Grade 9

Intermediate

5 min

Lesson 3 of 12

The Law of Conservation of Mass

Not Started

Understand the fundamental principle that matter is neither created nor destroyed during a chemical reaction.

If you burn a 5kg log of wood, you are left with only a small pile of ash weighing less than 1kg. Where did the rest of the matter go? Did it simply vanish from existence, or is it just hiding?

Learning Objectives

1.Explain the Law of Conservation of Mass in terms of atoms and total mass.
2.Calculate the mass of a missing reactant or product using experimental data.
3.Analyze why a reaction in an open container might appear to lose or gain mass.

The Golden Rule of Chemistry

In the late 1700s, chemist Antoine Lavoisier discovered a fundamental truth: matter is neither created nor destroyed during a chemical reaction. This is the Law of Conservation of Mass. Think of atoms like LEGO bricks; you can take a castle apart and build a spaceship, but you still have the exact same number of bricks. In a chemical reaction, the reactants (starting materials) break their bonds and rearrange to form products (new substances). Because no atoms are created or destroyed, the total mass at the start must equal the total mass at the end. Mathematically, we express this as:

Mass_{total\ reactants} = Mass_{total\ products}

If you react $10g$ of substance A with $5g$ of substance B in a sealed container, what will be the total mass of the products formed?

1. Identify the total mass of reactants: $10g + 5g = 15g$ . 2. Apply the Law: Since mass is conserved, the product mass must match the reactant mass. 3. Result: The total mass of the products is $15g$ .

Quick Check

If a reaction starts with $50g$ of total reactants and produces $30g$ of a solid product and some gas, what must be the mass of the gas produced?

Subtract the known product mass from the total starting mass.

Answer

20g

Atoms: The Unchanging Building Blocks

Why does the mass stay the same? It’s all about the atoms. During a reaction, the chemical bonds between atoms break and new bonds form. However, the identity and number of atoms remain constant. If you start with 4 atoms of Hydrogen and 2 atoms of Oxygen as reactants, you will end with exactly 4 atoms of Hydrogen and 2 atoms of Oxygen in the products, even if they are now bonded together as water ( $H_2O$ ). This is why we balance chemical equations: to ensure the number of atoms on the left side ( $reactants$ ) matches the number of atoms on the right side ( $products$ ).

In the reaction

2Mg + O_2 \rightarrow 2MgO

, if

48.6g

of Magnesium (

Mg

) reacts completely to produce

80.6g

of Magnesium Oxide (

MgO

), how much Oxygen (

O_2

) was consumed?

1. Set up the equation: $Mass_{Mg} + Mass_{O_2} = Mass_{MgO}$ 2. Plug in the known values: $48.6g + x = 80.6g$ 3. Solve for $x$ : $x = 80.6g - 48.6g$ 4. Result: $32.0g$ of Oxygen was used.

Quick Check

True or False: In a chemical reaction, new atoms are created to form the new products.

Atoms are only rearranged, never created or destroyed.

Answer

False

The Mystery of 'Disappearing' Mass

Sometimes, it looks like the law is being broken. If you bake a cake or burn a candle on a scale, the mass seems to decrease. This usually happens in an open system, where matter can escape to the surroundings. Most often, the 'missing' mass is actually a gas (like $CO_2$ or water vapor) that floated away into the air. If you were to perform the same reaction in a closed system (like a sealed flask), you would see that the mass stays exactly the same. The gas is still there; you just can't see it easily!

A student places a beaker containing $100g$ of hydrochloric acid on a scale. They add $10g$ of calcium carbonate marble chips. The mixture fizzes violently. After the fizzing stops, the scale reads $106g$ . Explain this result.

1. Calculate expected mass: $100g (acid) + 10g (chips) = 110g$ . 2. Compare to actual mass: $110g (expected) - 106g (actual) = 4g$ difference. 3. Analyze the 'loss': The reaction produced Carbon Dioxide gas ( $CO_2$ ). 4. Conclusion: $4g$ of $CO_2$ escaped into the atmosphere because the beaker was an open system.

Key Takeaways

1The Law of Conservation of Mass states that the total mass of reactants always equals the total mass of products.
2Chemical reactions involve the rearrangement of atoms, but no atoms are created or destroyed.
3Apparent changes in mass in 'open systems' are usually due to gases entering or leaving the reaction container.
4To find a missing mass, use the formula: $Mass_{total\ in} = Mass_{total\ out}$ .

Quick Check

Multiple Choice

Who is the scientist credited with establishing the Law of Conservation of Mass?

Multiple Choice

If $12g$ of Carbon reacts with $32g$ of Oxygen to form Carbon Dioxide, what is the mass of the Carbon Dioxide produced?

True/False

In a closed system, the mass of the contents will change if a gas is produced.

What's Next?

Review Tomorrow

In 24 hours, try to explain to a friend why a burnt match weighs less than an unburnt match, and where that 'lost' mass actually went.

Practice Activity

Look at a balanced chemical equation (like $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ ) and count the atoms of each element on both sides to verify they are equal.

Browse

Browse

The Law of Conservation of Mass

Learning Objectives

The Golden Rule of Chemistry

Atoms: The Unchanging Building Blocks

The Mystery of 'Disappearing' Mass

Key Takeaways

Quick Check

What's Next?

The Law of Conservation of Mass

Learning Objectives

The Golden Rule of Chemistry

Atoms: The Unchanging Building Blocks

The Mystery of 'Disappearing' Mass

Key Takeaways

Quick Check

What's Next?