In reality, there is no substance H2Cr4O2 in the list of recognized and studied chemicals. This may be an error in notation of the chemical formula.
Considering the chemical formula mentioned, the teacher may have intended to introduce the students to the chemical substances Cr2O3 (chromium(III) oxide) or H2CrO4 (chromic acid), both of which relate to the element Chromium (Cr).
Cr2O3, also known as chromium(III) oxide, is a solid substance with a green to black color and is insoluble in water. This substance is often used for plating colors on metals, or as a non-metal in chemical reactions.
H2CrO4, also known as chromic acid, is a liquid substance with a brown-red to deep black color. It is a strong oxidizing agent and can cause fires when in contact with flammable substances. Chromic acid is also widely used in the plating industry and for cleaning metals.
Therefore, if you intend to introduce one of these chemical substances, make sure that you have accurately identified the chemical formula to avoid confusion.