Transformations

Level 1

What is a transformation?

A transformation is the passage of a system from an initial state to a final state. By "state", we mean all the observable characteristics of an object:

  • its temperature (hot, cold),
  • its position (top, bottom),
  • its volume (large, small),
  • its mass (heavy, light),
  • its shape (round, square),
  • its physical state (solid, liquid, gas).

  Example: An ice cube melting, water heating up, or a ball moving are transformations.

The two main families of transformations

Physical transformation

A physical transformation is a transformation that does not change the nature of matter. Molecules and atoms remain identical, only the appearance changes.

  Examples:

  • Melting ice: H2O solid → H2O liquid
  • Dissolving salt in water: the salt is still present, dissolved in water
  • The air in a balloon expanding when heated

Chemical transformation

A chemical transformation is a transformation that changes the nature of matter. The molecules of the reactants disappear and new molecules (the products) appear.

  Examples:

  • Combustion of paper: paper + dioxygen → ash + smoke
  • Cooking an egg: proteins permanently change structure
  • Rusting: iron + dioxygen → iron oxide

How to distinguish them?

  • Can we easily go back?
    Yes → probably physical / No → probably chemical
  • Do new substances appear?
    Yes → chemical / No → physical

Changes of state (physical)

Changes of state are physical transformations: melting, solidification, vaporisation, condensation, sublimation. The molecules remain the same; only their arrangement changes. It is therefore a physical transformation

  Example: Liquid water → water vapour (vaporisation) when water boils.

Dissolution, expansion, deformation

Dissolution: a solid spreads out in a liquid. The solid is still present but dispersed (reversible by evaporation).
Expansion / contraction: volume change linked to temperature (rails, thermometers).
Deformation: the shape is changed (bending, stretching) without changing the nature.

In all these situations, the molecules remain the same; only their arrangement changes. These are physical transformations.

Signs of a chemical transformation

Signs that often indicate a chemical transformation:

  • Gas formation: appearance of bubbles that are not due to boiling (effervescence).
  • Appearance of a solid (precipitate): a solid forms within a solution that was clear.
  • Permanent colour change: the colour changes and does not revert (to be distinguished from a simple mixture of shades).
  • Significant temperature change: the reaction releases heat (exothermic) or absorbs heat (endothermic).
  • Light emission: certain reactions, such as combustion, produce light.

  Example: 
Vinegar + baking soda → bubbles (CO2): sign of a chemical transformation because a new substance is formed