Practice

Density = mass ÷ volume = 540 ÷ 200 = 2.7 g/cm³ — about aluminium.

No. It is still H₂O, the mass has not changed, and freezing reverses it. A change of state, not of substance.

The proton number names the element: 11 protons is sodium. Mass number = 11 + 12 = 23.

Oxygen has 8 protons, so a neutral atom has 8 electrons: 2 in the first shell, 6 in the second.

Group 1, period 3 is sodium. Group 1 means 1 electron in the outer shell — which is why it gives that electron away so readily.

They have 2 outer electrons. Losing both leaves a full shell, so the ion carries charge 2+.

Sodium loses its 1 outer electron → Na⁺. Chlorine gains it to complete its shell → Cl⁻. Opposite charges attract: an ionic bond.

Two bonds, one to each hydrogen. Each bond is a shared pair of electrons.

Moles = mass ÷ molar mass = 36 ÷ 18 = 2 mol.

0.5 × 6.02 × 10²³ = 3.01 × 10²³ particles. The mole is a count, not a weight — the count is the same for every substance.

Carbon is already balanced. Hydrogen: 4 on the left, so 2H₂O. Oxygen: 2 + 2 = 4 on the right, so 2O₂. CH₄ + 2O₂ → CO₂ + 2H₂O.

Zinc chloride and hydrogen. The gas gives a squeaky pop with a lit splint.

The equation's ratio is 1 : 1, so 0.5 mol of Mg makes 0.5 mol of H₂.

8 g of H₂ = 8 ÷ 2 = 4 mol. The ratio 2 : 2 means 4 mol of H₂ makes 4 mol of H₂O. Mass = 4 × 18 = 72 g.

Double the pressure, half the volume: 30 cm³. Squeezing the same particles into half the space doubles the hits on the walls.

Cold particles move more slowly and hit the walls less hard, so the pressure inside drops. The outside air pushes the balloon in until the pressures match again — at a smaller volume.

250 cm³ = 0.25 dm³. Concentration = 25 ÷ 0.25 = 100 g/dm³.

The same amount of solute now sits in five times the volume, so the concentration falls to one fifth.

Each pH step is a factor of 10. Five steps from 7 down to 2 is 10⁵ = 100,000 times.

The acid names the salt: sulfuric acid makes a sulfate, and the metal is potassium — potassium sulfate, plus water.

Exothermic — it gives out energy to the surroundings, and that energy arrives as heat.

Crush the lump, warm the mixture, or use a stronger acid. Crushing works because more of the solid's surface is exposed, so collisions happen more often.

Carbon: 3CO₂. Hydrogen: 4H₂O. Oxygen on the right: 6 + 4 = 10 atoms, so 5O₂. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

In graphite, some electrons are delocalised — free to move between the layers and carry charge. In diamond every electron is locked into a bond.