6.2 pH scale and indicators SNC2D P

Water Molecules Water is an important part of all life. The earth’s surface is about 70% water Humans are made up of about 66% water and the amount of water needed varies depending on an organism. Nevertheless, water is one of the basic requirements for life on earth.

Water Molecules A water molecule is made up of three atoms: two hydrogen atoms and one oxygen atom (H2O) The oxygen atom tends to be highly attracted to the electrons of the hydrogen atom. This is due to the large number of protons in its nucleus. As a result, oxygen pulls the hydrogen’s electrons closer causing it to have a stronger negative charge. These partial charges enable water molecules to bind to one another.

Water Molecules Water molecules are also able to dissociate from one another. The dissociation of water molecules contributes to the formation of acids and bases. When water dissociates hydrogen ions and hydroxide ions are released. Remember!! The presence of hydrogen ions (H+) indicates an acid, the presence of hydroxide ions (OH -) indicates the presence of a base.

Water Molecules Thus in pure water, there is an equal concentration of hydrogen (H+) and hydroxide ions (OH-). The amount of H+ ions to OH- ions determines whether the substance is an acid, base or neutral. A pH scale helps measure the amount of H+ ions that are present.

The Potential Hydrogen (pH) The pH scale is a numerical scale, ranging from 0 to 14, that is used to classify solutions as acidic, basic, or neutral. A pH scale can help to monitor of a variety of solutions (pool water, foods and beverages, and solutions from industrial processes) to ensure that they are safe.

Acidic: pH < 7 Basic: pH > 7 Neutral: pH = 7 (Page 230) Acidic: pH < 7 Basic: pH > 7 Neutral: pH = 7 The pH scale ranges from 0 to 14, where 0 is the pH of very strong acids and 14 is the pH of very strong bases. Copyright © 2010 McGraw-Hill Ryerson Ltd.

Acidic Solutions: pH < 7 All acidic solutions have values that are less than 7 on the pH scale. Acidic solutions have a greater amount of hydrogen ions than hydroxide ions. The lower the pH number the more acidic the solution, thus the greater the number of hydrogen ions.

Basic Solutions: pH > 7 All basic solutions have values that are above 7 on the pH scale. Basic solutions tend to have many more hydroxide ions than hydrogen ions. The greater the pH number the greater the number of hydroxide ions and the more basic the solution.

Neutral Solutions: pH > 7 Neutral solutions have a pH value of 7. Neutral solutions have an equal number of hydrogen and hydroxide ions. Pure water is considered to be neutral.

A pH value relates to the concentration of hydrogen ions in a solution A pH value relates to the concentration of hydrogen ions in a solution. Values increase or decrease exponentially (by a power of 10) as you move up or down the scale.

Learning Check! Questions 1-6 clicker technology

Determining the pH of a Solution There are a variety of ways to determine the pH of a solution. 1) pH Meter 2) Litmus Paper 3) Universal Indicator 4) pH paper

Example 1: pH Meter pH Meters have a sensor or probe that electronically produces a precise (real time) reading of the pH of a solution that is displayed digitally on the meter.

Example 2: pH Indicators pH Indicators are substances that change colour to show the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in a solution.

pH indicator: Litmus Paper Red and Blue Litmus Paper – A chemically treated indicator paper. Blue litmus turns red in acids. Red litmus turns blue in bases. This simple indicator can determine whether a solution is acidic, basic, or neutral.

pH indicator: Universal Indicator Universal Indicator and pH Paper are composed of a mixture of indicators that change to different colours under different pH conditions. These indicators cover the entire pH range from 0-14. pH paper has a colour scale which can be compared to the colour change to determine the pH As the pH increases the colour changes to help quickly identify the pH.

Other pH Indicators These indicators change colour wihtin a small range of pH values. This is useful if more precise measurements are required.

pH Indicators in Nature A variety of plants contain juices that can act as natural acid-base indicators.

Learning Checkpoint! Questions 7-9 on clicker technology