4. Now you have the final product. Start
testing the pH of different solutions
Red cabbage juice belongs to the
anthocyanin family of chemicals
Red cabbage contains a pigment
This pigment contains the natural pH
indicator in red cabbage.
12. pH indicators
pH indicators are chemicals that
change color when an acid or base is
added to it
The name pH means “the potential of
It refers to the ability of a chemical to
donate or accept hydrogen ions from
pH is a measure of how acidic or basic
a solution is.
The pH scale ranges from 1 to 14.
The pH 7 is neutral.
The color of the juice changes
because of its hydrogen ion
To compute the pH of a
Acid comes from the Latin word
“acidus” meaning sour because acids
generally have a sour taste.
Acids donate hydrogen ions in an
aqueous solution thus increasing the
hydrogen ions in the solution which
gives a low pH (less than 7).
Bases generally have a bitter taste and a
slimy or soapy feel to the skin.
Bases increase the concentration of
hydroxide ions [OH-] which decreases the
[H+] in the solution hence giving a high pH.
19. pH of some common
1.0 Battery Acid (sulfuric 5.8-6.4 peas
acid) 6.4 cow's milk
1.8-2.0 limes 6.5-7.5 human saliva
2.2-2.4 lemon juice 7.0 distilled water
2.2 vinegar (acetic acid) 7.3-7.5 human blood
2.9-3.3 apple juice, cola 8.3 baking soda
3.7 orange juice 9.2 borax
4.0-4.5 tomatoes 11.0 laundry ammonia
5.6 unpolluted rain 12.0 lime water
21. Invisible Ink
An invisible inkis any substance that can be
used for writing, which is not visible to the naked
The process of rendering the ink visible usually
requires either heat or a pH indicator.
Solutions of iron, silver or copper salts were used
It has been used for espionage, most notably by
the British and American armies during the
◦ The secret writing was placed between the lines of an
innocent letter, in case they were intercepted by the
◦ They would mark their letters written in invisible ink
with “F” for fire and “A” for acid.
Squeeze lemons on a bowl then add a few drops of
Use the juice as ink, then write your message by
using a cotton bud.
Let it dry.
To reveal the secret message, heat the paper by
using a candle, lamp, or ironing it.
24. Chemistry Behind It
Lemon juice is a weak organic acid
The juice contains 5-6% citric acid
When heated, the carbon compound
breaks down, which then produces a
The compound reacts with
air, undergoing oxidation.
Another explanation is that the acid
weakens the paper fibers, which causes
the weak parts to burn first.
25. Citric Acid
Citric acid is a natural acid that is
commonly found in a variety of fruits.
Other uses of citric acid:
◦ Food additive and preservative
◦ Cleaning agent
◦ Widely used in cosmetics
Other citrus fruits such as orange
and lime contain high concentration
of citric acid, so they can also be
used as invisible inks.
29. A basic solution of sodium bicarbonate
(NaHCO3), which is baking soda, and
sodium carbonate (Na2CO3), which is
soda ash is prepared. These
substances are water-soluble resulting
in a clear solution with an appearance
the same as plain water. The pH of the
solution is about pH = 9 (basic)
whereas the pH of plain water is about
30. The wine glass contains a few drops of
phenolphthalein indicator. This is a
liquid that turns pink in basic solutions.
Larger quantities of phenolphthalein in
solution will produce a darker color;
therefore the solution appears to be
31. The milk glass contains a small
quantity of saturated barium chloride
solution. With the addition of excess
base to this saturated solution, the
barium chloride will precipitate the
carbonate ions from the solution
forming barium carbonate
(BaCO3), which is insoluble in water.
The precipitate disperses throughout
the glass as a white solid and gives the
appearance of milk.
32. The beer glass contains about 5mL
acid (HCl, which is a strong acid) and
a few crystals of sodium dichromate
(Na2Cr2O7). The BaCO3 from the milk
glass is soluble in acidic solutions
(pH 4) so the solids re-dissolve in the
acidic solution to form a clear liquid.
However, Na2Cr2O7 dissolves to give a
pale orange color, which resembles the
appearance of a mug of beer.
125 mL Erlenmeyer flask
50 mL ethyl alcohol
less than 1 mL of thymolphthalein indictor
less than 1 mL of 0.1 M sodium hydroxide
white cotton shirt
35. Pour 50 mL of the ethyl alcohol in the 125 mL
Add 2-3 droppers full of thymolphthalein indicator
to the ethyl alcohol.
Add just enough sodium hydroxide (about 2 drops)
so a dark blue color is created. Some adjustments
may be needed on the amount of sodium
hydroxide to add.
