Class Notes 2-2 Water and Solutions

Part I – Polar Molecules What is a substance in which the atoms are held together by a covalent bond? MOLECULE

What is the name for a molecule in which the charges are distributed evenly? NONPOLAR

Example of a nonpolar molecule METHANE

Example of a nonpolar molecule Diagram METHANE

Example of a nonpolar molecule Diagram METHANE H C C H H H H

What is the name for a molecule where the charges are unevenly distributed? POLAR

What is an example of a polar molecule from the text? WATER

WATER Number of protons in oxygen? 8 8 +

Number of electrons in the outer shell of oxygen? WATER 6 8 + O

Number of protons in hydrogen? WATER 1 1 + 8 + H O

WATER 1 Number of electrons in the outer shell of hydrogen? H O 8 1 +

What would make oxygen happy? WATER A full outer shell 1 + 8 + H O

Example of a polar molecule What would make oxygen happy? WATER A full outer shell 1 + H 8 + O

Example of a polar molecule What would make oxygen happy? WATER A full outer shell 1 + 1 + H H 8 + O

One eight OXYGEN How many protons does hydrogen Have? How many protons does oxygen have? eight Which of these two atoms (Hydrogen or oxygen) would have a greater attraction for the electrons? OXYGEN

The electrons are pulled closer to the oxygen atom The electrons are pulled closer to the oxygen atom. What is the charge on the oxygen atom in a water molecule? NEGATIVE O H H

The hydrogen atom has less pull on the electrons than oxygen. + + What is the charge on the hydrogen atom? POSITIVE

Two reasons why molecules are polar 1. 2.

Two reasons why molecules are polar 1. unequal charge O H H + +

Two reasons why molecules are polar 1. unequal charge O H H + + 2. asymmetrical shape

Two reasons why molecules are polar unequal charge O H H + + asymmetrical shape C H Symmetrical

Two reasons why molecules are polar unequal charge O H H + + asymmetrical shape polar C H Symmetrical O Asymmetrical Non-polar H H

Part II What is the name for the bond between the positive end of one polar molecule and the negative end of another polar molecule? hydrogen bond

O is a strong chemical bond Which bond is weaker, the hydrogen bond or the covalent bond? Hydrogen bond H O + hydrogen bond covalent bond is a strong chemical bond

Part III Characteristics water #1 What property of water results from its attraction to the ions in a salt? solvent Water dissolves the salt.

Many compounds dissolve in water. A solution is formed when one substance dissolves in another. A solution is a homogeneous mixture. Solvents dissolve other substances. Solutes dissolve in a solvent. solution

Diagram: Dissolves O H H + + + Water Na Cl Salt

Diagram: Water dissolves salt: O O O Na Cl O O O H O + H O + H O + Na + H O + Cl H O + H O + The (–) O of water is attracted to the (+) Na The (+ ) H of water is attracted to the (-) Cl

Cohesion Adhesion Characteristic #2 (and a hint for the lab) What is the attraction of one water molecule for another water molecule? Cohesion What is the attraction of one water molecule for another (non-water) molecule? Adhesion

Cohesion diagram: One molecule is attracted to another molecule. O polar to another molecule. polar H O +

Four polar properties of water good solvent (the "universal" solvent) 1. 2. 3. 4. cohesion O high boiling point (100 C) expands when freezes

The Solubility Rule Polar is soluble in Nonpolar is soluble in Like dissolves in like

Some compounds form acids or bases. An acid releases a hydrogen ion when it dissolves in water. high H+ concentration pH less than 7 more acidic stomach acid pH between 1 and 3

A base removes hydrogen ions from a solution. low H+ concentration pH greater than 7 bile pH between 8 and 9 more basic

A neutral solution has a pH of 7. pure water pH 7

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