Q3P - Chemistry: Molecular Nature Of Matter And Change

Question

Which of the following are pure substances? Explain. 

(a) Calcium chloride, used to melt ice on roads, consists of two elements, calcium and chlorine, in a fixed mass ratio. 

(b) Sulfur consists of sulfur atoms combined into octatomic molecules. 

(c) Baking powder, a leavening agent, contains 26% to 30% sodium hydrogen carbonate and 30% to 35% calcium dihydrogen phosphate by mass.

(d) Cytosine, a component of DNA, consists of H, C, N, and O atoms bonded in a specific arrangement.

Step-by-Step Solution

Answer

Out of the given four options, calcium chloride, sulfur and cytosine are pure substances while baking powder is a mixture.

Step 1: Definition of pure substances

A pure substance is made up of only one kind of particle. The constituents of a pure substance cannot be distinguished or separated by physical methods. Pure substances can further be classified as elements and compounds.

Step 2: Identification

1. By the definition of compounds, we can say that calcium chloride is a compound as it is made by combining two different elements, calcium and chlorine, in a fixed mass ratio. Thus, calcium chloride is a pure substance.
2. As Sulfur consists of only sulfur atoms combined into octatomic molecules so it is an element. Thus, sulfur is also a pure substance.
3. In baking powder, the proportions of the components are not fixed, so it is a mixture. Hence, baking powder is not a pure substance. 
4. As in cytosine atoms of different elements are bonded in a specific arrangement, this is a compound and a pure substance.