Name two ways to determine if a solution is acidic.

A common task in chemistry labs is to identify whether a given solution is acidic, neutral or basic, which are determined by a solution's pH level. Depending on your laboratory's equipment and what information you are given, here are five ways to find out what type of solution you have.

Table of Contents Show

What pH Tells You
Take the Litmus Test
Probe for the Answer
Memorize Some Solutions
Look at the Formula
Key Concepts
Deciding if a Solution is Neutral
Deciding if a Solution is Acidic
Deciding if a Solution is Basic
Examples with Worked Solutions
How do you test if a solution is acidic?
What are two common ways to identify acids and bases?

What pH Tells You

A solution's pH will be a number between 0 and 14. A solution with a pH of 7 is classified as neutral. If the pH is lower than 7, the solution is acidic. When pH is higher than 7, the solution is basic. These numbers describe the concentration of hydrogen ions in the solution and increase on a negative logarithmic scale. For example, If Solution A has a pH of 3 and Solution B has a pH of 1, then Solution B has 100 times as many hydrogen ions than A and is therefore 100 times more acidic.

Take the Litmus Test

If your lab has litmus paper, you can use it to determine your solution's pH. When you place a drop of a solution on the litmus paper, the paper changes color based on the pH of the solution. Once the color changes, you can compare it to the color chart on the paper's package to find the pH. With unknown solutions, you should wear gloves, put on eye protection and work under a fume hood to be safe.

Probe for the Answer

A pH meter will make short work of identifying the pH of your solution. These meters have a glass probe that measures a solution's ion concentration. To use a pH meter, place a small portion of your solution in a beaker or test tube, rinse the probe of the pH meter, and then place the probe into your solution. Within seconds, the readout will tell you the pH. After taking your measurement, rinse the probe again and place it back in its storage solution.

Memorize Some Solutions

A few solutions are familiar liquids or are commonly used in science labs and it's handy to memorize what kind of solutions they are. Water and blood are both neutral. Many household cleaners, such as bleach and ammonia, are basic, as is sodium hydroxide. Citric juices, coffee and wine are acidic. Solutions with the word "acid" in them, such as stomach acid and hydrochloric acid are acidic.

Look at the Formula

Although it isn't a perfectly reliable way of identifying a solution, in some cases a solution's molecular formula can help you identify whether it is acidic or basic. Use this method if you have no other way to definitively identify a solution. If the solution's formula begins with H, such as HCl or H2SO4, it is usually acidic. If the solution ends in -OH, such as NaOH or KOH, it is often basic.

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Key Concepts

A solution is defined as neutral if the concentration of hydrogen ions is the same as the concentration of hydroxide ions1:
neutral solution: [H+] = [OH-]
where [H+] = concentration of hydrogen ions in solution
and [OH-] is the concentration of hydroxide ions in solution

A solution is defined as acidic if the concentration of hydrogen ions is greater than the concentration of hydroxide ions:
acidic solution: [H+] > [OH-]
where [H+] = concentration of hydrogen ions in solution
and [OH-] is the concentration of hydroxide ions in solution

A solution is defined as basic2 if the concentration of hydrogen ions is less than the concentration of hydroxide ions:
basic solution: [H+] < [OH-]
where [H+] = concentration of hydrogen ions in solution
and [OH-] is the concentration of hydroxide ions in solution

Summary:

acidic neutral basic
[H+] > [OH-] [H+] = [OH-] [H+] < [OH-]

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Deciding if a Solution is Neutral

A solution is neutral if the concentration of hydrogen ions in solution is the same as the concentration of hydroxide ions in the solution.

neutral solution: [H+] = [OH-]

If you know the concentration of hydrogen ions and hydroxide ions in a solution, then these must be the same in order for the solution to be neutral:
example: an aqueous solution contains 0.15 mol L-1 H+(aq) and 0.15 mol L-1 OH-(aq)
[H+(aq)] = [OH-(aq)] = 0.15 mol L-1 so solution is neutral

