Separate a substance change from a change in spacing

In a change of state, such as melting, freezing, boiling or condensing, the particles stay the same and only their spacing, arrangement and movement change. In a chemical change, new substances with different particles form. A particle question often hides this choice inside a short description.

This lesson completes the module on particle models and changes of state. It uses ideas from boiling and evaporation and heating curves.

How do you decide which change has happened?

Ask one question: are the particles at the end the same kind as at the start?

• If yes, the substance is unchanged. Only the spacing, arrangement and movement are different. The change is physical, and it is usually reversible by heating or cooling.
• If no, a new substance has formed. The atoms have been rearranged into particles with a different formula. The change is chemical.

In a diagram, the particle picture tells you. Melting ice gives the same V-shaped H₂O molecules, now free to slide past each other. Electrolysing water gives two kinds of particle that were not there before: H₂ and O₂.

Mass is useful too. Because no particles are lost, the mass of a sealed sample stays the same through a change of state. One mole of water has a mass of 18 g as ice, liquid or steam (2 × 1 + 16 = 18).

Worked example

Two changes are described. The second uses a balanced equation, and the amounts are invented for practice.

Change 1: 36 g of ice at 0 °C is warmed until it becomes steam.

Change 2: The 36 g of water is then split into hydrogen and oxygen by a chemical process, described here only at concept level.

Question: For each change, say whether the particles stay the same, and give the amount and mass of each substance.

Step 1, Change 1. The particles are H₂O before and after. Only the spacing and movement change, so this is a change of state. The mass of steam is still 36 g.

Step 2, amount of water. Molar mass of H₂O = 2 × 1 + 16 = 18 g/mol. Amount = 36 ÷ 18 = 2 mol.

Step 3, Change 2. New particles form, so this is a chemical change. The balanced equation is 2H₂O → 2H₂ + O₂. Check the atoms: hydrogen 4 on the left and 4 on the right, oxygen 2 on the left and 2 on the right.

Step 4, amounts. From the equation, 2 mol H₂O gives 2 mol H₂ and 1 mol O₂.

Step 5, masses. H₂: 2 × 2 = 4 g. O₂: 1 × 32 = 32 g. Total = 4 + 32 = 36 g, which matches the starting mass, so the answer is consistent.

The mass is the same in both changes. What differs is whether the particles were replaced. You can check a balance like this with the equation balance reasoning trainer and the amounts with the mole and equation-ratio tutor.

The mistake to watch for

The typical slip is to draw or describe steam as separate hydrogen and oxygen.

Mistaken answer: “When water boils it breaks into hydrogen and oxygen, which is why there are bubbles.”

Breaking the molecules would need a chemical change. In boiling the molecules stay whole and move apart. The correct reply: “The water molecules stay as H₂O. Only the spacing and movement change.”

The reverse slip also appears. A student writes that a chemical change “only changes the spacing”, when a new substance actually forms.

Check yourself

1. Classify each: (a) wax melting, (b) magnesium burning in air to form magnesium oxide, (c) iodine solid turning directly into gas when warmed.

2. Balance 2Mg + O₂ → 2MgO by counting atoms. Are both sides equal?

3. 9 g of water is boiled away completely. What mass of water vapour forms, and how many moles is this?

Where this leads next

You now have all the lessons for this module. Test them together with the mixed particle practice. The next module moves to atoms, elements and isotopes.

Students often follow the chemistry but hesitate at the question of which change has occurred. That is an area our teachers can rebuild in online one-to-one Chemistry tuition.