Poland Szkoła podstawowa, klasa VII Chemistry

1. Substances and their properties

Everyday substances and lab safety · Mixtures, elements and compounds

• Lab Safety and Safe Experiments – Dress for safety (goggles, coat, closed shoes, hair tied). Read labels and hazard pictograms. Heat gently with the mouth of the tube pointing away. Add acid to water, waft smells, never taste. Report every accident at once. Plan experiments step by step, assess the risks first and dispose of waste correctly.
• Mixtures and Their Separation – A mixture has two or more substances mixed without any fixed ratio, and each keeps its own properties. Homogeneous mixtures (solutions) look the same everywhere; heterogeneous ones do not. By particle size we get solutions (< 1 nm), colloids (1–1000 nm) and suspensions (> 1000 nm). Colloids scatter light (Tyndall effect). Concentration tells how much solute is in a solution. We separate mixtures by using a difference in their parts: evaporation, crystallisation, distillation, chromatography, sublimation, centrifugation and coagulation.

2. Internal structure of matter

Atoms and the periodic table · Chemical bonds · Valency and oxides

• Structure of the Atom – An atom has a tiny, heavy, positive nucleus made of protons and neutrons. Electrons move around it in fixed shells K, L, M, N. The number of protons (Z) tells the element; protons + neutrons give the mass number (A). Outer electrons decide valency. Isotopes share Z; isobars share A.

3. Chemical reactions

Physical and chemical changes; equations

Coming soon

4. Oxygen, hydrogen and their compounds; air

Oxygen, oxides and carbon dioxide · Hydrogen and hydrides · Air

• Oxygen – Oxygen (O, atomic number 8) is a colourless, odourless gas that makes up about 21% of air by volume. It exists as O₂ molecules. In the lab it is made by breaking down hydrogen peroxide with a manganese dioxide catalyst (2H₂O₂ → 2H₂O + O₂) or by heating potassium manganate(VII); in industry it comes from fractional distillation of liquid air. It relights a glowing splint. Oxygen supports burning and respiration and reacts with most elements to form oxides: metal oxides are basic, non-metal oxides are mostly acidic. Its other form, ozone (O₃), shields Earth from UV rays.
• Hydrogen: The Lightest Element – Hydrogen (H) is element 1: one proton and one electron. It exists as H₂ molecules, a colourless, odourless gas that is the least dense of all gases and hardly dissolves in water. In the lab it is made from a reactive metal and a dilute acid (Zn + 2HCl → ZnCl₂ + H₂) and collected over water. It burns with a squeaky pop to form only water. It reacts with non-metals to give hydrides such as HCl, NH₃ and H₂S, and it reduces some metal oxides. It has three isotopes: protium, deuterium and tritium. Uses: ammonia and fertilisers, margarine, rocket fuel and clean fuel cells.
• Composition of Air – Air is a mixture of gases. By volume, dry air is about 78% nitrogen, 21% oxygen, 0.93% argon and 0.04% carbon dioxide, with tiny amounts of other noble gases. Water vapour changes from 0 to about 4%. We can show air is a mixture and measure its oxygen by letting iron rust (or phosphorus burn) in a closed tube: water rises about one fifth. Pollutants such as smoke, sulfur dioxide, nitrogen oxides and carbon monoxide make air harmful, and we can reduce them.

5. Water and aqueous solutions

Water as a solvent · Solubility and concentration

• Water: The Small Molecule That Makes Life Possible – A water molecule (H₂O) is bent and polar: oxygen is slightly negative and the hydrogens slightly positive. Because of this, water molecules stick together with hydrogen bonds. These bonds explain why water dissolves many substances, stores a lot of heat, takes a lot of energy to evaporate, sticks together (cohesion) and to surfaces (adhesion), and why ice floats. Living things also use water as a reactant, for transport and to keep cells firm. Mineral salts dissolved in water supply ions like Na⁺, K⁺, Ca²⁺, Fe²⁺ and phosphate.
• Solutions: How Things Dissolve and How Much Can Dissolve – A solution is a uniform mixture of a solute dissolved in a solvent. In water (an aqueous solution) the solute breaks into particles too small to see, so it never settles and passes through filter paper. Concentration tells how much solute is present (mass % = solute ÷ solution × 100). Solubility is the most that can dissolve in 100 g of solvent at a given temperature; beyond it the solution is saturated. Evaporation, crystallisation and distillation separate solutions.