What Are Chemical Reactions?
Chemistry is all about change. When substances interact and transform into something entirely new, we call this a chemical reaction. Chemical reactions are happening all around you, every single day — from the food your body breaks down during digestion, to the fuel that powers a car engine, to the rust forming on a metal fence.
A process in which one or more substances (the reactants) are transformed into one or more new substances (the products). The new substances have different properties from the original ones.
In MYP Sciences, chemical reactions are a perfect example of the key concept of Change — specifically, how matter is transformed. The related concept of Transformation is at the heart of everything we study in this unit: how and why matter changes from one form into another.
We represent chemical reactions using word equations. The reactants are always written on the left-hand side, and the products are always written on the right-hand side, with an arrow pointing from reactants to products:
Think of a chemical reaction like a recipe. You start with ingredients (reactants), follow a process, and end up with a finished dish (products) — something completely different from what you began with!
Physical Changes vs Chemical Reactions
Not every change you observe is a chemical reaction. It is really important to distinguish between a physical change and a chemical reaction.
A change in which no new substances are formed. The substance may look different, but it is still the same material. Physical changes are usually easily reversed.
A change in which new substances are formed with different properties from the original reactants. Chemical changes are usually difficult or impossible to reverse.
Here is a quick comparison:
| Feature | Physical Change | Chemical Reaction |
|---|---|---|
| New substances formed? | ❌ No | ✅ Yes |
| Easily reversed? | Usually yes | Usually no |
| Example | Ice melting into water | Magnesium burning to form magnesium oxide |
| Example | Sugar dissolving in water | Copper carbonate decomposing when heated |
Is it physical or chemical?
- Melting chocolate → Physical change (it's still chocolate; cool it down and it solidifies again)
- Burning wood → Chemical reaction (ash and smoke are new substances; you cannot get the wood back)
- Dissolving salt in water → Physical change (evaporate the water and you get the salt back)
- Iron rusting → Chemical reaction (iron oxide is a new substance with different properties from iron)
A common trap: dissolving looks dramatic, but it is a physical change — no new substance is formed. Burning, on the other hand, always creates new substances and is always a chemical reaction.
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