Introduction

One of the fundamental principles in chemistry is the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction. This means that the total mass of the reactants (the substances you start with) is always equal to the total mass of the products (the substances you end up with).

Example: When a piece of magnesium burns in oxygen, the total mass of magnesium and oxygen before the reaction will equal the mass of the magnesium oxide produced after the reaction. This principle applies to all chemical reactions, whether they occur in a laboratory or naturally in the environment.

Understanding this concept helps students appreciate how chemical reactions obey a fundamental natural law and how to balance chemical equations.

Assessment Questions

Easy Questions

  1. What is the Law of Conservation of Mass?
  2. True or False: Mass can be created during a chemical reaction.
  3. What happens to the mass of the products in a chemical reaction compared to the mass of the reactants?
  4. In a chemical reaction, if you start with 50g of reactants, what will be the mass of the products?
  5. Fill in the blank: The total mass before and after a chemical reaction is always _.
  6. True or False: The mass of the reactants is always greater than the mass of the products.
  7. In a reaction where hydrogen reacts with oxygen to form water, what happens to the total mass of hydrogen and oxygen?
  8. What does the term ‘reactant’ mean in a chemical reaction?
  9. What do we call the substances that are formed as a result of a chemical reaction?
  10. True or False: The Law of Conservation of Mass applies to both physical and chemical changes.
  11. If 12g of carbon reacts with 32g of oxygen, what is the total mass of the product?
  12. What is meant by ‘mass is conserved’ in a chemical reaction?
  13. Fill in the blank: In a balanced chemical equation, the number of atoms of each element is the __ on both sides of the equation.
  14. True or False: A chemical reaction involves only the rearrangement of atoms, not the creation of new ones.
  15. In a closed system, what happens to the mass during a chemical reaction?
  16. What is the role of a chemical equation in describing a reaction?
  17. If the mass of the products is found to be less than the mass of the reactants, what might be the reason?
  18. True or False: Mass is always conserved in a chemical reaction in an open system.
  19. What is meant by an open system in terms of chemical reactions?
  20. Why is it important to balance a chemical equation?

Medium Questions

  1. Explain the Law of Conservation of Mass using an example.
  2. What would happen if the mass of the reactants in a chemical reaction was not equal to the mass of the products?
  3. Why must chemical equations be balanced in terms of both mass and atoms?
  4. What happens to the number of atoms of each element in a chemical reaction according to the Law of Conservation of Mass?
  5. Describe a situation where the Law of Conservation of Mass might appear not to be obeyed and explain why this happens.
  6. How does the Law of Conservation of Mass apply to everyday chemical reactions, such as burning fuel?
  7. In a reaction where 10g of reactant A reacts with 20g of reactant B, the product has a mass of 25g. What might explain the missing 5g?
  8. If 5g of magnesium reacts with 10g of oxygen, how much magnesium oxide is produced?
  9. Why is it necessary to conduct chemical reactions in a closed system to demonstrate the Law of Conservation of Mass?
  10. Write a balanced chemical equation for the reaction of hydrogen with oxygen to form water.
  11. In a chemical reaction, how do we determine the masses of the reactants and products?
  12. What is the significance of using a closed system in demonstrating the Law of Conservation of Mass?
  13. Explain why balancing chemical equations ensures that the Law of Conservation of Mass is obeyed.
  14. What is the mass of products formed when 50g of sodium reacts with chlorine gas?
  15. True or False: In a chemical reaction, the total number of atoms on the reactant side must equal the total number of atoms on the product side.
  16. Why does a balanced chemical equation represent the Law of Conservation of Mass?
  17. Describe the process of balancing a chemical equation.
  18. What happens to the mass of gases involved in a chemical reaction if the system is not closed?
  19. How does the Law of Conservation of Mass relate to the principle of a closed system in a laboratory?
  20. How can the Law of Conservation of Mass be applied to the process of photosynthesis?

Hard Questions

  1. Explain why mass seems to be lost in a chemical reaction involving gases in an open system.
  2. If a metal reacts with oxygen to form a metal oxide and the mass increases, is the Law of Conservation of Mass still obeyed? Explain.
  3. A reaction between 10g of calcium carbonate and 15g of hydrochloric acid produces 20g of products. What happened to the remaining 5g?
  4. Discuss how the Law of Conservation of Mass applies to the decomposition of water into hydrogen and oxygen.
  5. Explain how a balanced chemical equation reflects the Law of Conservation of Mass.
  6. In a chemical reaction involving gases, how can we ensure that the Law of Conservation of Mass is obeyed?
  7. Discuss why balancing a chemical equation is necessary to represent the Law of Conservation of Mass.
  8. In a reaction between 5g of zinc and 10g of hydrochloric acid, hydrogen gas is released. What will be the mass of the products?
  9. Describe an experiment that demonstrates the Law of Conservation of Mass in a closed system.
  10. How does the Law of Conservation of Mass relate to chemical stoichiometry?
  11. Explain why the mass of the products in a combustion reaction is greater than the mass of the fuel alone.
  12. In a reaction between 50g of copper and 20g of oxygen, 65g of copper oxide is formed. What might explain the loss of 5g of mass?
  13. Explain how the Law of Conservation of Mass applies to both chemical and nuclear reactions.
  14. If 100g of reactants form 90g of products, what could account for the missing 10g?
  15. Explain the role of gas production in causing mass discrepancies in reactions conducted in open systems.
  16. Describe how the Law of Conservation of Mass is demonstrated in a chemical equation for the reaction of methane with oxygen.
  17. In a reaction that produces a gas, why must the system be closed to measure mass accurately?
  18. If a chemical reaction in a laboratory appears to lose mass, what steps can be taken to explain this result?
  19. How does the concept of mass conservation apply to reversible reactions?
  20. What is the role of careful measurement in confirming the Law of Conservation of Mass during chemical experiments?

