Video Transcript
Which of the following diagrams
represents a correctly labeled pH scale?
To answer this question, we must
address two parts. First, do acids belong on the
left-hand side of the scale with a low pH or on the right-hand side of the scale
with a high pH? And secondly, does the
concentration of hydrogen ions increase or decrease with rising pH?
Let’s start by clearing some space
on screen to take a closer look at a proper pH scale. The pH scale, with values generally
falling between zero and 14, measures the acidity or basicity of a substance. Substances with a pH below seven
are acidic. The lower the pH, the more acidic a
substance is. For example, battery acid is more
acidic than stomach acid or lemons, as it is further to the left on the pH scale
with a lower pH value.
A Brønsted–Lowry acid produces
protons or, in other words, hydrogen ions when in solution. So the more acidic a substance, and
thus the further to the left on the pH scale, the higher the hydrogen ion
concentration. There is an inverse relationship
between the value of pH and hydrogen ion concentration. If the pH is low, the hydrogen ion
concentration is high.
When a substance has a pH of seven,
it is considered neutral; it is neither an acid nor a base. When the pH of a substance is above
seven, then it becomes basic. The stronger the base, the higher
the pH. For example, drain cleaner is a
very strong base with a pH of approximately 14. The higher the pH of a base, the
lower the hydrogen ion concentration.
Let’s now use this information to
examine our answer choices. The proper pH scale has acids with
a low pH and bases with a high pH. The concentration of hydrogen ions
increases as the pH gets lower. The only diagram that properly
displays this information is answer choice (A). Therefore, the correctly labeled pH
scale is answer choice (A).
