Which of the following, in powder form, is white? (A) NaCl, (B) K2Cr2O7, (C) Ca(MnO4)2, (D) Fe2O3, or (E) NiCl2.
Let’s look at the names of these substances before we answer the question. NaCl is sodium chloride; K2Cr2O7 is potassium dichromate; Ca(MnO4)2 is calcium permanganate; Fe2O3, iron (III) oxide; and NiCl2, nickel (II) chloride. These are the IUPAC names of the substances. All these substances are solids at room temperature. They are composed of metal cations and anions. Often, anions are made of nonmetals. However, the dichromate anion and the permanganate anion contain within themselves the metals chromium and manganese.
Because all these compounds are made up of positive and negative ions, we say these are ionic compounds. Ionic compounds all have similar properties. For example, they can exist as crystals. They have high melting points and boiling points. They are very hard and brittle, which means they shatter easily if you hit them with a hammer. And they do not usually conduct electricity in the solid state but do conduct very well when molten or dissolved in water.
The question asks, which of the following in powder form is white? Because these ionic substances have a very brittle nature, a hammer or mortar and pestle can be used to crush the crystals into a fine powder. So all of these substances in the answers can exist as powders. But which one forms a white powder? The color of an ionic substance usually arises from the presence of one of the ions in the compound. And usually, it’s from the metal-containing ion. Cr2O72−, the dichromate ion, gives dichromate compounds in orange color. MnO4−, the permanganate ion, gives permanganate compounds a purple color.
The Fe3+ ion, called the ferric ion, is red brown in color in compounds. It is the Fe3+ ion in iron (III) oxide that gives rusty metal its distinct red-brown color. Nickel (II) chloride is an example of a compound which has different colors whether water is present or not. When water is not present, the color of the compound is yellow brown. When water is present, the color of the compound appears green. This shows us that the metal cation does not always determine and give only one color for a particular compound. But sometimes, the presence of other compounds in the environment can affect the apparent color of a certain ionic compound.
Interestingly, sodium chloride does not actually have any color. It’s actually transparent. But when in the powder of form, there is so much reflection of light from the surfaces of the powder particles that it appears white. So next time you put some sodium chloride or salt on your food and you see the white powder, remind yourself that your eyes are being tricked because of reflection on those tiny transparent powder particles. So the substance which, in powder form, is white is NaCl.