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Phase diagram of water

Diagram:
A P-T diagram for pure water. The lines indicate the temperature and the pressure at which the solid, liquid, and vapor phases exist in equilibrium. All three phases exist in equilibrium only at the triple point; otherwise, there are a maximum of two phases.

In addition to the equilibrium curves (melting pressure curve, sublimation curve, vapor pressure curve), the diagram also includes the pressure and temperature data for the melting, boiling, triple, and critical points.
Attention: The axes of the diagram are not shown true to scale.

Information and ideas:
This diagram also reflects the density anomaly of water (lower density in the solid state than in the liquid state): The melting pressure curve shows a negative slope. The reason for the density anomaly is the hydrogen bonds.

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Steam pressure curve and phase diagram of water

Charts:
The steam pressure curves (p-V diagram) and the phase diagram (p-T diagram) of water are compared.

If you heat water to 100 °C at normal atmospheric pressure, it turns into steam. But what effect does raising or lowering the pressure have on the vaporization temperature?
The answer to this is given by the steam pressure curve (T-curves in the p-V diagram on the left) and the phase diagram (p-T diagram of the right) of the water. Steam pressure is the term for the pressure at which gas and liquid are in equilibrium, i.e. the same number of molecules evaporate as condense back into water. Above the critical temperature (numerical values are given) the water is always gaseous, regardless at what temperature, and it can be treated as a real gas (Van der Waals equation, formula is given). At every temperature below the critical temperature there is a steam pressure for which there is a two-phase zone (liquid and gaseous). In the liquid phase range it is possible to recognize from the steep rise in the curves that liquid substances are barely compressible.
The critical temperature must not be confused with the triple point temperature (see p-T diagram). This characterizes the values of temperature and pressure at which all phases (solid, liquid and gaseous) are present simultaneously.

Information and ideas:
At what temperature does water boil on Mount Everest? So-called "Steam pressure tables" provide information about this. It would also be interesting to refer to the phase transition points as temperature critical points. At the phase transition from liquid to gaseous the energy applied does not initially lead to an increase in temperature. The same applies to the melting of ice. Not until all the water has evaporated or melted does the temperature rise further.

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;: Chemical energy

Chart:
Chemical energy as binding energy between atoms (as a potential curve in the illustration).

Chemical energy is present both in the bond between atoms and molecules as well as in the potential for chemical bonding. This energy can be released in the form of heat during the bonding process or when those bonds are broken. This "heat of reaction" is also referred to as reaction enthalpy (H). The release of heat (dH < 0) is referred to as an exothermic reaction. An endothermic reaction is when heat is absorbed (dH > 0).
Every mixture of source materials that can react to produce end products can be regarded as a potential source of chemical energy.
Microscopically speaking, this chemical energy can be found in the bonds between individual atoms, as illustrated in the potential curve.

Information and ideas:
Chemical energy is a form of energy that is easy to store - whether in the human body or in batteries. An additional example is hydrogen as a chemical energy store for renewable energy sources.

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Stores for electrical energy

Overview graphic:
Examples for direct and indirect stores for electrical energy are shown and the stored energy form is designated.

Electrical energy should if possible be generated at precisely the time at which it is needed. This is because electrical energy is difficult and expensive to store. A distinction is made between direct and indirect stores for electrical energy. Electrical energy can only be stored directly in capacitors. With indirect storage, the electrical energy can be converted into a different form of energy which can then be stored.

Information and ideas:
Students should think about the economical use of the energy stores shown (for example: How much energy can be stored? Can the energy store be used without any problems? Where do losses occur?).

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Thermal energy

Diagram:
Formulas for the thermal energy of gases and temperature as a function of their molar heat capacity at constant volume.

The thermal or internal energy of a substance is the sum of the kinetic energies of its atoms or molecules. This energy is measurable as temperature. If you supply heat to the substance, the particle speed increases and the temperature rises. In the case of molecular gases, the supply of heat in addition to the translatory motion can excite other forms of motion (rotation and oscillation). This finds expression in the stepped curve of the molar heat capacity (diagram at the right). The molar heat capacity of a substance is the amount of energy required to raise 1 mole of a substance by 1°C. For gaseous substances, the following applies: If the gas particles move only linearly (translation), the amount of heat that is required to raise the gas by 1°C remains constant at 3R/2. In the case of molecular gases, the molecules start to rotate when a specific temperature is reached. In this area (linear increase in the diagram), more energy must be supplied to raise the temperature by 1°C, since the energy goes not only into the translatory motion, but also into exciting the rotation. If all particles are made to rotate, the energy required to raise the temperature by 1°C is again constant at 5R/2. The rise at the point of transition from rotation to oscillation can be explained in a similar way.

Information and ideas:
The overview graphic summarizes the topic of thermal energy using the example of gases. You will find detailed explanations as well as an explanation of the heat in solid bodies in the guideline "What is energy?"

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Excitation energy of a water molecule

Chart:
Water can absorb heat energy in the form of vibrations or movement of its molecules. This energy content depends on the physical state: steam contains more energy than liquid water, for example.

The material surrounding us takes on different physical states depending on pressure and temperature (in Kelvin): solid, liquid or gaseous. This also applies to water: During a phase change from solid to liquid and liquid to gas respectively the energy of the water molecules increases without the temperature rising - the diagram for water shows plateaus. The values of these plateaus are approx. 6 kJ/mol (melting heat) and approx. 40,7 kJ/mol (vaporization heat) respectively.

Information and ideas:
Ideally suited for explaining the topic of phase equilibrium.