Chemistry (KS3)
UK Key Stage 3 chemistry: atoms and elements, the periodic table, bonding, reactions, acids and bases, and materials.
Ämne: Kemi · Nivå: Högstadium (13–15) · 408 kort
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- All matter is made of tiny particles. In solids these particles vibrate in fixed positions, in liquids they slide past each other, and in gases they move freely at high speeds.
- Melting (solid to liquid), freezing (liquid to solid), evaporation/boiling (liquid to gas) and condensing (gas to liquid) are physical changes of state. The particles are rearranged but not chemically changed.
- Sublimation is when a solid changes directly into a gas without becoming a liquid first. Solid carbon dioxide (dry ice) and iodine are common examples.
- Diffusion is the gradual spreading of particles from areas of high concentration to areas of low concentration. It happens fastest in gases and slowest in solids.
- Atoms are extremely small — about a tenth of a nanometre across. They are built from three sub-particles: protons, neutrons and electrons.
- Protons carry a charge of +1, electrons -1, and neutrons are neutral. Protons and neutrons sit in the nucleus while electrons move in shells around it.
- The atomic number of an element is the number of protons in its nucleus. The mass number is the total number of protons plus neutrons.
- Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, so they have different mass numbers.
- An element is a pure substance made of only one type of atom. There are roughly 100 known elements, each represented by a one or two letter chemical symbol such as H, O, Na, Cl, Fe, Cu and Au.
- A compound is formed when two or more elements are chemically bonded together in fixed proportions. Water (H2O), sodium chloride (NaCl) and carbon dioxide (CO2) are everyday examples.
- A mixture contains two or more substances that are not chemically combined. Mixtures can be separated by physical methods such as filtration, distillation or chromatography.
- The periodic table arranges elements in order of increasing atomic number. Vertical columns are called groups and horizontal rows are called periods. Elements in the same group tend to have similar chemical properties.
- Group 1 of the periodic table contains the alkali metals — lithium, sodium and potassium are the common examples. They are soft, have low densities and react vigorously with water.
- Group 7 contains the halogens — fluorine, chlorine, bromine and iodine. They are coloured, reactive non-metals that form salts when they react with metals.
- Group 0 (sometimes called Group 8) contains the noble gases — helium, neon and argon. Their full outer electron shells make them very unreactive.
- Dmitri Mendeleev published an early version of the periodic table in 1869. He deliberately left gaps for elements that had not yet been discovered, and later predictions matched the real elements that filled them.
- Metals are typically shiny, malleable, ductile and good conductors of heat and electricity. Most are solid at room temperature, with mercury being a notable liquid exception.
- Non-metals are usually dull, brittle if solid, and are poor conductors of heat and electricity. Graphite (a form of carbon) is an unusual non-metal that does conduct electricity.
- In an ionic bond a metal atom transfers electrons to a non-metal atom, forming charged ions that attract each other. In a covalent bond two non-metal atoms share electrons.
- In a chemical reaction atoms are rearranged but never created or destroyed. The total mass of the reactants always equals the total mass of the products — this is the conservation of mass.
- State symbols in a chemical equation show the physical state of each substance: (s) for solid, (l) for liquid, (g) for gas and (aq) for an aqueous solution dissolved in water.
- The pH scale measures how acidic or alkaline a solution is. It runs from 0 to 14: below 7 is acidic, exactly 7 is neutral, and above 7 is alkaline.
- Igneous rocks form when molten magma or lava cools and solidifies. Granite cools slowly underground and has large crystals; basalt cools quickly at the surface and has small crystals.
- The Earth's atmosphere is roughly 78% nitrogen, 21% oxygen, around 1% argon, and about 0.04% carbon dioxide, with small amounts of water vapour and other gases.
- Alloys are mixtures of a metal with at least one other element. Steel is mostly iron with a little carbon, brass is copper and zinc, and bronze is copper and tin.
- Filtration separates an insoluble solid from a liquid using filter paper. Distillation separates a liquid from a solution (or from other liquids with different boiling points) by evaporating it and then condensing the vapour.
- When a pure substance is heated, its temperature stops rising while it melts or boils. The energy goes into breaking the forces between particles, not raising the temperature. This produces flat plateaus on a heating curve at the melting point and boiling point.