If you know the concentration of the acid and the base in a neutralisation reaction, you will need to calculate the concentration of hydrogen ions and hydroxide ions in the resulting solution in order to decide if the resulting solution is neutral or not.

example: 0.050 L of 0.02 mol L-1 hydrochloric acid is added to 0.025 L of 0.04 mol L-1 sodium hydroxide, will the resulting solution be neutral?

acid base
name
formula hydrochloric acid
HCl(aq) sodium hydroxide
NaOH(aq)
moles of each
= concentration (mol L-1) × volume (L) 0.02 × 0.050
= 0.0010 mol 0.04 × 0.025
= 0.0010 mol
strength strong acid strong base
Dissociation Equation HCl(aq) → H+(aq) + Cl-(aq) NaOH(aq) → Na+(aq) + OH-(aq)
moles H+ and OH- moles H+ = moles HCl
= 0.0010 mol moles OH- = moles NaOH
= 0.0010 mol
assuming no reaction occurs:
final solution concentrations
= moles ÷ total volume (L) [H+(aq)]
= 0.0010 ÷ (0.050 + 0.025)
= 0.013 mol L-1 [OH-(aq)]
= 0.0010 ÷ (0.050 + 0.025)
= 0.013 mol L-1
Compare [H+(aq)]
and [OH-(aq)] in final solution [H+(aq)] = [OH-(aq)]
Decide if the final solution is neutral Final solution is neutral because [H+(aq)] = [OH-(aq)]

If you know the pH of an acid and the pH of a base in a neutralisation reaction, you will need to calculate the concentration of hydrogen ions and the concentration of hydroxide ions in each solution in order to determine if the final solution is neutral or not.

example: At 25°C, 1 L of an aqueous acidic solution with a pH of 2.0 is added to 1 L of an aqueous basic solution with a pH of 12.0.
Is the resulting solution neutral?

aqueous acidic solution aqueous basic solution
pH at 25oC 2.0 12.0
calculate relevant concentrations for acidic solution and basic solution [H+(aq)] = 10-pH
= 10-2.0 = 0.010 mol L-1 [OH-(aq)] = 10(14-pH)
= 10-(14-12) = 0.010 mol L-1
calculate relevant moles for each solution
moles = concentration × volume moles (H+(aq)) = 0.010 × 1
= 0.010 mol moles (OH-(aq)) = 0.010 × 1
= 0.010 mol
assuming no reaction occurs:
concentrations in final solution
=moles ÷ total volume [H+(aq)] = 0.010/(1 + 1)
= 0.005 mol L-1 [OH-(aq)] = 0.010/(1 + 1)
= 0.005 mol L-1
Compare concentrations [H+(aq)] = [OH-(aq)]
Decide if final solution will be neutral Solution is neutral because [H+(aq)] = [OH-(aq)]

Deciding if a Solution is Acidic

A solution is acidic if the concentration of hydrogen ions in the solution is greater than the concentration of hydroxide ions in the solution.

acidic solution: [H+] > [OH-]

If you know the concentration of hydrogen ions [H+] and the concentration of hydroxide ions [OH-] in the solution, then the solution is acidic if [H+] > [OH-]
example: The concentration of hydrogen ions in a solution is 0.025 mol L-1 and the concentration of hydroxide ions in the same solution is 0.012 mol L-1. Is the solution acidic?
[H+] = 0.025 mol L-1
[OH-] = 0.012 mol L-1
0.025 > 0.012 so [H+] > [OH-] and so the solution is acidic

If you know the concentration of an aqueous acid and an aqueous base taking part in a neutralisation reaction, you will need to calculate the concentrations of H+(aq) and OH-(aq) in the final solution in order to decide if the final solution is acidic or not.

example: 0.100 L of 0.02 mol L-1 hydrochloric acid is added to 0.025 L of 0.04 mol L-1 sodium hydroxide, will the resulting solution be acidic?

