Chlorophyll

Why are plants green?

Difficulty:
Danger:
Duration:
20 minutes
Experiment's video preview

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
  • Conduct the experiment in a well-ventilated area and away from sources of ignition.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

Where do I get 96% alcohol (ethanol) solution?

Alcohol may be purchased at a pharmacy or synthesized using a laboratory technique, as follows. You’ll need three candles and hard liquor or 40–60% ethanol solution. The rest is included in your Chemistry of plants experiment set and in the Starter kit.

  1. Insert a metal adapter into a one-holed rubber stopper.
  2. Slide a silicon tube onto the adapter.
  3. Insert a funnel into a flask and pour in 40 mL of hard liquor or 40–60% ethanol solution.
  4. Close the flask with the rubber stopper.
  5. Fill a beaker half way with cold water. Put a clean test tube into the beaker.
  6. Set three candles onto the stove and light them. Cover the stove with a flame diffuser.
  7. Set the flask onto the flame diffuser. Insert the free end of the tubing into the test tube and wait until it fills up to 2/3 with liquid.
  8. Extinguish the candles.
  9. Pour the liquid from the test tube into the beaker with crushed green leaves. Now, continue the experiment following the instructions.

Step-by-step instructions

Leaves owe their greenish color to the green pigment chlorophyll. Chlorophyll is almost insoluble in water, but does dissolve in many organic solvents, like ethanol.

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When enough chlorophyll has dissolved in the ethanol, take two samples of the chlorophyll solution.

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There is a magnesium ion Mg2+ (green one) in each of the chlorophyll molecules, which readily “drops out” in the presence of acid. As a result, pheophytin molecules form, which are much more pale (even yellowish) than the bright-green chlorophyll.

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Copper ion Cu2+ (brown one) from copper salt CuSO4 readily fits the vacancy the magnesium ion has left in the chlorophyll molecule. The resulting pheophytin copper complex strongly resembles the initial chlorophyll color.

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It’s difficult to distinguish the chlorophyll sample from the pheophytin copper complex one. But the latter is more stable, so if you leave the samples under a light source, the chlorophyll will soon decompose and lose its color.

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Disposal

Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

Why do we use a solvent?

A chlorophyll molecule has a long hydrophobic (meaning “afraid of water”) tail that prevents it from dissolving in water. However, chlorophyll readily dissolves in alcohol (or acetone). Therefore it is necessary to use a solvent such as alcohol or acetone to extract the chlorophyll from the shredded leaves.

Learn more

Chlorophyll also dissolves in oils. That is why oils, such as rapeseed or olive, often feature a distinct greenish color. In order to remove this coloration, oils can be treated with an alkali. The alkali causes the chlorophyll molecule to lose its hydrophobic tail and, as a consequence, its ability to dissolve in oils.

Chlorophyll demonstrates an even higher solubility (compared to alcohol and acetone) in gasoline-like liquids. However, gasoline doesn’t effectively extract chlorophyll from leaves. This is because in plant cells, chlorophyll molecules are strongly bound to protein molecules. In order to break the chlorophyll - protein bond, a solvent must contain water. However, water doesn’t mix with carbohydrates such as gasoline, kerosene or petroleum. Therefore these solvents are not suitable for extracting chlorophyll from plants.

Why did the green solution become paler after we added citric acid?

This occurs when citric acid is added because the hydrogen ions H+ force the magnesium ions Mg2+ out of the chlorophyll molecules. As a result, chlorophyll turns into pheophytin. In comparison with chlorophyll, pheophytin has a darker but less bright color.

Learn more

Pheophytinization is a very common phenomenon. This overcomplicated term stands for a process of chlorophyll discoloration due to the loss of magnesium ions Mg2+ in the presence of acids. You might have noticed that fresh green vegetables turn darker when cooked. This is a direct result of pheophytinization! The effect of pheophytinization is particularly obvious in pickling: once the acidic (usually vinegar) marinade is added to cucumbers, their bright green color turns a darker, richer brownish green tone.

What happens when we add CuSO4?

By adding copper sulfate CuSO4 to the solution, we introduce copper ions Cu 2+ into the test tube. These ions integrate themselves into the chlorophyll molecules, taking the place of the magnesium Mg2+ ions. A chlorophyll molecule with copper has a bright green color – as a result, the solution becomes green. Even after several days, a copper-chlorophyll complex solution retains its bright color. On the contrary, a magnesium-chlorophyll solution will decompose much more rapidly.

Learn more

The reaction of copper ions Cu2+ with pheophytin in a solution yields copper pheophytin. This compound is registered as E141, a certified food color. Copper pheophytin may be added to food products as green food coloring in strictly regulated doses because it contains copper – a heavy metal that is hazardous if consumed over 5 mg a day. The Food and Drug Administration in the USA limits the use of E141 in the food industry exclusively to the coloring powder mixtures for citrus soft drinks. The permitted dose must be less than 0.2% by dry mass of a product. In Europe, Russia, and most Asian, African, and South American countries, E141 (copper complex of chlorophyll) is approved for use in confectionery, canned vegetables, cosmetics, and pharmaceutical products.

What other metals can take place of magnesium in chlorophyll?

Both the salts of zinc Zn2+ and mercury Hg2+ also form green-colored compounds with chlorophyll. However, reactions with these ions take much longer and require special conditions. As well, the green color of these complexes isn’t as bright as those with copper. Moreover, as we know, mercury compounds are highly toxic and aren’t suitable for conducting experiments at home.

Why did the chlorophyll solution turn pale?

With time, the magnesium-chlorophyll complex solution undergoes a photochemical oxidation. That’s why the solution loses its bright color. On the contrary, the copper-chlorophyll complex is much more stable than its natural “predecessor”. It resists oxidation and preserves the color of the solution for much longer.

What leaves would work best for this experiment?

Any fresh green leaves will work for this experiment. However, some will produce better results than others. Before you start, make sure that the plant isn’t poisonous. It’s best to avoid leaves from plants with milky sap, such as spurge, dandelion or leaves from your mom’s favorite ficus tree. In order to check whether a plant has milky sap, cut a leaf and look at the cut edge. If it releases white (or sometimes yellow, off-white or reddish) opaque drops, it isn’t suitable for the experiment. In addition, plants with thick juicy leaves (such as sedum, kalanchoe or spiderwort) produce pale solutions because their leaves have a very low chlorophyll content.