GED Science
Chemical Reactions & pH
In a chemical reaction, atoms swap partners to form new substances — but no atom is ever created or destroyed, so both sides of the equation must carry identical atom counts. pH is a separate 0–14 ruler measuring how acidic or basic a solution is.
The words first
- reactants → products
- what you start with → what you end with
- conservation of mass
- atoms in = atoms out, always
- coefficient vs subscript
- 2H₂O: the 2 in front counts molecules; the small ₂ counts atoms inside — you may change coefficients, NEVER subscripts
- acid / base / neutral
- pH below 7 / above 7 / exactly 7
The method
COUNTING ATOMS (the whole skill)
2H₂O means two water molecules:
H: 2 × 2 = 4 O: 2 × 1 = 2
CHECKING A BALANCE: CH₄ + 2O₂ → CO₂ + 2H₂O
left: C=1 H=4 O=4
right: C=1 H=4 O=2+2=4 ✓ balanced
SIGNS A CHEMICAL CHANGE HAPPENED
gas bubbles · color change · heat/light given off ·
a solid appearing in a liquid
(melting and dissolving are PHYSICAL — same substance still)
THE pH RULER
0 ──── 7 ──── 14
acid neutral base
lemon 2 · water 7 · soap 10 · bleach 13
acid + base mixed → they neutralize toward 7
Worked all the way through
The problem
Is this equation balanced: N₂ + 3H₂ → 2NH₃?
- Step 1 — Count the left side.N₂ gives 2 N. 3H₂ gives 3 × 2 = 6 H.
- Step 2 — Count the right side.2NH₃: N = 2 × 1 = 2. H = 2 × 3 = 6.
- Step 3 — Compare.N: 2 = 2 ✓. H: 6 = 6 ✓. Balanced — every atom that went in comes out.
On the board
left: N = 2, H = 6
coefficient × subscript
right: N = 2, H = 6
both columns match
balanced ✓
mass conserved
Answer
Yes — 2 nitrogens and 6 hydrogens on each side
A second one, in a different form
A gardener's soil tests at pH 4.5, too acidic for tomatoes. Should she add lime (a base) or vinegar (an acid)?
current: 4.5 — acid side of 7
goal: move toward neutral
raise the pH
adding a base neutralizes acid
lime ✓ (vinegar would lower it more)
Answer
Lime — a base raises the pH toward neutral
The usual mistake
Treating a one-unit pH difference as small. It is ten times the acidity.
Check yourself
Count atoms with coefficient × subscript, one element at a time, in two tidy columns. And when balancing yourself, only the big front numbers may change — touch a subscript and you have invented a different chemical.
Practice it
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