acid base
name
formula hydrochloric acid
HCl(aq) sodium hydroxide
NaOH(aq)
moles of each
= concentration (mol/L) × volume (L) 0.20 × 0.100
= 0.020 mol 0.04 × 0.025
= 0.0010 mol
strength strong acid strong base
Dissociation Equation HCl(aq) → H+(aq) + Cl-(aq) NaOH(aq) → Na+(aq) + OH-(aq)
moles H+ and OH- moles H+ = moles HCl
= 0.020 mol moles OH- = moles NaOH
= 0.0010 mol
assuming no reaction occurs:
final solution concentrations
= moles ÷ total volume (L) [H+(aq)]
= 0.020 ÷ (0.100 + 0.025)
= 0.016 mol L-1 [OH-(aq)]
= 0.0010 ÷ (0.100 + 0.025)
= 0.008 mol L-1
Compare [H+(aq)]
and [OH-(aq)] in final solution [H+(aq)] > [OH-(aq)]
Decide if the final solution is acidic Final solution is acidic because [H+(aq)] > [OH-(aq)]

example: At 25°C, 2 L of an aqueous acidic solution with a pH of 3.0 is added to 2 L of an aqueous basic solution with a pH of 9.0. Is the resulting solution acidic?

aqueous acidic solution aqueous basic solution
pH at 25°C 3.0 9.0
calculate relevant concentrations for acidic solution and basic solution [H+(aq)] = 10-pH
= 10-3.0 = 0.0010 mol L-1 [OH-(aq)] = 10(14-pH)
= 10-(14-9) = 0.000010 mol L-1
calculate relevant moles for each solution
= concentration × volume (L) mol(H+(aq)) = 0.0010 × 2
= 0.0020 mol mol(OH-(aq)) = 0.000010 × 2
= 0.000020 mol
assuming no reaction occurs:
calculate final concentrations
= moles ÷ total volume [H+(aq)] = 0.0020/(2 + 2)
= 0.0005 mol L-1 [OH-(aq)] = 0.000020/(2 + 2)
= 0.000005 mol L-1
Compare concentrations [H+(aq)] > [OH-(aq)]
Decide if final solution will be acidic Solution is acidic because [H+(aq)] > [OH-(aq)]

Deciding if a Solution is Basic

A solution is basic if the concentration of hydrogen ions in the solution is less than the concentration of hydroxide ions in the solution:

basic solution: [H+] < [OH-]

If you know the concentration of hydrogen ions [H+] and the concentration of hydroxide ions [OH-] in the solution, then the solution is basic if [H+] < [OH-]
example: The concentration of hydrogen ions in a solution is 0.025 mol L-1 and the concentration of hydroxide ions in the same solution is 0.070 mol L-1. Is the solution basic?
[H+] = 0.025 mol L-1
[OH-] = 0.070 mol L-1
0.025 < 0.070 so [H+] < [OH-] and so the solution is basic

If you know the concentration of an aqueous acid and an aqueous base taking part in a neutralisation reaction, you will need to calculate the theoretical concentrations of H+(aq) and OH-(aq) in the final solution in order to decide if the final solution is basic or not.

example: 0.100 L of 0.02 mol L-1 hydrochloric acid is added to 0.250 L of 0.09 mol L-1 sodium hydroxide, will the resulting solution be basic?

acid base
name
formula hydrochloric acid
HCl(aq) sodium hydroxide
NaOH(aq)
moles of each
= concentration (mol/L) × volume (L) 0.20 × 0.100
= 0.020 mol 0.09 × 0.250
= 0.0225 mol
strength strong acid strong base
Dissociation Equation HCl(aq) → H+(aq) + Cl-(aq) NaOH(aq) → Na+(aq) + OH-(aq)
moles H+ and OH- moles H+ = moles HCl
= 0.020 mol moles OH- = moles NaOH
= 0.0225 mol
assuming no reaction occurs:
concentrations in final solution
= moles ÷ total volume (L) [H+(aq)]
= 0.020 ÷ (0.100 + 0.250)
= 0.057 mol L-1 [OH-(aq)]
= 0.0225 ÷ (0.100 + 0.250)
= 0.064 mol L-1
Compare [H+(aq)]
and [OH-(aq)] in final solution [H+(aq)] < [OH-(aq)]
Decide if the final solution is basic Final solution is basic because [H+(aq)] < [OH-(aq)]

example: At 25°C, 500 mL of an aqueous acidic solution with a pH of 3.0 is added to 500 mL of an aqueous basic solution with a pH of 13.0. Is the resulting solution basic?

