Copper sulfate reacts with lactose. The solution turns bright yellow.

20 minutes
Experiment's video preview



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
  • Observe safety precautions when working with boiling water.
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

May I use sugar substitutes in this experiment?

Artificial sweeteners (sugar substitutes) may be of various chemical nature: alcohols, acids, and even proteins. The goal of our experiment is to demonstrate the difference between two disaccharides, so we cannot use sugar substitutes.

We already have one disaccharide in the set – lactose. It is a reducing disaccharide. Now, you need to find the second reagent – sucrose, or regular sugar. It is a non-reducing disaccharide.

Conduct the experiment to see the difference between them! For in-depth understanding of these complex organic compounds, remember to read the experiment description.

Step-by-step instructions

  1. Take two vials labeled with numbers. Fill the vial #1 halfway with water and add there one small spoon of sugar.
  2. Close the vial securely and shake it until all the sugar is dissolved.
  3. Take the vial #2 and fill it halfway with lactose solution.
  4. Add one big measuring spoon of 2M Na2CO3 solution into each vial.
  5. Add 4 drops of 0.4M CuSO4 solution into each vial.
  6. Close both vials tightly and shake them.
  7. Secure both vials in test tube holders.
  8. Insert the vials into a beaker and pour in boiling water to cover them. Wait 3–5 min.
  9. Compare the color of resulting solutions.
  10. To repeat this experiment, make sure to thoroughly wash the vials.
Graphical step-by-step instruction


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

Scientific description

What is the difference between sucrose and lactose?

Both sucrose and lactose belong to the same class of chemical compounds – carbohydrates. What is more, they have the same chemical formula - C12H22O11. However, the structures of these sugars have important differences. Firstly, they consist of different fragments: sucrose – of glucose and fructose, and lactose – of glucose and galactose. In addition, these fragments in each sugar are bound together in different ways.

Why do the solutions of sucrose and lactose turn blue?

The molecules in these carbohydrates contain hydroxyl groups or OH-groups. When we add copper sulfate to the solutions of sucrose and lactose, copper ions Cu2+ bind with OH-groups in these sugars and form complex compounds with them. These compounds have a bright blue color, which can be observed in the experiment.

Why does the lactose solution turn yellow while the sucrose solution does not?

Sucrose and lactose consist of two structural fragments, which are bound together in different ways in each of the carbohydrates. In the case of lactose, such binding makes it vulnerable to oxidizers: for example, copper(II) ions. Copper (II) ions Cu2+ slowly oxidize lactose and copper (I) oxide forms. In pure form, it is a red powder and when in a solution – a reddish suspension (a chemical one!).

Cu2+ + C12H22O11 → Cu2O↓ + C12H22O12