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

Chalk (Calcium carbonate - CaCO3) is composed of Calcium, Carbon, and Oxygen. When heated strongly in a flame, it decomposes into Calcium oxide (CaO) and Carbon dioxide (CO2) gas, resulting in a mass decrease and the observed crumble into fine white dust. This change indicates that chalk is a compound, not an element, as its properties and composition alter during the heating process, and it consists of different elements combined in a specific ratio.

Step-by-step solution

Understand the components of chalk

Chalk is primarily made up of Calcium carbonate (CaCO3). Calcium carbonate consists of the elements Calcium (Ca), Carbon (C), and Oxygen (O) combined together in a specific ratio.

Examine the effect of heating on chalk

When chalk (Calcium carbonate) is heated strongly in a flame, the chemical reaction occurs, and it decomposes into Calcium oxide (CaO) and Carbon dioxide (CO2) gas. The balanced chemical equation for the process is as follows: \[CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)\]

Analyze the mass decrease

As we can see from the chemical equation, when the chalk is heated, it decomposes into Calcium oxide and Carbon dioxide gas. The mass decrease observed is a result of CO2 gas released into the atmosphere, which is not part of the remaining solid chalk. Therefore, the mass of the remaining chalk (Calcium oxide) will be lesser than the original.

Identify if chalk is a compound or an element

Chalk (Calcium carbonate) consists of different elements (Calcium, Carbon, and Oxygen) combined together in a specific ratio. When it undergoes a chemical change due to heating, it decomposes into other compounds, such as Calcium oxide and Carbon dioxide. Since its properties and composition alter during this process and have a mixture of distinct elements, we can conclude that chalk is a compound, not an element.

Key concepts

Calcium Carbonate

Calcium carbonate, with the chemical formula \( CaCO_3 \) is a widely abundant substance found in rocks as the minerals calcite and aragonite most notably as limestone, which is the principal constituent of chalk. Beyond its geological presence, \( CaCO_3 \) is also a crucial component in many biological organisms such as seashells and is commonly used in various industrial applications, including as a construction material, in agriculture, and as a key ingredient in products like paint and antacids.

Its structure is composed of a calcium ion (\( Ca^{2+} \) ) bonded to a carbonate ion (\( CO_3^{2-} \) ), with the latter being a triangular planar arrangement of one carbon atom surrounded by three oxygen atoms. This ionic compound demonstrates physical properties that readily indicate its nature as a compound rather than a pure element.

If we improve the exercise by focusing on the individual elements within \( CaCO_3 \)—calcium, carbon, and oxygen—and explore how their properties differ from the properties of calcium carbonate itself, we can deepen the student's understanding of why \( CaCO_3 \) is a compound and not a mixture or pure element.

Chemical Reaction

A chemical reaction is a process that involves the rearrangement of the molecular or ionic structure of a substance, as opposed to a change in physical form or a nuclear reaction. During a chemical reaction, bonds between atoms are broken and new ones are formed, transforming the reactants into products with different properties from the original substances.

In the context of chalk and its heating, the process is a textbook example of endothermic reaction—where heat is absorbed from the surroundings—evidenced by the need for a strong flame to initiate the change. The decomposition of \( CaCO_3 \) into calcium oxide (\( CaO \) ) and carbon dioxide (\( CO_2 \) ) is a classic demonstration of such a reaction, as it clearly shows a marked transformation in the substance's composition and chemical properties.

An enhancement to the exercise could involve having students predict the outcome of heating \( CaCO_3 \) based on the types of chemical reactions, and then compare their predictions to the actual results. By doing this, not only do students understand 'what' happens, but they also begin to grasp the 'why' behind the chemical changes.

Decomposition

Decomposition is a type of chemical reaction in which a single compound breaks down into two or more elements or new compounds. The decomposition of calcium carbonate upon heating is emblematic of this type of reaction. It's classified as a thermal decomposition, which, as the name suggests, requires heat to break the chemical bonds holding the compound together.

As \( CaCO_3 \) is heated, the increase in temperature supplies enough energy to overcome the bond energy in the carbonate compound, culminating in its breakdown into calcium oxide (\( CaO \) ) and carbon dioxide (\( CO_2 \)), an inorganic carbon compound. The calcium oxide remains as a solid, while the carbon dioxide—a gas at these conditions—escapes into the atmosphere.

For a clearer understanding in an educational setting, this exercise could be further improved by discussing how the reaction rate of decomposition is influenced by factors like particle size, temperature, and catalysts. Through experimental demonstration or simulation, students could observe these factors in action, enhancing both their theoretical and practical understanding of how decomposition works.