Why does dissolved salt in water conduct electricity, but sugar does not?
Key idea
The Law of Conservation of Mass
Vinegar and baking soda react in a sealed balloon: total mass before and after the fizzing reaction stays exactly the same.
Proposed by Antoine Lavoisier, 1789
Matter can neither be created nor destroyed in a chemical reaction — only rearranged. If mass seems to change, check whether a gas has escaped the system.
Key idea
Checking the Law with Real Numbers
Reactants | Products
Calcium carbonate: 4.0 gCarbon dioxide: 1.76 g
Hydrochloric acid: 2.92 gWater: 0.72 g
Calcium chloride: 4.44 g
Total = 6.92 gTotal = 6.92 g
Key idea
The Law of Constant Proportions
Water from a river, a borewell, or the ocean
once purified — always contains hydrogen and oxygen in the exact same mass ratio.
Hydrogen : Oxygen = 1 : 8, always
Proposed by Joseph Proust. Decompose 9 g of pure water from any source, and you'll always get 1 g of hydrogen and 8 g of oxygen — regardless of where the water came from.
Key idea
Using the Fixed Ratio to Predict Amounts
Sodium chloride (NaCl) always contains sodium and chlorine in a mass ratio of 23 : 35.5.
If 46 g of sodium reacts completely...Chlorine needed = (35.5 ÷ 23) × 46 = 71 g. The ratio holds no matter the total quantity involved.
Key idea
Dalton's Atomic Theory
Postulate
All matter is made of tiny particles called atoms.
Atoms are indivisible
they cannot be created or destroyed in a reaction.
Atoms of a given element are identical in mass and chemical properties.
Key idea
Dalton's Atomic Theory — continued
Atoms of different elements differ in mass and chemical properties.
Atoms combine in simple whole-number ratios to form compounds.
The relative number and kinds of atoms in a compound are constant.
Key idea
How Atoms Combine
Atoms combine to complete their outermost (valence) shellthe resulting arrangement has lower, more stable energy. This holding force is a chemical bond.
Sharing electronsAtoms share one or more pairs of valence electrons — a covalent bond.
Transferring electronsOne atom gives up electrons, another accepts them — an ionic bond.
Key idea
Covalent Bonds: Sharing to Complete a Shell
Hydrogen (H—H)
Each H atom has 1 electron in the K-shell (needs 2). Two atoms share one electron each — a single covalent bond forms H₂.
Chlorine (Cl—Cl) and Oxygen (O=O)
Cl needs 1 electron → single bond, Cl₂. Oxygen needs 2 electrons → shares two pairs, forming a double bond, O₂.
Key idea
Covalent Bonds Between Different Elements
1
Hydrogen chloride (H—Cl)
Both H and Cl need just 1 more electron — they share a single pair to form HCl, a covalent compound.
→
2
Water (H₂O)
Oxygen needs 2 electrons, but each hydrogen offers only 1 — so two hydrogen atoms each share with the one oxygen atom, forming H₂O.
Key idea
Naming Covalent Compounds
Prefix | Meaning | Example
mono- (usually dropped on the first element)
1 — CO — carbon monoxide
di-
2 — CO₂ — carbon dioxide
tri-
3 — PCl₃ — phosphorus trichloride
hexa-
6 — SF₆ — sulfur hexafluoride
Key idea
Ionic Bonds: Sodium Chloride
Sodium loses an electron → Na⁺Sodium (2, 8, 1) loses its single valence electron, becoming a positively charged cation with 11 protons and 10 electrons.
Chlorine gains an electron → Cl⁻Chlorine (2, 8, 7) gains that electron to complete its octet, becoming a negatively charged anion. Opposite charges attract — an ionic bond.
Key idea
Ionic Compounds Form Crystal Lattices
Ionic compounds don't exist as single molecules
ions pack into a repeating 3-D crystal structure.
Every ion, surrounded
In sodium chloride, each Na⁺ ion is surrounded by six Cl⁻ ions, and each Cl⁻ by six Na⁺ ions — a regular, repeating pattern called a crystal lattice.
Write the symbols, write their valencies, then cross the valencies over as subscripts.
Elements & valencies | Formula
H (1) and Cl (1)
HCl
H (1) and S (2)
H₂S
C (4) and Cl (1)
CCl₄ — carbon tetrachloride
Key idea
Writing Formulae: Ionic Compounds
Cation first, then anioncross the charge numbers, then simplify to the smallest whole-number ratio.
Ions | Formula
Ca²⁺ and Cl⁻CaCl₂
Al³⁺ and O²⁻Al₂O₃
Mg²⁺ and OH⁻Mg(OH)₂ — brackets group repeated polyatomic ions
Key idea
Properties of Ionic and Covalent Compounds
| Ionic compounds | Covalent compounds
Solubility
Usually soluble in water — Usually soluble in kerosene, petrol
Conducts electricity (solid)
No — ions are fixed in place — No
Conducts electricity (dissolved/molten)
Yes — ions become free to move — Usually no
Melting & boiling points
High (strong inter-ionic attraction) — Low
Key idea
Molecular Mass
1
Water (H₂O)
H = 1 u, O = 16 u. Molecular mass = (1 × 2) + (16 × 1) = 18 u.
→
2
Carbon dioxide (CO₂)
C = 12 u, O = 16 u. Molecular mass = (12 × 1) + (16 × 2) = 44 u.
Key idea
Formula Unit Mass
Ionic compounds don't form molecules
so we add up the mass of one formula unit instead: the simplest whole-number ratio of ions.
Sodium oxide (Na₂O)
Na = 23 u, O = 16 u. (23 × 2) + (16 × 1) = 62 u.
Calcium nitrate, Ca(NO₃)₂
Ca = 40 u, N = 14 u, O = 16 u. 40 + {(14 + 48)} × 2 = 164 u.
Chapter · Key terms to remember
Key Terms
Law of Conservation of Mass
Mass is unchanged before and after a chemical reaction.
Law of Constant Proportions
A compound's elements always combine in a fixed mass ratio.
Molecule
A neutral group of atoms that exists independently.
Chapter · Key terms to remember
More Key Terms
Chemical bond
The force holding atoms together in a molecule or crystal.
Covalent bond
Formed by atoms sharing electrons.
Ionic bond
Formed by atoms transferring electrons (forming ions).
Chapter · Key terms to remember
More Key Terms
Molecular mass / Formula unit mass
Total mass of a covalent molecule / an ionic formula unit.
Valency
Combining capacity — the charge or bonds an atom forms.
Questions for your notebook
Write these down, then discuss
1
You want an ionic compound where total positive charge is 6+ and total negative charge is 6−. Which combination is correct? (i) 2 Al³⁺ and 3 Cl⁻ (ii) 3 Mg²⁺ and 1 PO₄³⁻ (iii) 2 Fe³⁺ and 3 O²⁻ (iv) 3 Ca²⁺ and 2 SO₄²⁻
2
Write the chemical formulae for: (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide
3
Write the formulae of the compounds formed from: (i) Ca²⁺ and Br⁻ (ii) Al³⁺ and CO₃²⁻ (iii) K⁺ and SO₄²⁻ (iv) NH₄⁺ and Cl⁻
Project as-is — students copy the questions, then the class discusses answers together.