IP Combined Science Notes (Lower Sec, Year 1-2): 02) Particle Model of Matter & States

Linking observations (melting ice, condensation on glassware) to invisible particles allows you to predict changes in state, density, and pressure with confidence.

Learning targets

• Describe particle arrangement, motion, and energy across solids, liquids, and gases.
• Interpret heating and cooling curves, identifying latent heat regions.
• Apply the density formula with consistent SI units and explain anomalous water behaviour.
• Predict effects of thermal expansion in solids, liquids, and gases.

1. Particle arrangement summary

Use diagrams that show particles as spheres with consistent spacing. Label arrows to indicate kinetic energy changes when heating/cooling.

2. Heating curve interpretation

A typical heating curve for ice:

• Region AB: Temperature rises from \( \pu{ -20 ^\circ C} \) to \( \pu{0 ^\circ C} \). Particles gain kinetic energy.
• Region BC: Melting occurs at constant temperature. Energy breaks bonds (latent heat), not raising temperature.
• Region CD: Water temperature climbs above \( \pu{0 ^\circ C} \) once fully liquid.

Worked example — Steam condensing

Steam at \( \pu{100 ^\circ C} \) releases latent heat when condensing. Explain in particulate terms:

1. Particles in gas phase lose potential energy as they come closer.
2. Kinetic energy may drop slightly if heat is removed faster than released by condensation.
3. Temperature remains constant during phase change because energy removal equals latent heat released.

3. Density calculations

Density is mass per unit volume:

\[ \rho = \frac{m}{V}. \]

Remember to express mass in \( \pu{kg} \) and volume in \( \pu{m3} \).

Worked example — Irregular solid using displacement

A stone is submerged in a measuring cylinder. Water level rises from \( \pu{30.0 mL} \) to \( \pu{45.5 mL} \).

• Volume of stone: \( \pu{15.5 cm3} = 1.55 \times 10^{-5} \pu{m3} \).
• Mass measured by electronic balance: \( \pu{52.8 g} = 5.28 \times 10^{-2} \pu{kg} \).

\[ \rho = \frac{5.28 \times 10^{-2}}{1.55 \times 10^{-5}} = 3.41 \times 10^{3} , \pu{kg.m-3}. \]

State answer to 3 s.f., matching the precision of the input data.

4. Thermal expansion

• Solids: Small expansion, exploited in bimetallic strips for thermostats. Different metals expand at different rates causing bending.
• Liquids: Expand more than solids. Mercury thermometers rely on volume expansion.
• Gases: Greatest expansion. Heating a sealed gas increases pressure via more frequent, forceful collisions.

For water, maximum density occurs at \( \pu{4 ^\circ C} \). Between \( \pu{0 ^\circ C} \) and \( \pu{4 ^\circ C} \), expansion occurs as hydrogen bonds rearrange — crucial for aquatic ecosystems.

Try it yourself

1. Sketch a particle diagram comparing solid and liquid water. Label kinetic energy changes when heating from \( \pu{-10 ^\circ C} \) to \( \pu{15 ^\circ C} \).
2. A gas in a \( \pu{2.5 L} \) container is heated from \( \pu{25 ^\circ C} \) to \( \pu{70 ^\circ C} \) at constant volume. Explain qualitatively how pressure changes.
3. Calculate the density of ethanol if \( \pu{45.0 g} \) occupies \( \pu{57.0 mL} \). Express answer in \( \pu{kg.m-3} \) to 3 s.f.

Move on to separation techniques at https://eclatinstitute.sg/blog/ip-combined-sciences-lower-sec-notes/IP-Combined-Science-Lower-Sec-03-Elements-Compounds-Mixtures-and-Separation.