Mass Conservation: Basic Laws of Chemistry - Chemistry (Grade 10)

The System Keeps Its Mass

When a chemical reaction happens, atoms do not vanish or appear from nothing. They are rearranged into new substances. That is why the mass of the whole system before a reaction equals the mass of the whole system after the reaction.

m_{\text{system before}} = m_{\text{system after}}

The key word is system. In a closed container, every substance remains in the part being weighed. In an open container, some gas may leave, so the balance reading can appear to change even though the total mass of the substances is still conserved. OpenStax Chemistry 2e describes conservation of matter as no detectable change in the total amount of matter during chemical or physical changes in openstax.org.

Lavoisier's Balance

Before modern chemistry, combustion was often explained with phlogiston, as if something left a material when it burned. Antoine Lavoisier challenged that idea through careful weighing. He tracked the mass of reactants and products, including gases, so combustion could be read as a reaction with oxygen, not as the loss of a mysterious substance.

Science History Institute summarizes Lavoisier's role in establishing the law of conservation of mass and connecting combustion with oxygen in sciencehistory.org.

Closed-System Balance

Switch between a closed and an open system. The correct law always counts every substance, not only the part that is still visible inside the container.

Closed-System Balance

Compare what the balance reads when all substances stay in the container and when some substance leaves the system boundary.

In a closed system, products and leftover reactants stay inside the part being weighed.

What is counted: The container, products, and leftover reactants are one system.
Mass reading: $10.00 + 2.00 = 6.08 + 5.92$ , so the total mass stays $12.00\ \mathrm{g}$ .

Zinc and Sulfur

A student heats $10.00\ \mathrm{g}$ of zinc with $2.00\ \mathrm{g}$ of sulfur. After the reaction, $6.08\ \mathrm{g}$ of zinc sulfide forms and $5.92\ \mathrm{g}$ of zinc remains unreacted.

The simplified equation is:

\mathrm{Zn(s)} + \mathrm{S(s)} \rightarrow \mathrm{ZnS(s)}

The product mass does not have to equal the mass of every starting substance if one reactant is left over. First count only the zinc that actually reacted.

\begin{aligned} m_{\mathrm{Zn\ reacted}} &= 10.00 - 5.92 = 4.08\ \mathrm{g} \\ m_{\mathrm{reactants\ reacted}} &= 4.08 + 2.00 = 6.08\ \mathrm{g} \\ m_{\mathrm{product}} &= 6.08\ \mathrm{g} \end{aligned}

So mass conservation is confirmed for the part that actually reacts: the mass of zinc that reacts plus the mass of sulfur equals the mass of zinc sulfide formed.

If the whole container is counted, the mass is also unchanged:

\begin{aligned} m_{\mathrm{before}} &= 10.00 + 2.00 = 12.00\ \mathrm{g} \\ m_{\mathrm{after}} &= 6.08 + 5.92 = 12.00\ \mathrm{g} \end{aligned}

Reading Mass Carefully

Keep three questions separate.

What is the product mass? Only the new product counts, so it is $6.08\ \mathrm{g}$ of zinc sulfide.
Is the system mass conserved? Yes, when the product and leftover reactant are counted together.
Can the reading appear lower? Yes, if gas leaves an open container and is not weighed.

Britannica explains that chemical equations must be balanced because the same number of atoms of each element appears on both sides, and mass calculations follow conservation of matter in britannica.com.

Once total mass is protected, the next question becomes sharper: if two elements form the same compound, does their mass ratio stay fixed? That leads directly to the law of constant composition.

The System Keeps Its Mass

m_{\text{system before}} = m_{\text{system after}}

Lavoisier's Balance

Science History Institute summarizes Lavoisier's role in establishing the law of conservation of mass and connecting combustion with oxygen in sciencehistory.org.

Closed-System Balance

Switch between a closed and an open system. The correct law always counts every substance, not only the part that is still visible inside the container.

Closed-System Balance

Compare what the balance reads when all substances stay in the container and when some substance leaves the system boundary.

In a closed system, products and leftover reactants stay inside the part being weighed.

What is counted: The container, products, and leftover reactants are one system.
Mass reading: $10.00 + 2.00 = 6.08 + 5.92$ , so the total mass stays $12.00\ \mathrm{g}$ .

Zinc and Sulfur

A student heats $10.00\ \mathrm{g}$ of zinc with $2.00\ \mathrm{g}$ of sulfur. After the reaction, $6.08\ \mathrm{g}$ of zinc sulfide forms and $5.92\ \mathrm{g}$ of zinc remains unreacted.

The simplified equation is:

\mathrm{Zn(s)} + \mathrm{S(s)} \rightarrow \mathrm{ZnS(s)}

The product mass does not have to equal the mass of every starting substance if one reactant is left over. First count only the zinc that actually reacted.

\begin{aligned} m_{\mathrm{Zn\ reacted}} &= 10.00 - 5.92 = 4.08\ \mathrm{g} \\ m_{\mathrm{reactants\ reacted}} &= 4.08 + 2.00 = 6.08\ \mathrm{g} \\ m_{\mathrm{product}} &= 6.08\ \mathrm{g} \end{aligned}

So mass conservation is confirmed for the part that actually reacts: the mass of zinc that reacts plus the mass of sulfur equals the mass of zinc sulfide formed.

If the whole container is counted, the mass is also unchanged:

\begin{aligned} m_{\mathrm{before}} &= 10.00 + 2.00 = 12.00\ \mathrm{g} \\ m_{\mathrm{after}} &= 6.08 + 5.92 = 12.00\ \mathrm{g} \end{aligned}

Reading Mass Carefully

Keep three questions separate.

What is the product mass? Only the new product counts, so it is $6.08\ \mathrm{g}$ of zinc sulfide.
Is the system mass conserved? Yes, when the product and leftover reactant are counted together.
Can the reading appear lower? Yes, if gas leaves an open container and is not weighed.

Once total mass is protected, the next question becomes sharper: if two elements form the same compound, does their mass ratio stay fixed? That leads directly to the law of constant composition.

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