Artikel Kimia

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What is Density Posted: 03 Feb 2012 07:30 AM PST The density of any substance is equal to its mass divided by its volume; D=m/v. Scientist express the densities of solids as grams per cubic centimeter (g/cm3), liquids as grams per milliliter (g/mL0, and gases as gram per liter (g/L). The density of a substance changes with its temperature, generally as the temperature of a subtance increases, its density decreases. Liquids have higher density than gases and a lower density than solids, generally, liquids approximately 1000 times more dense than gases and 5% to10% less than solids. Water is notable exception; liquid water at 0 C is 1,09 times more dense than solid water (ice).

What is Surface Tension Posted: 03 Feb 2012 02:28 AM PST Surface tension is a measure of the resistance to keep necessary to keep the molecules at the surface of a liquid from expanding the liquid”s surface area; this phenomenon is responsible for the ability of a drop of liquid to keep its shape. The molecules within a liquid are attracted in all directions to surrounding molecules by intermolecular forces; such as van der Waals forces and hyrogen bonding; however; molecules on the liquid surface are attracted laterally and downward, but not upward. This lack of balance in the forces of attraction causes surface molecules to crowd together and form a skin on the surface of the liquid (for example, surface tension causes water to bead up on a freshly waxed car; this same property makes a soap bubble round). The attraction of surface molecules to molecules within the liquid is called cohesive force; strong attraction of surface molecules to unlike molecules outside the liquid is called adhesive force. As the temperature of a liquid increases, its surface tension decreases. Wetting agents or surfactants are chemicals that artificially decreases the surface tension of a liquid. The molecules of the wetting agent attract the liquids surface molecules and thus overcone its normal cohesive forces. The resultants decrease in surface tension permits the liquid to spread out (for example, a detergent is considered a wetting agent).

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