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Flashcards · J249 · Topic 1

Matter flashcards

28 cards on Topic 1, subtopics P1.1 to P1.3. Pick a subtopic or study them all.

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P1.1 The particle model

Thomson’s plum pudding model of the atom
A ball of positive charge with tiny negative electrons embedded in it. There is no nucleus.
Alpha scattering (Geiger and Marsden): the three results and what each showed
Most went straight through: the atom is mostly empty space. Some were deflected through large angles: the centre is positive. A very few bounced back: the centre is tiny, dense and holds nearly all the mass.
What did Bohr add to the nuclear model?
Electrons orbit at fixed distances (energy levels). This explained atomic stability and light emitted at particular wavelengths.
Why does a scientific model of the atom change?
New experimental evidence cannot be explained by the old model. The new model must explain both the old and the new evidence.
Size of an atom, and of its nucleus compared with it
An atom (or small molecule) is about 1 × 10⁻¹⁰ m across. The nuclear radius is about 10 000 times smaller, yet holds almost all the mass.
Density: definition, equation and units
Mass per unit volume. ρ = m ÷ V, in kg/m³ (or g/cm³). 1 g/cm³ = 1000 kg/m³.
A metal cylinder has volume 35 cm³ and mass 273 g. Density?
273 ÷ 35 = 7.8 g/cm³ = 7800 kg/m³
How do you find the volume of an irregular solid?
By displacement: measure the rise in water level in a measuring cylinder, or use a eureka can. Read at the bottom of the meniscus with your eye level with it.
Why is a gas much less dense than its solid or liquid?
The particles are the same, but in a gas they are far apart, so a given volume contains far fewer particles. The particles are not lighter or smaller.

P1.2 Changes of state

What happens to the mass when a substance melts, boils or sublimes?
It is conserved. The same particles are present; only their arrangement and motion change.
Physical change vs chemical change
Physical: no new substance; reversing it gives back the original properties (ice melting). Chemical: a new substance forms; the original properties do not return (iron rusting).
What does heating do to a substance below, and at, a change of state?
Below: particles gain kinetic energy and the temperature rises. At a change of state: particles gain potential energy as they separate; the temperature stays constant.
Specific heat capacity: definition and equation
The energy needed to raise the temperature of 1 kg of a substance by 1 °C (J/kg °C). E = m × c × Δθ
Specific latent heat: definition and equation
The energy needed to change 1 kg of a substance from one state to another at constant temperature (J/kg). E = m × L
Specific latent heat of fusion vs of vaporisation
Fusion: solid ↔ liquid (melting, freezing). Vaporisation: liquid ↔ gas (boiling, condensing).
0.80 kg of steel (c = 490 J/kg °C) is heated by 60 °C. Energy needed?
E = 0.80 × 490 × 60 = 23 520 J ≈ 24 000 J
0.15 kg of ice at 0 °C becomes water at 20 °C. Energy? (L = 334 000 J/kg, c = 4200 J/kg °C)
Melting: 0.15 × 334 000 = 50 100 J Warming: 0.15 × 4200 × 20 = 12 600 J Total = 62 700 J
In a specific heat capacity experiment, how does heat lost to the surroundings affect the result?
The temperature rise is smaller than it should be, so the calculated c is too high. Lag (insulate) the apparatus.

P1.3 Pressure

What causes gas pressure?
Gas particles colliding with the surfaces of the container. Pressure is the force per unit area from these collisions.
A sealed gas is heated at constant volume. Explain what happens to its pressure.
The particles move faster, so they hit the walls more often and harder. The force on the walls increases, so the pressure increases.
Pressure and volume of a gas at constant temperature
p₁V₁ = p₂V₂ (pressure × volume = constant, for a fixed mass of gas). A bigger volume gives a lower pressure because the particles hit the walls less often.
Gas at 120 kPa and 0.50 m³ is compressed to 0.20 m³ at constant temperature. New pressure?
p₂ = 120 × 0.50 ÷ 0.20 = 300 kPa
Why does a bicycle pump warm up when you compress the air?Higher
Work is done on the gas: particles rebound faster from the moving piston, so their average kinetic energy, and so the temperature, rises.
Why does atmospheric pressure decrease with height?
There is less air above, so less weight pressing down, and the air is thinner, so there are fewer particles per cubic metre.
Pressure due to a column of liquidHigher
p = h × ρ × g (Pa, m, kg/m³, N/kg). Pressure increases with depth and with the density of the liquid.
Pressure due to the water at a depth of 35 m? (ρ = 1000 kg/m³, g = 10 N/kg)Higher
p = 35 × 1000 × 10 = 350 000 Pa
What causes upthrust?Higher
The bottom of an immersed object is deeper than the top, so the pressure and the upward force on it are greater. The resultant force is upwards.
When does an object float in a liquid?Higher
When the upthrust equals its weight. A fully submerged object floats up if its density is less than the liquid’s and sinks if it is greater.

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