Indicators and dry ice: demonstration
Dry ice is added to indicator solutions, producing bubbles and a ‘fog’ along with a gradual
color change. The experiment is a great way to demonstrate neutralization reactions and pH
changes, as well as to highlight that carbon dioxide forms weakly acidic solutions.
Read our standard health & safety guidance
Substance
pH
Background: Acids and Bases? Alkalis?
Stomach acid
An acid is a compound which donates a hydrogen ion (H+) to
another compound. This other compound is called a base, and
in a proton acceptor. In this case Dry Ice and Water react to
form our acid and we also use acetic acid aka vinegar, while
ammonia (NH3), sodium hydroxide (NaOH) or sodium
bicarbonate(NaHCO3) aka baking soda is the base. The term
alkali technically refers to a basic, ionic salt, but has become a
synonym for the word ‘base’ especially in reference to soluble
bases.
The relative strength of acids and bases is measured on a
logarithmic scale know as pH, which is based roughly on the
concentration of H+ ions in a solution; Acids will have a low
pH (less than 7) and bases will have a high pH (greater than 7).
The further the pH is from 7 the stronger the acid or base is.
An indicator is used to measure the pH of a substance. Common
indicators are litmus paper, phenolphthalein, methyl orange,
phenol red, bromothymol blue, bromocresol green and
bromocresol purple.
A solution with a pH of 7 is known as neutral. Neutralization is
the process in which an acid and a base are combined to form a
salt and a neutralized base. In this the reaction with sodium
hydroxide the salt is Sodium Carbonate (Na2CO3) and the
neutralized base is water (H20).
1.5 – 2.0
Lemon juice
2.4
Cola
2.5
Vinegar
2.9
Orange juice
3.5
Tomato Juice
4.0
Coffee/Tea
5.0
Milk
6.5
Pure Water
7.0
human saliva
Blood
6.5 – 7.4
7.34 – 7.45
Seawater
7.7 – 8.3
Hand soap
9.0 – 10.0
Household ammonia
11.5
Bleach
12.5
Household Lye(NaOH)
13.5
Lesson organization
This demonstration can be used when discussing acids, alkalis, indicators, or the properties of
carbon dioxide. With the appropriate audience, it could also be used to introduce a discussion of
the pH changes that take place during the titrations of weak acids with strong and weak alkalis and hence buffers.
It is also a good fun demonstration for more general audiences.
Apparatus and chemicals
The teacher requires:
Eye protection
Measuring cylinders (1 dm3) - as many as the number of indicators to be used (see note 2)
Expanded polystyrene cool-box to store the dry ice (see note 1)
Tongs or large spoon/scoop for transferring dry ice
Long stirring rod
Gloves (leather or insulated) for handling dry ice
Dry ice – allow 100 g for each indicator
Access to a range of indicator solutions. Suitable ones include:
Universal Indicator
Phenolphthalein
Thymolphthalein
Thymol blue
Phenol red
Bromothymol blue
Dilute ammonia solution and/or dilute sodium hydroxide solution (Irritant), 0.1 mol dm-3
Technical notes
Dry ice (solid carbon dioxide). Refer to CLEAPSS Hazcard 20.
Ammonia solution. Refer to CLEAPSS Hazcard 6 and Recipe Card 4.
Sodium Hydroxide solution (Irritant). Refer to CLEAPSS Hazcard 91.
Indicators (various hazards including Highly flammable). Refer to CLEAPSS Hazcard 32.
1 Never put dry ice in a sealed container.
2 If 1 dm3 measuring cylinders are not available, 1 dm3 ‘tall form’ beakers are suitable
substitutes. The measuring cylinders or beakers should be glass rather than plastic – the color
change is much easier to see.
3 0.1 mol dm-3 ammonia solution should be adequate for this demonstration.
Procedure
SAFETY: Wear eye protection and use gloves to handle the dry ice since it can cause severe
frost burns.
1. For each indicator, fill a large measuring cylinder with water to the 1 dm3 mark, or a large
beaker to within 5 cm of the top. Add enough indicator to give an easily visible color.
2. Add a few drops of ammonia solution or sodium hydroxide solution to give an alkaline
solution. Stir to mix the solution thoroughly.
3. Add a few lumps of dry ice. These will sink to the bottom and bubble as gaseous carbon
dioxide is produced. A spectacular fog is produced at the top of the cylinder. After several
minutes, the color of the indicator will change.
Teaching notes
In this demonstration carbon dioxide dissolves in and reacts with water to produce hydrogen ions
(H+). The acidic solution produced then reacts with and neutralizes the alkali present.
Carbon dioxide dissolves reversibly in water:
CO2(g) ⇔ CO2(aq)
(This is the basis of the fizz you get when taking the top off a bottle of carbonated water – the
CO2 comes out of solution when the pressure is released.)
Some of the dissolved CO2 reacts reversibly with water to form an acidic solution:
CO2(aq) + H2O(l) ⇔ HCO3-(aq) + H+(aq)
This acidic solution then reacts with the alkali present.
If the alkali is sodium hydroxide, the equation for the neutralization reaction is:
HCO3-(aq) + H+(aq) + Na+(aq) + OH- (aq) → Na+(aq) + HCO3- (aq) + H2O(l)
If the alkali is ammonia solution, the equation for the neutralization reaction is:
HCO3-(aq) + H+(aq) + NH4+(aq) + OH-(aq) → NH4+(aq) + HCO3-(aq) + H2O (l)
**Note: Unlike sodium hydroxide ammonia is a weak alkali and the color change takes place
more slowly. Ammonia itself reacts reversibly with water. NH3(g) + H2O(l) ⇔ NH4+(aq) + OH (aq)
The final pH reached is about 4.5. It is best to use indicators which change color at pH values
above this, or use Universal Indicator.
The expected color changes (alkali – acid) for the suggested indicators are:
Indicator
Phenolphthalein:
Thymolphthalein:
Thymol blue:
Phenol red:
Bromothymol blue:
Initial
pink
blue
Blue
red
blue
-
Final
colourless
colourless
yellow
yellow
blue
pH Range
8.2-10.0
8.3-10.6
8.0-9.6
6.8-8.4
6.0-7.6
You may want to demonstrate color changes at lower pH values. If so add a few drops of
concentrated hydrochloric acid at the end.