Metadata: Vatten, is, varme, svensk, lærarversion
Warming ice (Teacher version)
How do the temperature change when you warm ice?
Abstract
In this experiment, we measure the temperature when we heat ice to boiling, and let it boil a few minutes and then
let it cool.
Aim
The aim for the students’ to get an understanding of water molecules’ structure and why ice floats on water. They
shall understand why the temperature is constant at the phase transitions, but also that pure water never has a higher
temperature than 100 ° C. There is also an excellent opportunity to address the concepts of linear and exponential
change, and the concept of positive/negative slopecoefficient with help from data logger tools.
Theoretical background
Water is a unique solution because it has a higher melting point, boiling point and vapor heating than most other
liquids. A water molecule consists of two hydrogen atoms bound to one oxygen atom. The angle that the molecule
forms amounts to 104.5 ° and is determined by the oxygen atom’s outer orbitals. Hydrogen and oxygen atoms are
held together by covalent bonds.
Forces between water molecules
Moreover, there are strong attraction forces that hold together the
molecules. The basis for this is that oxygen nucleus is more electronegative than the hydrogen atom, and therefore attracts the
electrons stronger. This means that there are more electrons in the
vicinity of the oxygen atom and that there is an electron deficiency at
the hydrogen atoms. Electron distribution makes the resulting strong
electrostatic forces between the oxygen atoms in a water molecule
and hydrogen atoms in another. This electrostatic bonding is called
hydrogen bonding and is weaker than covalent bonds. If there were no
hydrogen bonds, water would not freeze until at least minus 100 ° C.
In liquid water at room temperature each water molecule forms at an
average 3.4 hydrogen bonds with other water molecules, the lifetime
of hydrogen bonds, however, is short (less than 1 ns). When water
freezes to ice, each water molecule is forming 4 hydrogen bonds with
other molecules, and is formed as a regular crystal pattern of
hexagonal rings. The regular crystal pattern makes the ice take up
more space than water, hence ice is not as dense as water and
therefore ice floats. See picture below.
Tips for the teacher! When to illustrate
hydrogen bonds and why the ice takes up
more space, you can use the students to
illustrate this. Let some students stretch out
their arms and tie their hands, explaining
that the body except the arms is an oxygen
atom and the arms are the covalent bonds,
the tied hands are hydrogen atoms.Then let
the students come together as closely as
possible. Then let the students get together
in the crystal structure. That is, all will try to
lay hands on another student's head, no
more than two hands per head. What
formation used most space?
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Metadata: Vatten, is, varme, svensk, lærarversion
In water vapor there is
no hydrogen bonds between the water molecules. Water molecules will continue to interact, but only with elastic
collisions and they will have too high speed to form hydrogen bonds.
Technical equipment
This experiment uses a stainless steel temperature sensor. Temperature sensors do usually not need to be
calibrated. It is good to choose a scale on the y-axis, using so much of the surface in the coordinate system as possible,
and this can be done by selecting the auto-scale (see manual).
Equipment
• Data logger with stainless steel temperature sensor
• Beaker
• Ice, crushed
• burner or hot plate
Arrangement
A beaker is filled with crushed ice and a temperature sensor is linked to the data logger. The more crushed the ice is,
the better the outcome is, especially in the beginning of the experiment. Tip! There are ice-slush-sets that can be
used. See picture below.
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Questions for discussions
1. What could you see in the beaker when the temperature was constant in the beginning? Where did all the energy
go?
2. What could you see in the beaker when the temperature varied?
3. What could you see in the beaker when the temperature was at constant increase? Use the dataloggertool and to
find out how fast the temperature is increased.
4. What happens to the added energy when the water boils?
5. Examine with the help of the dataloggertool , how the temperature reduction occurs at cooling.
Results
A typical temperature-time graph for the heating of water can look like the graph below. If you have coarse crushed
ice, larger air pockets are formed between the iceparts, which affects the graph appearance with variations in
temperature at the beginning of the experiment. Air pockets are warmer than the ice/ice water:
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Comments on the graphs:
When warming ice, the temperature is constant initially, since all thermal energy is used to break the hydrogen bonds
as the ice melts. When all is melted the added energy is used for increasing the speed of water molecules, as
manifested by a steady increase in temperature, ie a linear change. When the water have reached the boiling point,
temperature will be constant, since all the energy now is required for breaking the remaining hydrogen bonds so that
water molecules can leave the liquid phase and become "free" in the gas phase. As long as there is water in the
beaker, the temperature is constant. When all the water have left the beaker the energy heats the pots and beakers,
which can reach very high temperatures and get cracked by the high temperature.
When you stop warming the water decreases in temperature. All processes of cooling are exponentially decreasing
because in a hot materal there are many molecules with high energy which can provide the substance by evaporation
when the material is hot. The remaining molecules are the less energy-rich molecules and the temperature decreases
rapidly at first. After a while we have only the energy-poor molecules remain and the number of energy-rich
molecules that can provide the substance is rapidly decreasing.
Variationer
Examine other substances such as ethanol and saline.
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