Particles in motion and quick revision

4.1 Particles are always moving

Drop a few grains of potassium permanganate into a glass of water. Pink streaks spread out from the grains, and after some time the whole glass is evenly pink. The water particles are in constant motion. They pull particles away from the grain and carry them through the liquid, spreading them until the mixture is uniform. The same process explains why sugar dissolves and why a drop of ink colours a whole glass.

The motion of particles changes with temperature. In the chapter's test, a grain of potassium permanganate is dropped into hot water, room-temperature water and ice-cold water. The colour spreads fastest in the hot water, more slowly at room temperature, and slowest in the cold water. Heating gives the particles more energy, so they move faster.

The same idea explains how a smell travels. Burn an incense stick in one corner of a room and the fragrance reaches every corner after a little while. The particles of the fragrance spread through the air, helped along by collisions with moving air particles. This spreading of particles from a region of higher concentration to a region of lower concentration is called diffusion.

4.2 Soap particles at work

Particles explain everyday cleaning too. When clothes stained with oil are washed with soap, many soap particles surround each oil particle. The soap particle has one end that grabs the oil and another that mixes with water. The soap particles lift the oil away from the fabric and carry it into the water, so the cloth comes out clean.

4.3 Thermal energy and the state of matter

The state of a material depends on the energy its particles have. This internal energy of motion is called thermal energy. In a solid, the thermal energy is low, so the particles stay close together and only vibrate. At the melting point, the added energy is used to overcome the attractions, and particles break free of their fixed places and begin sliding past one another.

The table summarises the three states.

4.4 Common mistakes to avoid

• Thinking matter is continuous: even solid objects are built from particles with spaces between them.
• Believing that grinding changes a substance: breaking or grinding changes size and shape, not the substance.
• Saying solids have no spaces between particles: spaces exist, but they are small.
• Thinking gases have no mass: gases are matter, and they have particles and weight.
• Saying liquids have a fixed shape: liquids have a fixed volume but take the shape of their container.
• Believing evaporation happens only at the boiling point: it happens at all temperatures, just slowly below the boiling point.
• Thinking diffusion needs stirring: particles spread on their own because of their motion.