Question

Which of the following statements best explains the law of conservation of mass? (a) \(100 \mathrm{~g}\) of water is heated to give steam. (b) \(A\) sample of \(\mathrm{N}_{2}\) gas is heated at constant pressure without any change in mass. (c) \(36 \mathrm{~g}\) of carbon combines with \(32 \mathrm{~g}\) of oxygen to form \(68 \mathrm{~g}\) of \(\mathrm{CO}_{2}\) (d) \(10 \mathrm{~g}\) of carbon is heated in vacuum without any change in mass.

Short answer

Option (c) best explains the law of conservation of mass as it shows the total mass of reactants equals the total mass of the products in a chemical reaction.

Step-by-step solution

Understanding the Law of Conservation of Mass

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. It means that the total mass of reactants before the reaction is equal to the total mass of products after the reaction.

Analyzing Option (a)

Heating water to give steam is a physical change, not a chemical reaction. So, while this demonstrates that mass is conserved during phase changes, it doesn't directly relate to the law as it applies to chemical reactions.

Analyzing Option (b)

Heating a sample of N2 gas at constant pressure without any change in mass highlights the conservation of mass. However, it doesn't involve a chemical reaction, as there is no change in the chemical composition of the substance.

Analyzing Option (c)

The reaction between carbon and oxygen to form carbon dioxide is a chemical reaction. The statement indicates that the masses of reactants add up to the mass of the product (36 g + 32 g = 68 g), which aligns with the law of conservation of mass.

Analyzing Option (d)

Heating carbon in a vacuum without any change in mass may indicate that mass is conserved, but as there is no chemical reaction presented, it does not serve as a good example for the law of conservation of mass.

Choosing the Best Statement

Based on the understanding of the law of conservation of mass, option (c) is the statement that best explains this law, as it refers to a chemical reaction where the total mass of reactants equals the total mass of the products.

Key concepts

Chemical Reactions

At the heart of chemistry lies the study of chemical reactions, a process where substances known as reactants are transformed into new substances called products. During a chemical reaction, the atoms in the reactants are rearranged through breaking and forming of bonds to create different substances. From the burning of a candle to the rusting of iron, chemical reactions are fundamental to countless events in our daily lives.

It is crucial to differentiate between chemical and physical changes. Physical changes, like melting ice into water, do not result in new substances forming. Conversely, chemical reactions always produce new substances. For example, when hydrogen reacts with oxygen, it forms water, which has different properties compared to the original gases. This distinction is vital to understand the application of the law of conservation of mass in chemistry.

Mass Conservation in Chemistry

The principle of mass conservation in chemistry, often called the law of conservation of mass, was first articulated by Antoine Lavoisier in the late 18th century. It asserts that mass is neither created nor destroyed during a chemical reaction. Instead, the mass of the reactants must equal the mass of the products. This concept is a cornerstone of chemical equations and stoichiometry, which is the calculation of reactants and products in chemical reactions.

In practice, this means that when planning an experiment or analyzing a reaction, chemists must carefully balance each side of the equation, ensuring that the mass and number of atoms of each element are conserved. This balance is not only a fundamental law of nature but also a practical tool for predicting the amounts of substances that will react and what quantities of products can be expected.

Reactants and Products Mass Balance

Understanding the mass balance of reactants and products is inherent in grasping the law of conservation of mass. When a chemical reaction occurs, the reactants undergo a transformation to create products. Due to the conservation of mass, if we accurately measure the mass of each reactant and sum these values, we should arrive at the total mass of the products.

This mass balance can be illustrated through simple equations, as was shown in the textbook exercise option (c): the reaction of carbon with oxygen to form carbon dioxide. Before the reaction, the mass of carbon (36 g) and oxygen (32 g) totals 68 g. Post-reaction, the mass of the carbon dioxide produced is also 68 g — demonstrating that the mass before and after the reaction is balanced. Through the careful application of this principle, chemists can predict and verify the outcomes of reactions, ensuring that all atoms that start a reaction are accounted for in the products.