Year 10 Chemistry: Conservation of Mass and the Quantitative Interpretation of Chemical Equations

Introduction to Conservation of Mass

The Conservation of Mass is a very important principle in chemistry. It tells us that in any chemical reaction, the total mass of the substances involved does not change. In other words, matter cannot be created or destroyed; it can only change forms.

Key Points:

• Mass is conserved: The mass of the reactants (the starting substances) is equal to the mass of the products (the substances formed).
• Chemical equations: We use chemical equations to represent reactions. These equations must be balanced to show that mass is conserved.

Understanding Chemical Equations

A chemical equation uses symbols to show what happens during a reaction. For example, if we mix hydrogen gas (H₂) with oxygen gas (O₂), they react to form water (H₂O). The equation for this reaction is:

$$

2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O}

$$

Breaking it Down:

• Reactants: The substances on the left side (2 H₂ and O₂).
• Products: The substances on the right side (2 H₂O).
• Balanced Equation: The number of atoms for each element is the same on both sides.

Example of Conservation of Mass

Let’s say we have:

• 4 grams of hydrogen
• 32 grams of oxygen

When they react, we get 36 grams of water. Notice how the total mass before the reaction (4 g + 32 g = 36 g) equals the total mass after the reaction (36 g). That’s conservation of mass!

Tips and Tricks for Balancing Equations

1. Count Atoms: Before you start balancing, count the number of atoms of each element in the reactants and products.
2. Use Coefficients: Adjust the numbers in front of the chemical formulas (these are called coefficients) to balance the atoms.
3. Start with Complicated Molecules: If a compound has more than one element, balance it first before the simpler ones.
4. Check Your Work: After balancing, make sure the numbers of each type of atom are the same on both sides.

Practice Questions

Easy Level Questions

1. What does the Conservation of Mass state?
2. Write the equation for the reaction of hydrogen and oxygen to form water.
3. Name the reactants in the equation: $$2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O}$$ .
4. How many grams of water are produced from 4 grams of hydrogen and 32 grams of oxygen?
5. What is a chemical equation?
6. True or False: Matter can be created in a chemical reaction.
7. How many oxygen atoms are on the left side of the equation $$2 \text{H}_2 + \text{O}_2$$ ?
8. What do we call the numbers in front of the chemical formulas?
9. If you have 10 grams of reactants, how many grams of products will there be?
10. What is the product when magnesium reacts with oxygen?

Medium Level Questions

11. Balance the equation: $$\text{C} + \text{O}_2 \rightarrow \text{CO}_2$$ .
12. What happens to the mass of substances in a closed system during a reaction?
13. Write a balanced equation for the combustion of propane ( $$\text{C}_3\text{H}_8$$ ).
14. How do you check if a chemical equation is balanced?
15. If 5g of A reacts with 10g of B to give 12g of C, what is the mass of the unreacted substance?
16. Name one real-life example of the Conservation of Mass.
17. In the reaction $$\text{2H}_2 + \text{O}_2 \rightarrow 2H_2O$$ , how many hydrogen atoms are there in total on the left side?
18. How can you tell if a chemical equation is not balanced?
19. What is the significance of balancing chemical equations?
20. How many molecules of water are produced from 2 molecules of hydrogen and 1 molecule of oxygen?

Hard Level Questions

21. If the balanced equation is $$2 \text{Na} + \text{Cl}_2 \rightarrow 2 \text{NaCl}$$ , how many sodium atoms are on each side?
22. What is the total mass of products if 10g of A reacts with 15g of B and forms 20g of C and 5g of D?
23. Explain why it is important to balance chemical equations in terms of conservation of mass.
24. Write the balanced equation for the reaction: $$\text{Al} + \text{O}_2 \rightarrow \text{Al}_2\text{O}_3$$ .
25. How would you explain the importance of the Conservation of Mass to someone who doesn’t know about chemistry?
26. If you start with 20g of reactants and produce 15g of products, what happened to the remaining mass?
27. How do you balance the equation $$\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$$ ?
28. What is the molar mass of water ( $$\text{H}_2\text{O}$$ )?
29. Describe a scenario where the Conservation of Mass might be observed in a laboratory setting.
30. In a chemical reaction, if the mass of the reactants is greater than the mass of the products, what might explain this discrepancy?

Answers and Explanations

Easy Level Answers

1. The total mass of reactants equals the total mass of products.
2. $$2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O}$$ .
3. $$\text{H}_2$$ and $$\text{O}_2$$ .
4. 36 grams of water.
5. A way to represent chemical reactions using symbols.
6. False.
7. 2 oxygen atoms.
8. Coefficients.
9. 10 grams of products.
10. Magnesium oxide.

Medium Level Answers

11. $$\text{C} + \text{O}_2 \rightarrow \text{CO}_2$$ (already balanced).
12. The total mass remains the same.
13. $$\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$$ .
14. Count the atoms of each element on both sides.
15. 3 grams of unreacted substance.
16. Burning wood (the mass of ash is less than the original wood).
17. 4 hydrogen atoms.
18. If the number of each type of atom differs on both sides.
19. It shows that matter is conserved during reactions.
20. 2 molecules of water.

Hard Level Answers

21. 2 sodium atoms on each side.
22. 25 grams of products.
23. To ensure that the mass before and after a reaction remains equal.
24. $$4 \text{Al} + 3 \text{O}_2 \rightarrow 2 \text{Al}_2\text{O}_3$$ .
25. It explains that matter is never lost in reactions, only changed.
26. It might have evaporated or escaped.
27. $$4 \text{Fe} + 3 \text{O}_2 \rightarrow 2 \text{Fe}_2\text{O}_3$$ .
28. 18 g/mol.
29. Mixing vinegar and baking soda (produces gas).
30. There might have been a gas produced that escaped into the air.

By understanding the Conservation of Mass and how to balance equations, you will have a strong foundation in chemistry that will help you as you learn more about this fascinating subject!