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Adhesion Cohesion Surface Tension Polarity Adhesion Cohesion Surface tension Polarity Water molecules have an area that is negatively charged and another positively charged, and this will be responsible for many of the water properties The molecules of water are united by chemical bonds called Hydrogen Bonds Cohesion One of the properties of water that depend on how strongly the molecules are attached to each other is called Cohesion. Read the Khan Academy text and underline the definition of: cohesion and surface tension Cohesion refers to the attraction of molecules for other molecules of the same kind, and water molecules have strong cohesive forces thanks to their ability to form hydrogen bonds with one another. Cohesive forces are responsible for surface tension, the tendency of a liquid’s surface to resist rupture when placed under tension or stress. READ “Properties of Water” handout and complete the lab Ex of cohesion: Rain drops (water is attracted to itself forming drops) Water forms drops also when in contact with hydrophobic surfaces and tend to spread in hydrophilic surfaces. Adhesion Adhesion is another property of water. Adhesion is the attraction of molecules of one kind to molecules of another kind. It is strong for water because the water molecule has a positive and a negative charge. The meniscus in a graduated cylinder is formed because of Adhesion. The water molecules are attracted to the glass molecules Ex of adhesion: water climbing on paper towel Movement of water to the top of plants in the xylem. Chapter 21: Section 2: Polar Substances Water is one of the most important substances on Earth. We are 80% water!! We studied a few properties of water: Density, cohesion, adhesion and specific heat In this section we will discuss another property of water: Water is a polar substance and it is considered a Universal Solvent • Water is a polar molecule • What does it mean to be a polar molecule or substance???????????? Important background information Atom composition – the components of atoms have charges Neutrons Chemical Bonds -Atoms form molecules - To form molecules, atoms are united by chemical bonds. -These bonds happen among electrons, the negative particles of atoms. To form molecules, atoms can transfer or share electrons • When atoms transfer electrons, the resulting molecules are called : Ionic Compounds – and these molecules always have a charge • When atoms share electrons, the resulting molecules are called molecular compounds or simply molecules. These molecules can be: • Polar • Non Polar • Polar – the distribution of electrons is uneven – molecule has a charge. Ex: water • Non Polar - when the distribution of electrons is even – molecule does not have a charge Polar molecules such as water have an area that is negatively charged and another positively charged. This polarity allows water to dissolve many different solutes. Universal Solvent: Water is called the universal solvent because it dissolves many different types of substances. This property is a result of its charge. • Water (the solvent) will dissolve solutes that also have a charge • The solutes that have a charge can be ionic compounds or other polar molecules. Aqueous Solutions A solution in which water is the solvent is called an aqueous solution. “like dissolves like.” Polar solvents dissolve polar solutes Nonpolar solvents dissolve nonpolar solutes. Water which is polar will dissolve polar and ionic substances. These substances are hydrophilic Non polar substances are hydrophobic • Why oil and water don’t mix? • Oil molecules are nonpolar, so polar water molecules are not attracted to them. The effect of the detergent: Detergents are bipolar substances - they have a polar side and another side that is non-polar. The polar side is attracted to the water and the non-polar side is attracted to the oil droplet, and surrounds it. We can clean greasy dishes or oil from clothes because the molecules of oil will be washed away with the molecules of the detergent. • Why ducks have oil in their feathers? Ducks have a special gland that produces an oil, which ducks rub over their feathers. This oil, that will not mix with water, creates a protective barrier that stops feathers from becoming soaked. This barrier insulates ducks from water and cold temperatures. .
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