Topic 4.1 Notes – Introduction for Reactions

1. Physical Changes vs Chemical Changes

What a Physical Change Is

A physical change alters form or appearance but keeps the chemical composition the same. The particles are still the same atoms bonded in the same way.

Common examples:

• Phase changes
  $\ce{H2O(s) -> H2O(l) -> H2O(g)}$
  Ice, water, and steam are all still $\ce{H2O}$.
• Dissolving (forming mixtures)
  Salt dissolving in water separates ions, but it’s still $\ce{NaCl}$ chemically.
• Cutting, crushing, grinding
  Smaller pieces, same substance.

No new substance forms. Many physical changes are reversible by physical means (like freezing or evaporating), but reversibility alone does not define them. The key is composition does not change.

What a Chemical Change Is

A chemical change produces one or more new substances with different compositions.

At the particle level:

• Bonds break
• Atoms rearrange
• New bonds form

For example:

$$\ce{2Mg(s) + O2(g) -> 2MgO(s)}$$

Magnesium and oxygen become magnesium oxide. Different formula, different properties.

Evidence of a Chemical Change

These are clues. One by itself is suggestive, not absolute proof.

• Temperature change (heat absorbed or released)
• Light produced (like burning magnesium)
• Gas formation (bubbles not from boiling)
• Precipitate formation (solid forms from two solutions)
• Color change (not just dilution)

On tests, they love giving you lab observations and asking whether a chemical reaction occurred. Always ask yourself: Did the composition change?

Physical vs Chemical Change Comparison

If identity changes, it’s chemical. If identity stays the same, it’s physical.

2. What a Chemical Reaction Is

A chemical reaction is the process that causes a chemical change. Reactants become products through rearrangement of atoms.

Two core ideas:

• Atoms are conserved (law of conservation of mass).
• Matter is reorganized, not created or destroyed.

When you balance equations later in this unit, you’re enforcing conservation of atoms. If 4 oxygen atoms start on the left, 4 must end on the right.

Chemical reactions are about rearrangement, not disappearance.

3. How to Read a Chemical Equation

Take this example:

$$\ce{2Al(s) + 3Cl2(g) -> 2AlCl3(s)}$$

Parts of a chemical equation

Each piece of the equation has a specific meaning.

Break it down:

• Reactants are on the left.
• Products are on the right.
• The arrow (→) means “yields.”
• Coefficients (big numbers in front) show relative amounts and are adjusted to balance.
• Subscripts (small numbers in formulas) define the substance itself.
  Changing a subscript changes the compound.

Students often try to balance by changing subscripts. That changes the identity. Only coefficients can change.

4. The Five Main Types of Chemical Reactions

These are patterns. Recognizing them helps you predict products.

a. Synthesis (Combination)

General form:
$$\ce{A + B -> AB}$$

Two or more substances combine into one product.

Example:
$\ce{CaO + CO2 -> CaCO3}$

Pattern: many → one.

b. Decomposition

General form:
$$\ce{AB -> A + B}$$

One compound breaks into simpler substances.

Example:
$\ce{2KClO3 -> 2KCl + 3O2}$

Pattern: one → many.

c. Combustion

For hydrocarbons:

$$\ce{C_xH_y + O2 -> CO2 + H2O}$$

Must include $\ce{O2}$ as a reactant.

Example:
$\ce{2C2H6 + 7O2 -> 4CO2 + 6H2O}$

If you see a carbon-hydrogen compound reacting with oxygen, think combustion immediately.

d. Single Replacement

General form:
$$\ce{AB + C -> AC + B}$$

One element replaces another.

Example:
$\ce{Fe + CuSO4 -> FeSO4 + Cu}$

Often involves electron transfer. One switch happens.

e. Double Replacement

General form:
$$\ce{AB + CD -> AD + CB}$$

Two ionic compounds exchange partners.

Example:
$\ce{Pb(NO3)2 + 2KI -> PbI2(s) + 2KNO3}$

Often forms:

• A precipitate
• Water (acid-base)
• A gas

Two switches happen.

5. Connecting Evidence to Reaction Types

When lab observations show:

• Solid forms from two solutions → usually double replacement.
• Metal reacts with acid and bubbles form → often single replacement.
• Hydrocarbon burns in oxygen → combustion.
• One compound breaks apart when heated → decomposition.
• Two elements combine → synthesis.

On quizzes, they’ll describe what you see in a beaker. Translate observation → composition change → reaction type.