Solid objects have a freezing point

Water forum 2: water and its properties (teacher's booklet)

Water and its properties WASSER-FORUM Accompanying information for the classroom Background information State of aggregation Classical states of aggregation Fixed: the molecules swing only a little about their fixed position in the crystal lattice. When the temperature rises, they vibrate more violently and require more space. Exception: the density anomaly of the water. Liquid: The molecules move so strongly that the particles move freely through the liquid. Gaseous: The kinetic energy of the molecules is so high that they are no longer connected, but rather fly around in a room and are evenly distributed. Therefore, unlike liquids and solids, gas can be compressed. Non-classical states of aggregation These states of matter, which only occur under extreme conditions, include, for example, plasma, atomic gas, Bose-Einstein condensate, superfluid, etc. Special phase transitions In addition to the known transitions from one state of aggregation to the next, water can also have a liquid state of aggregation "skip". Sublimation: On the surface of the ice, water molecules, which have a higher kinetic energy, can change directly into the gaseous state even at temperatures below freezing point. Resublimation: When gaseous water molecules hit an extremely cold surface, so much energy is withdrawn from them that they change directly into the solid state (e.g. in the case of hoar frost or ice flowers). Properties of water Surface tension and drop shape Due to the dipole properties of the water molecules, water has a high surface tension. This is due to the fact that the water molecules on the water surface distribute their binding forces more to the horizontally neighboring molecules, since there are no water molecules above them with which they could form a bond. This creates a kind of skin that can carry very light objects. The shape of the water droplet is also determined by the surface tension. Without contact with other substances, e.g. when falling freely in a vacuum, water forms small liquid bodies which, due to their cohesion and surface tension, take up the smallest possible space - that of a sphere. The proverbial drop shape usually only occurs briefly when the drop is detached from a larger body of liquid or through friction. The surface tension can be reduced e.g. by surfactants (soaps), which have a hydrophilic (water-loving) and a hydrophobic (water-repellent) part in their molecular structure. The following experiment is used to illustrate this: On the surface of the water in a wide bowl, talcum powder (cornstarch is also possible) is sprinkled, which is evenly distributed over the surface. If you dip a finger moistened with washing-up liquid in the middle of the surface, the talc will move jerkily towards the edge of the bowl. Explanation: The binding forces evenly distributed in the horizontal are weakened so strongly at the point of impact of the surfactant that the non-weakened molecules of the surface contract like a cut rubber band. Capillary forces In narrow cavities, e.g. in a thin glass tube, the water rises against the force of gravity. This so-called capillarity results from the cohesive forces in the water and the adhesive forces between the water and the vessel wall. The surface tension of the water also plays a role here. If you put a glass tube in a water glass, the forces of attraction between the glass walls and the water molecules are so great that the water in the glass tube rises so high that the gravitation cancels these forces of attraction. The water column rises the higher the narrower the diameter of the tube. The capillary forces act e.g. during water transport in plants or in the cavities of kitchen paper. This property can also be observed when carefully dipping a piece of sugar into the cube. The capillary forces can be visualized well in class with two simple experiments:

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