aqueous acidic solution aqueous basic solution
pH at 25°C 3.0 13.0
calculate relevant concentrations
for each solution [H+(aq)] = 10-pH
= 10-3.0 = 0.0010 mol L-1 [OH-(aq)] = 10(14-pH)
= 10-(14-13) = 0.10 mol L-1
calculate relevant moles
= concentration × volume (L) moles(H+(aq)) = 0.0010 × 0.5
= 0.0005 mol moles(OH-(aq)) = 0.10 × 0.5
= 0.05 mol
assuming no reaction occurs:
concentrations in final solution
= moles ÷ total volume [H+(aq)] = 0.0005/(0.5+0.5)
= 0.0005 mol L-1 [OH-(aq)] = 0.05/(0.5+0 .5)
= 0.05 mol L-1
Compare concentrations [H+(aq)] < [OH-(aq)]
Decide if final solution will be basic Solution is basic because [H+(aq)] < [OH-(aq)]

Examples with Worked Solutions

Question 1. A solution is known contain 1.23 × 10-3 mol L-1 hydrogen ions and 1.23 × 10-4 mol L-1 hydroxide ions.
Is the solution acidic, basic or neutral?

Extract the data from the question:
[H+] = 1.23 × 10-3 mol L-1
[OH-] = 1.23 × 10-4 mol L-1

Compare [H+] and [OH-]
[H+] > [OH-]

Decide if the solution is acidic, basic or neutral:
Solution is acidic because [H+] > [OH-]

Question 2. At 25°C, 10 mL of aqueous sodium hydroxide solution is added to 100 mL of aqueous ethanoic (acetic) acid solution.
The pH of the resulting solution is 3.4.
Is the solution acidic, basic or neutral?

Extract the data from the question:
volume of NaOH(aq) = 10 mL (not relevant to the question)
volume of CH3COOH(aq) = 100 mL (not relevant to the question)
pH of the final solution = 3.4 at 25°C

Calculate the concentration of hydrogen ions in the final solution:
[H+] = 10-pH = 10-3.4 = 3.98 × 10-4 mol L-1

Calculate the concentration of hydroxide ions in the final solution:
At 25°C Kw = [H+][OH-] = 1.0 × 10-14
[OH-] = 1.0 × 10-4 ÷ [H+] = 1.0 × 10-14 ÷ 3.98 × 10-4 = 2.51 × 10-11 mol L-1

Compare [H+] and [OH-] in the final solution:
[H+] = 3.98 × 10-4 mol L-1
[OH-] = 2.51 × 10-11 mol L-1
[H+] > [OH-]

Decide if the solution is acidic, basic or neutral:
The solution is acidic because [H+] > [OH-]

Question 3. 0.15 g of solid sodium hydroxide is added to 0.025 L of 0.020 mol L-1 HCl(aq).
Is the resulting solution acidic, basic or neutral?

Extract the data from the question:
mass NaOH = 0.15 g
volume of HCl(aq) = V(HCl) = 0.025 L
concentration of HCl(aq) = c(HCl(aq)) = 0.020 mol L-1

Calculate the concentration of hydrogen ions in the solution:
hydrochloric acid is a strong acid so it fully dissociates in water: HCl → H+(aq) + Cl-(aq)
[H+(aq)] = [HCl] = 0.020 mol L-1

Calculate moles of NaOH:
moles = mass ÷ molar mass
moles(NaOH) = 0.15 g ÷ (22.99 + 16.00 + 1.00) g/mol = 0.15 ÷ 39.99 = 3.75 × 10-3 mol

