Goal To observe and learn about the phenomenon of surface tension of water. Items required:

• water vessel • detergent, e.g. washing-up liquid (a few drops) • small object e.g. a pin, a leaf, a coin – a safety pin in the pictures

Method of the experiment:

1. Pour water into the vessel (roughly half of it)
2. Gently place a small object on the water surface. Observe the object floating on the water.
3. Remove the object.
4. Add a few drops of detergent to the dish and mix thoroughly.

5. Place the item on the water surface again.

Observations When the object is placed on the surface of clean water, it floats on the surface, even with a slight movement of the dish. When the detergent liquid is added, the object falls to the bottom and cannot be placed on the surface. Conclusions The forces of interaction between the particles of clean water are strong enough to keep small objects on the surface without any problems. The water molecules attract each other to form a "film" on the surface. When the detergent is added, its particles reduce the surface tension, i.e. they weaken the interaction between the water molecules. A film broken up in this way is not able to hold an object on the surface. Explanation Due to electromagnetic effects, water molecules attract each other. Each water molecule is subject to cohesion forces, which are balanced inside the solution (Fig. 1). On the surface, these forces are not balanced. The resultant force is directed perpendicularly to the liquid-solid, liquid-gas or liquid-liquid interface and is called surface tension force. Whether a liquid-liquid interface is created also depends on cohesive forces, namely whether the attractive forces between the molecules of both liquids are stronger or weaker than the cohesive forces of each liquid. If the cohesive forces are clearly greater, the liquids will not mix with each other, but a liquid-liquid boundary will form, as in the case of water and oil. Figure. 1: Cohesion forces acting on selected liquid molecules in the depths of the solution and on its surface, together with the resultant surface tension force (Fnap_pow) acting on molecules at the liquid boundary. The particles on the surface have more potential energy than those located deeper. According to the fundamental laws of nature, each particle system strives for a state with minimal energy, in this case by - among other things - minimising its surface area in relation to the volume of the liquid. This ratio is smallest for a sphere, which is why water drops falling in the form of rain take on a spherical shape (only air resistance extends it slightly). Mutual attraction of the same particles creates a "membrane" on the border of the media. This phenomenon allows liquid droplets to form, small objects to float on the surface or insects to move around it. The cohesive forces between the particles and the surface tension forces change under the influence of temperature or mixing of two liquids, resulting in a solution with slightly different intermolecular interactions. Substances which have a particularly strong influence on reducing surface tension forces are surfactants or detergents.