Chemistry: chemical reactions and conservation of mass (grade 10) · lesson

Mass is conserved

💡 Atoms are rearranged, never created or destroyed, so total mass stays the same.

In a sealed container, the mass of the products equals the mass of the reactants. 4.0 g of hydrogen and 32.0 g of oxygen make 36 g of water.

In an open container, mass can seem to drop. A gas forms and escapes into the air, so the scale no longer counts it. A burning candle loses mass for this reason.

🧠 Let's work it out

If a scale reading drops, ask: did a gas leave?
If the container were sealed, would the reading stay the same?
In a sealed bag, 10.0 g of reactants make 10.0 g of products.

What do you think comes next?

conservation of mass: the total mass stays the same in a reaction

🔗 Khan Academy: Chemical reactions · Khan Academy (free) ↗

Balancing equations

💡 A balanced equation has the same number of each kind of atom on both sides.

Coefficients are the big numbers in front of formulas. They count molecules. Subscripts are the small numbers inside a formula. Change coefficients to balance, never subscripts.

To count atoms, multiply the coefficient by the subscript. 2 O₂ has 2 × 2 = 4 oxygen atoms. Molecules can change in number; atoms cannot.

🧠 Let's work it out

Balance one element at a time, saving oxygen (or the element in the most places) for last.
For C₃H₈ + O₂: 3 C → 3 CO₂; 8 H → 4 H₂O; now oxygen on the right is 6 + 4 = 10, so 5 O₂.

What do you think comes next?

coefficient: the number in front of a formula
subscript: the small number after an atom in a formula

🔗 PhET: Balancing Chemical Equations · PhET Interactive Simulations (free) ↗

Signs and types of reactions

💡 A chemical change makes a new substance, and decomposition breaks one substance into simpler ones.

Signs of a chemical change include a new gas, a color change, a new solid, or heat or light given off. Melting, dissolving, and tearing are physical changes: the substance stays the same.

In synthesis, simple substances join into one. In decomposition, one compound breaks into simpler substances, like water splitting into hydrogen and oxygen.

🧠 How to think about it

Ask: is the substance at the end different from the one at the start? If it is still water or still sugar, the change was physical.

synthesis: two or more substances join into one
decomposition: one substance breaks into simpler ones

Why atoms combine in set ratios

💡 Electron transfer sets the ratio: atoms combine so electrons lost equal electrons gained.

Sodium has 1 valence electron to lose. Chlorine needs 1 to fill its outer shell. So they combine one to one, as NaCl.

Coefficients count atoms or molecules. They do not tell how reactive something is.

🧠 How to think about it

Match electrons given with electrons taken. If one atom gives 1 and the other takes 1, the ratio is 1 : 1.