Question

If a piece of hard white blackboard chalk is heated strongly in a flame, the mass of the piece of chalk will decrease, and eventually the chalk will crumble into a fine white dust. Does this change suggest that the chalk is composed of an element or a compound?

Short answer

The change in mass and the formation of fine white dust upon heating chalk suggests that it is composed of a compound, not an element. The compound is calcium carbonate (CaCO_3), which decomposes upon heating to form calcium oxide (CaO) and carbon dioxide (CO_2) gas.

Step-by-step solution

Recall the definitions of element and compound.

An element is a substance that cannot be broken down into simpler substances by chemical reactions. A compound, on the other hand, is a substance composed of two or more different elements chemically bonded together in a fixed proportion. Compounds can be broken down into their constituent elements by chemical reactions.

Analyze the observations

In this exercise, we are given that when a piece of chalk is heated, its mass decreases and it eventually crumbles into fine white dust. For a substance to lose mass when heated, a chemical change must occur that causes a component of the substance to be released as a gas. This would make the initial substance a compound, as elements do not typically undergo any chemical reactions under heating.

Identify the chalk composition

Based on our analysis in Step 2, we can conclude that chalk is more likely to be a compound. In fact, chalk is composed mainly of calcium carbonate (CaCO_3), a compound that decomposes upon heating to form calcium oxide (CaO) and carbon dioxide (CO_2) gas which escapes and causes the mass decrease. The final white dust is calcium oxide (CaO).

Answer the question

The observed change in the mass of the chalk when heated and its eventual transformation into fine white dust suggests that chalk is composed of a compound, and not an element. The compound in question is calcium carbonate (CaCO_3).

Key concepts

Chemical Reactions

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. During chemical reactions, the bonds between atoms are broken and new bonds form to make different molecules. It's important to note that in a chemical reaction, the mass is conserved, although it might not seem so when substances change states or when gases are involved. For example, when chalk is heated and it transforms into a white dust, a chemical reaction is taking place.

Substances like the calcium carbonate in chalk undergo reactions when subjected to heat, leading to a mass change observable in the exercise. This is a common chemical process known as thermal decomposition, which is one of the various types of chemical reactions. A substance will react differently depending on its composition and the conditions it is exposed to. In this case, heating causes the calcium carbonate to decompose.

Compound Decomposition

Decomposition of a compound is a type of chemical reaction where a single compound breaks down into two or more simpler substances. This can happen through various methods such as heating (thermal decomposition), electrical current (electrolysis), or through a reaction with another chemical (chemical decomposition).

The decomposition of calcium carbonate upon heating illustrates an essential teaching point in this area. When a compound composed of more than one element, like chalk, is heated, the energy provided can be enough to break the bonds holding the compound together. The result is the formation of new substances with different properties. In this exercise example, the compound calcium carbonate breaks down to form calcium oxide and carbon dioxide—a gas that escapes into the air, which is why there's a decrease in mass. Decomposition reactions are central to understanding how complex compounds can be transformed into their basic constituents.

Calcium Carbonate (CaCO3)

Calcium carbonate, with the chemical formula CaCO₃, is a widespread chemical compound found in rocks as minerals like limestone, and is the principal constituent of chalk. This substance is an example of an inorganic compound because it does not contain carbon-hydrogen bonds.

Understanding how CaCO₃ behaves when heated is crucial for grasping key chemistry concepts. When exposed to high temperatures, as mentioned previously, calcium carbonate decomposes in a reaction to produce calcium oxide (CaO) and carbon dioxide (CO₂). This reaction is significant not only in a laboratory or classroom setting but also industrially, as it provides a source of lime (CaO) and plays a pivotal role in the carbon cycle. This real-life application of a seemingly simple classroom exercise exemplifies the importance of chemistry in everyday processes.