Question

A sample of limestone $\left({\mathrm{CaCO}}_3\right)$ is heated in a closed vessel until it is partially decomposed. Write an equation for the reaction, and state how many phases are present.

Short answer

The phases present are solid and gas.

Step-by-step solution

Identify the Chemical Formula

The formula for limestone is ${\mathrm{CaCO}}_3$. When heated, it decomposes into calcium oxide $\mathrm{CaO}$ and carbon dioxide ${\mathrm{CO}}_2$.

Write the Chemical Equation

The decomposition reaction for limestone when heated is represented by the equation: $${\mathrm{CaCO}}_{3(s)}\to {\mathrm{CaO}}_{(s)}+{\mathrm{CO}}_{2(g)}$$ This shows solid calcium carbonate decomposing into solid calcium oxide and gaseous carbon dioxide.

Determine the Phases Present

Examine the equation: ${\mathrm{CaCO}}_3$ and $\mathrm{CaO}$ are solid (s), while ${\mathrm{CO}}_2$ is a gas (g). This indicates two phases: solid and gas.

Key concepts

Limestone Decomposition

Limestone, chemically known as calcium carbonate ${\mathrm{CaCO}}_3$, is a common mineral found in natural rock formations. When limestone is subjected to heat, a fascinating transformation known as decomposition occurs. During limestone decomposition, calcium carbonate breaks down into calcium oxide ($\mathrm{CaO}$), often referred to as quicklime, and carbon dioxide (${\mathrm{CO}}_2$) gas. This reaction is an excellent example of a thermal decomposition, where a substance breaks down into two or more simpler substances upon heating.

• The formula for limestone: ${\mathrm{CaCO}}_3$.
• Products of decomposition: $\mathrm{CaO}$ and ${\mathrm{CO}}_2$.
• Type: Thermal decomposition reaction.

The process is significant in industries such as construction for producing lime, which has pivotal roles in building materials and environmental applications.

Phase Identification

Phase identification in chemical reactions is crucial to understand the physical state of the reactants and products. Each substance in a chemical reaction can be in one of several phases: solid, liquid, gas, or aqueous (dissolved in water).
In the example of limestone decomposition:

• ${\mathrm{CaCO}}_3$ starts as a solid.
• $\mathrm{CaO}$ remains a solid after decomposition.
• ${\mathrm{CO}}_2$ forms as a gas.

Hence, the decomposition results in two distinct phases. Knowing these phases helps chemists predict the behavior of substances during the reaction, such as changes in pressure or volume due to gas production. Understanding the phase changes is also essential for practical applications, such as capturing the ${\mathrm{CO}}_2$ gas formed in industrial processes.

Chemical Equation

A chemical equation provides a concise description of a chemical reaction by showing the reactants and products along with their states. It serves as a universal language for chemists, allowing them to convey complex reactions quickly and clearly.
In the context of limestone decomposition, the chemical equation is written as:
$${\mathrm{CaCO}}_{3(s)}\to {\mathrm{CaO}}_{(s)}+{\mathrm{CO}}_{2(g)}$$
Here, the equation captures the transformation of solid calcium carbonate into solid calcium oxide and gaseous carbon dioxide. Each component in the equation is tagged with its phase:

• (s) indicates a solid phase.
• (g) indicates a gaseous phase.

Writing a balanced chemical equation ensures that the law of conservation of mass is followed, meaning the number of each type of atom on both sides of the equation matches. Thus, chemical equations not only show what substances are involved but also how they change and interact, providing a foundation for predicting the outcomes and energy changes of chemical reactions.