CHAPTER 2 “Chemistry of Life” Water & solutions (Section 3 only)

REVIEW OF SECTION 1 & 2 Matter is anything that occupies space and that has mass. Elements are substances that cannot be broken down chemically into simpler kinds of matter. An atom is the simplest part of an element Compounds are made up of two or more elements in fixed proportions.

Polarity of Water In a water molecule, the hydrogen and oxygen atoms share electrons to form covalent bonds. (Covalent bonds form when two atoms share one or more pairs of electrons) Water molecules as a whole have no net charge, but the oxygen end has a slight negative charge, and the hydrogen end has a slight positive charge.  For this reason, water is referred to as a polar molecule.   It has a positive end and a negative end

Solubility The polar nature of water allows it to dissolve polar substances. Water does not dissolve nonpolar substances because a weaker attraction exists between polar and nonpolar molecules

Hydrogen Bonding A hydrogen bond is the force of attraction between a hydrogen molecule with a partial positive charge and another atom or molecule with a partial or full negative charge. The polar nature of water also causes water molecules to be attracted to one another. The positively charged region of one water molecule is attracted to the negatively charged region of another water molecule.

COHESION ADHESION An attractive force that holds molecules of a single substance together is called cohesion. Related to cohesion is surface tension of water. The cohesive forces in water resulting from hydrogen bonds cause the molecules at the surface to be pulled downward into the liquid. As a result, water acts as if it has a thin “skin” on its surface. Adhesion is the attractive force between two particles of different substances, such as water molecules and glass molecules.

Specific Heat Water has a high specific heat. Specific heat is the amount of heat required to raise the temperature of one gram of a substance 1 degree Celsius. For example, water can absorb more solar energy without its temperature increasing. Go to the beach on a hot summer day, and notice the temperature of the sand and the water.  In the middle of the afternoon, the sand will feel hot, and the water cool.  At midnight, the sand will feel cool, and the water temperature will be about the same. The sand changes its temperature through the course of the day, but the water temperature doesn't change much.

SOLUTIONS A solution is a mixture in which one or more substances are uniformly distributed in another substance. A solute is a substance dissolved in the solvent. A solvent is the substance in which the solute is dissolved.

Acids & Bases As water molecules move around, they bump into one another. Some of these collisions are strong enough to result in a chemical change: one water molecule loses a proton(a hydrogen nucleus), and the other gains this proton. *H2O ↔ H+ + OH - The OH – ion is the hydroxide ion - The free H+ ion can react with another water molecule as shown below… H+ + H2O ↔ H3O+ The H3O+ is the hydronium ion

Acids If the number of hydronium ions in a solution is greater than the number of hydroxide ions, the solution is an acid Examples: Hydrochloric acid (HCl) in gastric juice Sulphuric acid (H2SO4) Nitric acid (HNO3) Carbonic acid in softdrink (H2CO3) Uric acid in urine Ascorbic acid (Vitamin C) in fruit Citric acid in oranges and lemons Acetic acid in vinegar Tannic acid (in tea and wine) Tartaric acid (in grapes)

Bases If the number of hydroxide ions in a solution is greater than the number of hydronium ions, the solution is a base Examples: Sodium hydroxide (NaOH) or caustic soda Calcium hydroxide ( Ca(OH)2 ) or limewater Ammonium hydroxide (NH4OH) or ammonia water Magnesium hydroxide ( Mg(OH)2 ) or milk of magnesia Many bleaches, soaps, toothpastes and cleaning agents

PH & Buffers Scientists have developed a scale for comparing the relative concentrations of hydronium ions and hydroxide ions in a solution, called the PH scale ranging from 0-14. Buffers are chemical substances that neutralize small amounts of either an acid or a base added to a solution.

INDICATORS An indicator, when added to an acid, a neutral substance or a base, will change different colors. INDICATOR COLOR IN ACID COLOR IN NEUTRAL SOLUTION COLOR IN BASE Litmus red purple blue Bromothymol Blue yellow Phenolphthalein clear Universal Indicator Yellow-green