Calculate theoretical concentration of NaOH when the NaOH is added to the acid, assuming no reaction occurs :
[NaOH(aq)] = moles ÷ volume (L) = 3.75 × 10-3 mol ÷ 0.025 L = 0.15 mol L-1

Calculate the concentration of OH- due to NaOH once NaOH is added to the acid:
NaOH is a strong base so it fully dissociates in water: NaOH → Na+(aq) + OH-(aq)
[OH-(aq)] = [NaOH] = 0.15 mol L-1

Compare [H+(aq)] and [OH-(aq)] in the solution:
[H+(aq)] = 0.020 mol L-1
[OH-(aq)] = 0.15 mol L-1
[H+(aq)] < [OH-(aq)]

Decide if the solution is acidic, basic or neutral:
The solution is basic because [H+(aq)] < [OH-(aq)]

Question 4. 28.0 mL of 0.012 mol L-1 HCl(aq) is added to 22.0 mL of 0.015 mol L-1 NaOH(aq).
Is the resulting solution acidic, basic or neutral?

Extract the data from the question:
volume of HCl(aq) = 28.0 mL = 28.0/1000 = 0.0280 L
concentration of HCl(aq) = 0.012 mol L-1
volume of NaOH(aq) = 22.0 mL = 22.0/1000 = 0.0220 L
concentration of NaOH(aq) = 0.015 mol L-1

Calculate the theoretical concentration of hydrogen ions in the final solution resulting from the hydrochloric acid, assuming no reaction occurs:
moles HCl = concentration (mol/L) × volume (L) = 0.012 mol/L × 0.0280 L = 3.36 × 10-4 mol
Hydrochloric acid is a strong acid so it fully dissociates in water: HCl → H+(aq) + Cl-(aq)
moles of H+(aq) = moles HCl = 3.36 × 10-4 mol
theoretical [H+(aq)] in the final solution = moles H+(aq) ÷ total volume of the solution in litres
= 3.36 × 10-4 mol ÷ (0.028 L + 0.022 L) = 6.72 × 10-3 mol L-1

Calculate the theoretical concentration of hydroxide ions in the final solution resulting from the sodium hydroxide, assuming no reaction occurs:
moles NaOH = concentration (mol/L) × volume (L) = 0.015 mol/L × 0.0220 L = 3.30 × 10-4 mol
Sodium hydroxide is a strong base so it fully dissociates in water: NaOH → Na+(aq) + OH-(aq)
moles of OH-(aq) = moles NaOH = 3.30 × 10-4 mol
theoretical [OH-(aq)] in the final solution = moles OH-(aq) ÷ total volume of the solution in litres
= 3.30 × 10-4 mol ÷ (0.028 L + 0.022 L) = 6.60 × 10-3 mol L-1

Compare [H+(aq)] and [OH-(aq)]
[H+(aq)] = 6.72 × 10-3 mol L-1
[OH-(aq)] = 6.60 × 10-3 mol L-1
[H+(aq)] > [OH-(aq)]

Decide if the solution is acidic, basic or neutral:
Final solution is acidic because [H+(aq)] > [OH-(aq)]

1. Since we are using the Arrhenius definition of acids, bases and neutralisation, it is quite acceptable to use H+ (or H+(aq)) to represent the hydrogen ion.

2. If the solution is aqueous, then we can use the terms alkali instead of base and alkaline instead of basic.

How do you test if a solution is acidic?

One of the quickest test to determine if a solution is an acid or a base is the litmus paper test. This involves dipping a special strip of paper (known as litmus paper) into the solution and observing the color of the paper. Litmus paper will turn red in acid solutions and blue in base solutions.

What are two common ways to identify acids and bases?

There are two ways to identify acids and bases, one way is sensory elevation which includes taste and touch. The other way to identify acids and bases are organic dyes which is a naturally occurring color pigment that changes color when exposed to acids or based. An example of the organic dye is the litmus test.