What Does It Mean for Pure Substances to Combine?
You already know that matter is made of elements — pure substances, each made of only one type of atom, that cannot be broken down into simpler substances by ordinary chemical means. But what happens when those pure substances meet each other? Sometimes they simply mix together, and sometimes something far more dramatic happens: they chemically react and form an entirely new substance.
Understanding how pure substances combine is at the heart of chemistry. It explains why iron goes rusty, why baking soda fizzes in vinegar, and how your body digests food.
A process in which chemical bonds between atoms are broken or formed, producing new substances (products) that are chemically different from the starting substances (reactants).
The starting substances that enter a chemical reaction. They are written on the left-hand side of a word equation.
The new substances formed as a result of a chemical reaction. They are written on the right-hand side of a word equation.
Think of atoms as LEGO bricks. When you take apart one model (breaking bonds) and rebuild the bricks into a completely different shape (forming new bonds), you've done something like a chemical reaction — you started with the same pieces but ended up with something new.
Physical Changes vs. Chemical Changes
Not every change you observe in matter is a chemical reaction. Scientists make an important distinction between physical changes and chemical changes.
A change in which no new substance is formed. The substance may change shape, size, or state, but its chemical identity stays the same. Physical changes are usually reversible.
A change in which one or more new substances are formed, with different chemical properties from the original substances. Chemical changes are usually irreversible.
Examples to compare:
| Change | Physical or Chemical? | Why? |
|---|---|---|
| Ice melting into water | Physical | Still H₂O — just a different state |
| Paper being cut | Physical | No new substance formed |
| Iron rusting | Chemical | New substance (iron oxide) formed |
| Baking a cake | Chemical | New substances formed; can't 'unbake' it |
| Salt dissolving in water | Physical | Salt can be recovered by evaporation |
A key test: can you get back to your original substance using a simple physical method (like cooling, filtering, or evaporating)? If yes, it's probably a physical change. If you'd need a chemical reaction to reverse it, it's a chemical change.
Dissolving is a physical change, not a chemical one. When salt dissolves in water, the salt and water are not chemically bonded — they are simply mixed. You can get the salt back by evaporating the water.
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