Both of these atoms can form a hydrogen bond with oxygen atoms of different water molecules. Every water molecule can be hydrogen bonded with up to three other water molecules See Fig. However, because hydrogen bonds are weaker than covalent bonds, in liquid water they form, break, and reform easily. Thus, the exact number of hydrogen bonds formed per molecule varies. Molecules of pure substances are attracted to themselves.
This sticking together of like substances is called cohesion. Depending on how attracted molecules of the same substance are to one another, the substance will be more or less cohesive. Hydrogen bonds cause water to be exceptionally attracted to each other.
Therefore, water is very cohesive. Our experience with water, however usually involves water touching something else or being acted upon by gravity. In space, water is able to form perfectly round spheres because the attraction of water to itself pulls the water into the shape with the least amount of surface area compared to the volume — a sphere.
A European Space Agency astronaut Pedro Duque of Spain watches a water bubble float between him and the camera, showing his image refracted, on the International Space Station. B A large water sphere made on a 5 cm diameter wire loop by U. Weird Science. Adhesion is similar to cohesion, but it involves unlike i. Water is very adhesive ; it sticks well to a variety of different substances. Water sticks to other things for the same reason it sticks to itself — because it is polar so it is attracted to substances that have charges.
Water adheres to many things— it sticks to plants, it sticks to dishes, and it sticks to your eyebrows when you sweat. In each of these cases water adheres to or wets something because of adhesion. The function of adhesives is to hold things together. Adhesives come in many forms, including glue, tape and sealant, and are used in a variety of everyday applications.
In addition, natural substances can act as adhesives, such as soda and caramel. The stickiness of the adhesive is caused by molecular bonds, and its strength depends on how much stress is needed to pull those bonds apart. Every substance contains molecules that have a positive or negative charge. For two substances to bond, their opposing charges need to be attracted to each other in the exact amount of charge.
For example, to produce a molecule of hydrogen chloride HCl , the positive charge of the hydrogen atom bonds with the negative charge of the chlorine atom, and the combination of the two creates a strong, neutral bond. When two materials are brought into contact with each other, the surface molecules interact, giving rise to attractive forces that may be physical, chemical or electrostatic corresponding to adsorption, covalent bonding or van der Waals forces, respectively.
When the molecules are similar, as in the case of two 'glue molecules,' the cohesive force causes the glue to stick to itself. When the molecules are dissimilar, as in the case of a glue molecule and a molecule of the substrate the surface the glue is sticking to , the adhesive force holds the glue to the substrate.
Hence, the 'stickiness' of tape is caused by a combination of the molecular forces of the glue material sticking to itself as well as holding onto the substrate. A pressure-sensitive adhesive is a sticky, viscous, liquidlike material that adheres to a surface using only pressure. To function well, it should have good adhesion to a surface and good cohesion, or internal strength. For good adhesion, it is important that the adhesive can readily flow out on the surface.
The degree, or freeness, of the flow often determines the intensity of adhesion. Degree of flow depends on the difference between the surface energy of the material and the surface energy of the adhesive. JPG Credit:. Play Download. Philip Broadwidth - Well David, that's a very good question. Previous What causes sudden personality changes? Next Is it more tooth-friendly to suck, or crunch a sweet? Related Content Chemistry Physics Technology. How does glue actually work?
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