Grade 8 Integrated Science Chapter 10 Lesson 3 on acid and base solutions, pH, hydroxide and hydronium ions, and measuring pH.

1. Chapter 10 Lesson 3: Acid and Base Solutions – p352-359 – page 1
What are acids and bases?
 Acids
o An acid is a substance that produces a _______________________(H3O+) when
dissolved in water
 Nearly all acid molecules contain one or more __________________atoms
(H).
 When the acid mixes with water, this hydrogen atom
_______________________________ from the acid.
 It quickly combines with a water molecule, resulting in a
_________________________________ atom.
 A hydronium ion, H3O+, is a ________________________charged ion
formed when an acid dissolves in water
Figure 1 Acids, such as hydrochloric acid, produce hydronium ions when they dissolve in water.

Bases
o A base is a substance that produces
___________________ ions (-OH) when dissolved in
water.
 When a hydroxide compound such as sodium
hydroxide (NaOH) mixes with water, hydroxide
ions _________________________ from the
bases and form hydroxide ions (-OH) in water.
 Some bases, such as ammonia (NH3), do not
contain _______________________ ions
 These bases produce hydroxide ions by
taking _________________________
atoms away from water, leaving
hydroxide ions (OH-).
Figure 2 Sodium hydroxide in water causes
the ions to separate leaves sodium ions,
hydroxide ions, and water
Figure 3 Ammonia in water causes ammonia to steal a hydrogen
ion from water creating hydroxide ions from the water
molecules.

2. Chapter 10 Lesson 3: Acid and Base Solutions – p352-359 – page 2
What is pH?
 The pH is an _______________________measure of the concentration of hydronium ions
(H3O+) in a solution.
o Inverse means that as one thing increases, another thing_____________________.
o In this case, as the concentration of ______________________ions increases, pH
decreases.
 A solution with a lower pH is more_____________________________.
 As the concentration of hydronium ions decreases, the
pH____________________________.
 A solution with a higher pH is more _______________________________.
Balance of Hydronium and Hydroxide Ions
 All acid and base solutions contain both hydronium and hydroxide ions.
 In a neutral solution, such as water, the concentration of hydronium and hydroxide ions
are _____________________________.
 What distinguishes an acid from a base is which of the two ions is present in the
____________________________ concentration
o ______________________have a greater concentration of hydronium ions (H3O+)
than hydroxide ions (OH-)
o Bases have a greater concentration of hydroxide ions than
_____________________ions.
Figure 4 Hydronium and Hydroxide Ion
Concentration



Acids
o [H3O+] ______ [OH-]
Neutral
o [H3O+] ______ [OH-]
Bases
o [H3O+] ______ [OH-]
*brackets around a chemical
formula mean concentration
The pH Scale
 The pH _____________________________ is used to indicate how acidic or basic a
solution is.
 The pH scale contains values that range below _________ and above ______________
o ________________ have a pH below 7
o Bases have a pH ________________ 7
o Solutions that are neutral have a pH of _____________

3. Chapter 10 Lesson 3: Acid and Base Solutions – p352-359 – page 3
How is the concentration of hydronium ions different in a solution with a pH of 1 from the
concentration of a solution with a pH of 2?
 A change in one pH unit represents a ________________________change in the acidity or
basicity of a solution.
o For example, if one solution has a pH of 1 and a second solution has a pH of 2, then
the first solution is not twice as acidic as the second solution; it is
_________________________ more acidic
 The _________________________ in acidity or basicity between two solutions is
represented by 10n, when n is the difference between the two pH values.
o For example, how much more acidic is a solution with a pH of 1 than a solution with
a pH of 3?
 First, calculate the difference, n, between the two pH values:
 n = 3-1 = _____
 Then use the formula 10^n, to calculate the difference in acidity:
 102= _____________
 A solution with a pH of 1 is _________ times more acidic than a solution
with a pH of 3

4. Chapter 10 Lesson 3: Acid and Base Solutions – p352-359 – page 4
How is pH Measured?
pH Indicators
 An indicator is a compound that changes
_______________ at different pH values when it reacts
with acidic or basic solutions
o When one or two drops of indicator are added to
a solution, the color of the solution changes. This
color can be matched to a set of standard colors
that correspond to pH.
 For example, bromthymol blue is an indicator that changes from
______________to green to blue between pH 6 and pH 7.6
pH Testing Strips
 pH can also be measured using pH testing strips
 The strips contain an indicator that changes to a variety of
________________ over a range of pH values

These are most commonly used to test water
pH Meters
 Although pH strips are quick and easy, they provide only an
_____________________ pH value. A more accurate way to
measure pH is to use a pH meter
 A pH meter is an electronic instrument with an
____________________that is sensitive to the hydronium ion
concentration in solution.

5. Chapter 10 Lesson 3: Acid and Base Solutions – p352-359 – page 5
Review Questions from the Scope
1. What happens when acids and bases dissolve in water?
2. How does the concentration of hydronium ions affect pH?
3. What methods can be used to measure pH?