling point is 100oC.
Note: This is an important slide that summarizes a lot of information. Be sure that students can explain it to each other over the course of the Investigation.
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SLIDE HEAT-2-7
- Tell students that we now turn to a subject more directly related to the experiments in Investigation 2.
- Inform students that common table salt, sodium chloride, is a very polar molecule indeed.
- It is the interaction of the positive charge on the sodium atom and the negative charge on the chloride atom that holds these atoms together. Thus, the solid sodium chloride that we sprinkle on food and use in Lab, consists of tightly and regularly packed sodium and chloride atoms as shown in this slide.
- Explain that the interaction between Na+ and Cl– is so weak, that water molecules actually can pull them apart from each other as they are mixed. This is shown on the next slide.
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SLIDE HEAT-2-8
- Explain that this slide depicts what happens when NaCl is dissolved in water.
- The negative (oxygen) sides of the polar water molecules interact with the positively charged sodium atoms.
- When a charged atom is dissolved in water, as it is here, it is referred to as an ion.
- Na+ is a positively charged ion.
- In water, chloride forms negatively charged ions.
- As the solid NaCl salt is dissolved in water, the charges of the water molecules begin to interact with the Na+ and Cl– ions and keep them in solution.
- This is a very common finding with many different salts in water.
- Explain that many of the hydrogen bonds between the water molecules are broken in the process of mixing with the salt.
- More and more salt can be added until there are no longer enough water molecules present to hold the Na+ and Cl– apart.
- At such a point, we would say that the solution is “saturated”.
- The addition of more NaCl would simply sink as a solid to the bottom of the container at saturation.
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SLIDE HEAT-2-9
- Tell students that this slide depicts the effect of salt on the water phase diagram.
- Emphasize how this diagram looks similar to the freshwater phase diagram.
- Explain that decreased temperature causes the solid formation and high temperatures induce boiling and gas formation. However, in order for the ridged, tightly packed ice structure of water to form, all of the dissolved sodium and chloride ions must first be released so that the water-water interactions and packing leading to ice formation may occur.
- Explain that as the water cools, therefore, one may see that the salt is no longer soluble and “falls out of solution”.
Note: This might not be easily seen in the Lab.
Note: Only once this happens can the appropriate interactions occur between water molecules for ice to form. Therefore, the presence of salt may be expected to inhibit ice formation until lower temperatures are reached.
- Show students that at the other extreme, we see that water interactions with the Na+ and Cl– ions must first be broken prior to boiling and steam formation.
- Consequently, some of the heat applied to the sample must overcome the interactions between the water molecule and the salt ions before boiling can occur.
- Tell students that, in Lab, they will measure the effect of various salt concentrations on the temperature at which water boils.
- Explain to students that, as noted in red above the solid and gas phase of water in this slide, both ice derived from saltwater (icebergs for example) as well as steam from boiling saltwater (by distilling salt water for example) can be consumed by humans in place of freshwater. This is because, as noted above, the salts in saltwater are excluded from ice and water vapor.
Note: Schemes to deliver freshwater to water-impoverished areas of the world through either transporting icebergs or distilling seawater have been entertained and may become economically feasible in the future.
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SLIDE HEAT-2-10
- Remind students that goggles should be worn when heating glassware.