- Density is mass per unit volume. Solids are usually densest because particles are packed tightly, liquids are slightly less dense, and gases have very low densities because their particles are far apart.
- Gas pressure is caused by particles colliding with the walls of their container. More frequent or harder collisions mean higher pressure.
- Heating a gas at constant volume increases its pressure because the particles move faster and hit the walls harder and more often. Reducing the volume at constant temperature also raises the pressure because the collisions happen more often.
- In the first 20 elements, electrons fill shells in the order 2, 8, 8. The first shell holds up to 2 electrons, the second up to 8, and the third up to 8 at this level.
- Common electronic configurations: sodium (Na) is 2,8,1; chlorine (Cl) is 2,8,7; argon (Ar) is 2,8,8. The number of outer electrons matches the group number in the periodic table.
- Models of the atom developed over time: Dalton imagined solid spheres, Thomson proposed a 'plum pudding' of electrons in a positive blob, Rutherford's gold foil experiment showed a tiny dense nucleus, and Bohr placed electrons in fixed shells.
- In Rutherford's gold foil experiment, most alpha particles passed straight through a thin gold sheet but a few were deflected at large angles. This showed the atom is mostly empty space with a tiny, dense, positively charged nucleus.
- Relative atomic mass (Ar) compares the mass of an atom of an element with one twelfth the mass of a carbon-12 atom. For example, Ar(H) ≈ 1, Ar(C) ≈ 12, Ar(O) ≈ 16.
- Group 1 elements get more reactive going down the group (Li, Na, K, Rb, Cs). Group 7 elements get less reactive going down the group (F, Cl, Br, I). The trends are explained by how easily atoms gain or lose outer electrons.
- Mendeleev arranged elements by atomic mass and grouped them by similar properties. He left gaps for undiscovered elements, including one he called eka-silicon. It was later identified as germanium, matching his predictions closely.
- Sodium reacts with chlorine to form sodium chloride: Na loses one electron to become Na+, and Cl gains one electron to become Cl−. The oppositely charged ions attract to form NaCl.
- Ionic compounds form giant lattices of repeating positive and negative ions. They have high melting points, are usually solid at room temperature, and conduct electricity when molten or dissolved in water but not when solid.
- Simple covalent molecules include hydrogen (H2), chlorine (Cl2), water (H2O), carbon dioxide (CO2), methane (CH4) and ammonia (NH3). Atoms share pairs of electrons to fill their outer shells.
- A balanced equation has the same number of atoms of each element on both sides. For example, 2H2 + O2 → 2H2O has 4 H and 2 O on each side.
- An exothermic reaction releases energy to the surroundings, often as heat. Combustion is exothermic. An endothermic reaction takes in energy from the surroundings. Photosynthesis is endothermic, absorbing light energy.
- The reactivity series places metals in order of how vigorously they react: K, Na, Ca, Mg, Al, (C), Zn, Fe, Sn, Pb, (H), Cu, Ag, Au. Carbon and hydrogen are often included as reference points.
- Metals above carbon in the reactivity series, such as aluminium, must be extracted by electrolysis of their molten compounds. Metals below carbon, such as iron, can be extracted by heating the ore with carbon to reduce it.
- Gas tests: hydrogen gives a squeaky pop with a lit splint, oxygen relights a glowing splint, and carbon dioxide turns limewater milky (cloudy white).
- The salt formed in neutralisation depends on the acid: hydrochloric acid gives chloride salts, sulfuric acid gives sulfate salts, and nitric acid gives nitrate salts.
- Crude oil is separated by fractional distillation in a tall column. Lighter, shorter-chain fractions such as petroleum gases and petrol leave near the top, and heavier fractions such as diesel, fuel oil and bitumen come out lower down.
- In chromatography, an Rf value is the distance moved by a spot divided by the distance moved by the solvent. Rf values can be used to identify substances and are always between 0 and 1.
- Complete combustion of a hydrocarbon needs plenty of oxygen and produces carbon dioxide and water. Incomplete combustion happens with limited oxygen and can produce carbon monoxide (a toxic gas) and soot (small carbon particles).
- For a fire you need three things, often called the fire triangle: fuel, oxygen and heat. Removing any one of them puts the fire out.