Place the rubber stopper in the flask while the
solution is being stored.
Fill the dropper with some of the blue indicator
Squirt the solution onto the white cotton shirt and
in a few seconds the blue "stain" will disappear.
(The residue comes off in the wash. This does not
seem to work well with paper).
36. The sodium hydroxide causes the indicator of
thymolphthalein solution to turn the dark blue
color. When the solution is squirted onto the
shirt, it reacts with the carbon dioxide in the air.
The carbon dioxides reacts with the water to
form an acid called carbonic acid. The carbonic
acid then reacts with the sodium hydroxide in a
neutralization reaction. This reaction forms
sodium carbonate, which is a washing
soda, and the stain disappears. The carbon
dioxide is the acid that neutralizes the
base, sodium hydroxide. The color of the
thymolphthalein is colorless in the presence of
37. Ice Cream in a Bag
What you'll need:
1 tablespoon sugar
1/2 cup milk or half & half
1/4 teaspoon vanilla
6 tablespoons rock salt
1 pint-size plastic food storage bag (e.g., Ziploc)
1 gallon-size plastic food storage bag
How to make it:
Fill the large bag half full of ice, and add the rock salt. Seal the bag.
Put milk, vanilla, and sugar into the small bag, and seal it.
Place the small bag inside the large one, and seal it again carefully.
Shake until the mixture is ice cream, which takes about 5 minutes.
Wipe off the top of the small bag, then open it carefully. Enjoy!
A 1/2 cup milk will make about 1 scoop of ice cream, so double the recipe if you
38. Why does salt do?
Adding salt to the ice lowers the
freezing point of the ice, so even more
energy has to be absorbed from the
environment in order for the ice to
melt. This makes the ice colder than it
was before, which is how the ice
cream freezes. The larger crystals of
the rock salt take more time to
dissolve in the water around the
ice, which allows for even cooling of
the ice cream.
39. Why salt?
You could use other types of salt instead of sodium
chloride, but you couldn't substitute sugar for the salt because
(a) sugar doesn't dissolve well in cold water and (b) sugar
doesn't dissolve into multiple particles, like an ionic material
such as salt.
Compounds that break into two pieces upon dissolving, like
NaCl breaks into Na+ and Cl-, are better at lowering the
freezing point than substances that don't separate into
particles because the added particles disrupt the ability of the
water to form crystalline ice. The more particles there are, the
greater the disruption and the greater the impact on particle-
dependent properties like freezing point depresssion, boiling
point elevation, and osmotic pressure. To make ice cream the
cream mixture needs to change from a liquid to a solid. This
process is called freezing (a phase change) and requires heat
to be removed from the mixture. The addition of salt to the ice
is needed for the phase change to take place. This is
because salt lowers the melting point of ice and in this
process requires heat from the surroundings (endothermic
change). The melting ice lose its heat energy and thus
freezes the ice cream.
41. In order to generate electricity, there must be a power
source and a complete circuit. When using a citrus fruit to
create electricity, these rules still apply. In a simple
experiment using a citrus fruit, the components of the circuit
include: a lemon or other fruit, wire, two different metal
element. The lemon in this circuit serves as the battery and
Two metals often used in this demonstration are zinc and
copper. The acidic juice of the lemon dissolves small
amounts of the two metals and their electrons react with
each other. The negatively charged ions travel through the
wires, creating an electrical current. (Electricity is the
movement of electrons.) This demonstration is a closed
circuit, which allows electrons to flow from the power source
and back again, with no breaks.
The lemon, with the zinc and copper, becomes a battery. A
battery is composed of two metals and an electrolyte. An
electrolyte is a conductive liquid; here, the lemon juice
performs this function. A chemical reaction takes place
between the metals within the citrus fruit. This creates
voltage, which pushes the electrons through the circuit.
A common misunderstanding is that citrus fruits create
electricity. What happens is the electrolyte (the citrus juice)
42. Fruit Charger
Lemon juice is highly concentrated with citic acid, which
gives the fruit a bitter taste, but also makes it a great
conductor of electricity.
When acids are dissolved into water, the acids break
apart into positive and negatively charged ions. These
ions have the power to conduct electricity through a
liquid like lemon juice.
Through a process known as oxidation-reduction, the
citric acid in the lemon juice and the metals pushed into
the flesh of the lemon will form a reaction which results
in an electric current. One metal will lose its electrons, a
process known as oxidation, and other metal gains
those lost electrons, a process known as reduction.
Because the citiric acid in the lemons is so highly
concentrated, all it takes is a few pieces of metal to
yield an electric output much like that of a battery's