Particles in Physical and Chemical Changes

Introduction

From the outside, physical and chemical changes can look alike: both may involve heating, colour changes or gases. The real difference is hidden at the level of particles. In a physical change, whole particles move around. In a chemical change, the particles themselves are taken apart and rebuilt. Thinking at this level turns the new-substance test into something you can picture and explain.

Core explanation

Two kinds of force. In a molecular substance such as water, there are two kinds of attraction:

- Chemical bonds inside each molecule. These are strong and hold the atoms together. In H₂O, two hydrogen atoms are bonded to one oxygen atom. - Intermolecular forces between separate molecules. These are much weaker and hold the molecules close to one another in a solid or liquid.

Physical changes: whole molecules move. In a change of state or in dissolving, only the weak forces between particles are overcome or re-formed. The molecules move further apart or closer together, but each molecule stays intact. Water molecules in steam are exactly the same H₂O molecules that were in the ice. The substance keeps its identity because its particles are unchanged.

Chemical changes: atoms are rearranged. In a chemical reaction, the strong bonds inside the reactant particles break, and new bonds form between atoms in different combinations. The particles of the product are different from those of the reactants. For example, when hydrogen burns:

hydrogen + oxygen → water

Hydrogen molecules (H₂) and oxygen molecules (O₂) break apart, and the atoms join as H₂O molecules. The same hydrogen and oxygen atoms are present, but they now form a new substance.

Atoms are conserved. In both kinds of change, no atoms are created or destroyed. In a physical change the molecules are conserved too; in a chemical change the atoms are conserved but the molecules are not.

Comparing the two.

Feature Physical change Chemical change --- --- --- What moves Whole particles Atoms between particles Forces affected Weak forces between particles Strong bonds within particles Particles after Same as before Different from before New substance No Yes Energy change Usually smaller Often larger

Energy scale. Because chemical bonds are strong, breaking and making them usually involves more energy than overcoming intermolecular forces. This is why burning fuels releases far more energy than, say, condensing steam of similar mass.

Step-by-step reasoning

To classify a change from a particle diagram:

1. Look at the particles before: what atoms are in each, and how are they joined? 2. Look at the particles after. 3. If every particle has the same atoms joined in the same way, only positions changed: physical. 4. If atoms are joined to different partners, new particles exist: chemical.

Visual explanation

Draw water as a red circle with two small white circles attached. For boiling, show the same three-circle molecules first packed closely, then spread far apart. For electrolysis, show the molecules split, with white circles now paired as H₂ and red circles paired as O₂. Only the second picture shows new particles.

Real-world analogy

A box of assembled construction-toy cars is a good model. Driving the cars into a different formation is a physical change: every car is intact. Taking the cars apart and using the pieces to build aeroplanes is a chemical change: the same pieces now make different objects.

Real-world example

Water can be split into hydrogen and oxygen by electrolysis, using electricity. This is a chemical change: the bonds in H₂O molecules are broken and new H₂ and O₂ molecules form. Boiling the same water uses far less energy per gram and only separates the molecules. Hydrogen produced by electrolysis is being developed as a fuel for vehicles.

Why?

Why is less energy needed to boil water than to split it into hydrogen and oxygen? Boiling only overcomes weak forces between molecules, while splitting water must break the strong covalent bonds inside each molecule.

Common misconception

"When water boils, the molecules break into hydrogen and oxygen atoms." Boiling does not break bonds inside the molecules. Steam is made of whole H₂O molecules. If the molecules split, the gas would be a mixture of hydrogen and oxygen, which it is not.

Worked example

Question: A diagram shows pairs of joined grey circles (N₂) and pairs of white circles (H₂) before a change, and groups of one grey circle joined to three white circles after. Is this physical or chemical?

Reasoning: After the change, the atoms are joined to different partners, forming new particles (NH₃, ammonia) that were not present before.

Answer: Chemical change, because bonds have broken and new bonds have formed to make a new substance.

Quick check

1. In a physical change, are the bonds within molecules broken? Answer: No, only the weaker forces between molecules are overcome or formed.

Exam focus

Examiners often show particle diagrams and ask you to identify the change. Look at whether atoms have new partners. Use "bonds are broken and new bonds are formed" for chemical changes and "particles only move apart or closer together" for physical ones.

Advanced insight

For substances made of giant structures, such as metals and ionic compounds, there are no separate molecules. Melting such a substance overcomes strong forces throughout the structure, which is why their melting points are high. The distinction still holds: melting leaves the same atoms or ions in the same substance, while a reaction produces a different combination.

Summary

Physical changes overcome or form weak forces between particles; the particles themselves stay intact, so no new substance forms. Chemical changes break strong bonds within particles and form new bonds, rearranging atoms into new particles. Atoms are conserved in both, but only in a physical change are the molecules conserved as well.

Practice questions

1. What is the difference between a chemical bond and an intermolecular force? Answer: A chemical bond is a strong force holding atoms together within a particle; an intermolecular force is a weak attraction between separate molecules. 2. Describe what happens to water molecules when ice melts. Answer: They gain energy, overcome some of the forces between them and slide past each other, but each molecule stays intact as H₂O. 3. Explain, in terms of atoms, why burning methane is a chemical change. Answer: Bonds in methane and oxygen molecules break and the atoms rejoin as carbon dioxide and water molecules, which are new substances. 4. Why do chemical changes usually involve larger energy changes than changes of state? Answer: They involve breaking and making strong chemical bonds rather than only overcoming weak forces between particles.