Answers

Easy Questions

  1. Mass cannot be created or destroyed in a chemical reaction.
  2. False.
  3. The mass of the products is equal to the mass of the reactants.
  4. 50g.
  5. The same.
  6. False.
  7. It stays the same.
  8. A substance that takes part in and undergoes change during a chemical reaction.
  9. Products.
  10. True.
  11. 44g.
  12. It means the total mass of the substances before and after the reaction remains the same.
  13. Same.
  14. True.
  15. The mass stays the same.
  16. It shows what happens to the reactants and the products in a chemical reaction.
  17. Some product may have escaped as a gas.
  18. False.
  19. An open system allows gases to escape, which can make the mass appear to change.
  20. To ensure the same number of atoms of each element are on both sides of the equation.

Medium Questions

  1. In the reaction of magnesium burning in oxygen, the mass of magnesium plus the mass of oxygen equals the mass of magnesium oxide produced.
  2. This would violate the Law of Conservation of Mass. Mass is always conserved in a chemical reaction.
  3. Because the mass and number of atoms of each element must remain constant according to the Law of Conservation of Mass.
  4. The number of atoms remains the same but they are rearranged to form new products.
  5. In an open system, gases may escape, making it seem like mass is lost.
  6. When fuel burns, the mass of the fuel and oxygen equals the mass of the gases and other products produced.
  7. Some of the mass may have escaped as a gas.
  8. 15g of magnesium oxide.
  9. To prevent gases from escaping, which ensures the total mass can be measured.
  10. 2H₂ + O₂ → 2H₂O.
  11. **By weighing the reactants and the products carefully.

**

  1. A closed system ensures no matter or gas can escape, so mass measurements are accurate.
  2. Balancing equations ensures that the same number of atoms are on both sides, thus following the Law of Conservation of Mass.
  3. 50g of sodium chloride.
  4. True.
  5. Because it ensures the mass of reactants and products is conserved.
  6. By adding coefficients to ensure the same number of each atom on both sides of the equation.
  7. The gas may escape, making the mass appear to decrease.
  8. In a closed system, gases cannot escape, so the total mass remains constant.
  9. The total mass of the products of photosynthesis equals the mass of carbon dioxide and water used in the process.

Hard Questions

  1. Mass is lost because the gases escape into the atmosphere, making it seem like mass is not conserved.
  2. Yes, the mass includes the oxygen from the air, which has combined with the metal to form the oxide.
  3. The missing mass may have been lost as a gas.
  4. The mass of the water equals the mass of the hydrogen and oxygen produced, demonstrating the Law of Conservation of Mass.
  5. Because the same number of atoms of each element must appear on both sides of the equation.
  6. By conducting the reaction in a closed system to prevent gases from escaping.
  7. Balancing equations ensures the mass of atoms is conserved according to the Law of Conservation of Mass.
  8. 15g (mass of zinc chloride and hydrogen gas combined).
  9. An experiment where a reaction takes place in a sealed container to show the total mass before and after is the same.
  10. Stoichiometry involves calculating the relative quantities of reactants and products in a chemical reaction while respecting the Law of Conservation of Mass.
  11. Because oxygen from the air combines with the fuel, adding to the mass of the products.
  12. Some mass may have been lost as gas or in other forms that were not measured.
  13. Both types of reactions follow the Law of Conservation of Mass because atoms are conserved, even if energy is released in different forms.
  14. Some of the mass may have escaped as a gas.
  15. Gas can escape from an open system, causing the mass to seem lower than it actually is.
  16. CH₄ + 2O₂ → CO₂ + 2H₂O shows that the total mass of methane and oxygen equals the mass of carbon dioxide and water.
  17. To prevent gases from escaping, which could make the mass seem to decrease.
  18. We can ensure the reaction is conducted in a closed system or account for any gases that might have escaped.
  19. Even in reversible reactions, the total mass of the reactants and products is conserved.
  20. Careful measurement ensures no mass is lost and confirms that the Law of Conservation of Mass is obeyed.