Water is not only for drinking

In winter, we experienced black ice and silver frost quite often. How to get rid of ice effectively? Gravel and rock salt are used most commonly. Rock salt is a chemical compound, which is calledsodium chloride and its formula is NaCl. Rock salt is not chemically clean substance but it contains another components. Calcic chloride is most commonly added to the sodium chloride, whose formula is CaCl2.

Solid state substances

In the solid state molecules are flocked together, and their mutual attraction is stronger than in liquid and gas state. Solid substances have regular inner setting – water in the form of ice or snow flakes is arranged into crystal lattice. Water occurs in solid state up to 0 °C. If salt is added in the water, the mutual size of the intermolecular forces changes. Forces between molecules are weaker and we can observe the melting of ice at the temperature lower than 0 °C.


Salt does not melt on ice but it needs water for its melting. The water that is needed comes from water vapour condensation. Water vapour, which is present in the air, condense on the surfaces of substances which have lower temperature. In this way water condensates on the surface of ice. In this newly-emerged water crystalline salt starts to melt and therefore a  very saturated solution occurs. When melting, the molecules of the melting dissolvent – water- squeeze between the molecules of the melting substance – ice and salt and spread them apart. This causes weakening of their mutual intermolecular force and the substance changes from solid to liquid state.

Solution of water and salt has a lower temperature then the water itself. Clean water melts and freezes at the temperature around 0 °C. Solution of salt in water freezes at the temperature much lower than –21, 2 °C, this temperature depends on the concentration of salt in the solution.

Due to the temperature of solidification of emerged solution being much lower than the temperature of solidification of ice, the solution remains liquid even at temperatures below 0 °C. Another salt and another ice will be dissolving in the solution until all the ice melts or until the solution will be so diluted that it will freeze at the set temperature. To make the ice melt further, more salt would have to be added. The more it freezes, the more salt is needed. If the outer temperature is really low, using the rock salt to treat the icy roads does not apply. Road salting is, from practical reasons, limited by the outer temperature being about –7 °C.

Surface tension

Another very interesting property of liquids is formation of surface tension. If we carefully put a thin needle or a razor on the free surface of water, we can observe the surface of the liquid slightly bending. The objects will not sink although the density of the substances that are the objects made from is bigger than the density of water. In the nature, a water measurer moves on the surface of water even though the density of its body is bigger. If the water measurer moves to another spot, the bending of the liquid disappears. The drop that appears at the end of the water tap that has not been tightened properly is gradually getting bigger, until a neck is created and the drop breaks off. The drop appears to us as a small filled up, flexible ball.

The examples show that free surface of a  liquid acts in the same way as a  thin, elastic membrane. In order to explain properties of liquid surface, we must consider activity happening among liquid molecules.

Liquid molecules mutually affect one another by attractive forces. The forces inside the liquid mutually cancel each other out. Whereas at the liquid´s surface the resultant of forces affecting the liquid´s molecules is vertical towards the free surface of the liquid and the molecules are drawn into the liquid. Molecules of gas above the free surface of a liquid also affect the molecules of a liquid at the free surface but their number is considerably lower so that their force affecting is significantly weaker.

The force which draws molecules into a liquid can be influenced by the liquid temperature or by adding surface-active agents.

Generally, the higher the liquid´s temperature is, the faster thermal movement of molecules, causing the force among molecules to be smaller. Surface-activeagents affect the force among molecules. After adding a surface-active agent in the water, for example a  washing detergent, the force among molecules gets smaller; therefore the solidity of surface level of water decreases. These effects are used when doing the washing or doing the dishes.

Water belongs among liquids, which wet a container´s walls. Water molecules which are on the edge between a container and a liquid are drawn into the container by bigger force. What occurs is bending of the free surface of a liquid. The shape of water in the container is a shape of a crescent.

If we gradually start throwing small, clean objects into the water, for example paper clips or coins, the volume of water that comes out is a volume that is the same as the volume of an object that was thrown in. The volume of a liquid that is pushed out stretches the surface layer of the liquid causing the surface of the liquid to bend upwards. We can keep throwing coins into the container so long as tensile stress of the surfaced membrane stretched by the liquid does not exceed the value of the surface tension of the set liquid and the surface membrane does not burst and the liquid does not flow over.