Particulate Nature of Matter · Lesson 5 of 6
Particles in Motion
“Observe particles spreading in water and air, then connect motion, heat, and the states of matter.”
• Use colour and fragrance observations as evidence of particle movement. • Predict how water temperature affects the spreading of potassium permanganate. • Connect thermal energy, particle movement, spacing, and attraction across the three states. • Explain the chapter’s soap-and-oil cleaning application using a simple particle model.
Colour spreads through water
Place a small grain of potassium permanganate in still water and watch over time. Pink streaks spread outward at first. Eventually the colour appears across the whole glass. Something is moving even though individual constituent particles are too small to see.
Moving water particles interact with the grain, helping separate and spread potassium permanganate particles. The visible colour maps where the dissolved substance has reached. It is evidence for continual movement and mixing, not evidence that the entire grain has moved unchanged across the glass.
Problem
Why is the water initially pink near the grain rather than uniformly pink?
- 1.The potassium permanganate has only begun to spread.
- 2.Its particles move away from the grain as the moving water particles interact with them.
- 3.With time, the colour becomes more evenly distributed through the water.
A teacher should handle potassium permanganate with a spoon or spatula. Do not touch it with bare hands or taste the water.
What difference does heating make?
Compare equal small grains in hot, room-temperature, and ice-cold water. The colour spreads fastest in hot water, more slowly at room temperature, and slowest in cold water. Greater thermal energy is associated with more vigorous particle movement, which helps the dissolved substance spread faster. Keep other conditions as similar as possible when comparing the three glasses.
| Water temperature | Expected relative rate of colour spreading | Particle explanation |
|---|---|---|
| Hot | Fastest | Water particles move most vigorously |
| Room temperature | Intermediate | Movement is less vigorous |
| Ice-cold | Slowest | Movement is less vigorous still |
Problem
One glass contains hot water and another cold water. The hot glass also has much more stirring. Can their different spreading rates be attributed only to temperature?
- 1.Stirring itself moves the liquid and can change spreading.
- 2.Keep stirring, grain size, and water amount comparable to isolate the effect of temperature.
- 3.With a fairer comparison, faster spreading in the hot glass supports the link between heat and particle movement.
Fragrance travels through air
When an incense stick is lit in one corner of a room, its fragrance is noticed nearby first and later farther away. Fragrance particles spread through the air. Constantly moving air particles collide with them and help carry them through the available space. Perfume provides a familiar comparison.
The nose detects the fragrance, not individual gas particles. Reaching a distant part of the room therefore gives indirect evidence that particles can move and mix. A gas has large spaces and very weak attraction between its particles, which allows movement through the room.
Problem
Why can someone smell perfume after it is sprayed some distance away?
- 1.The perfume gives off particles that enter the air.
- 2.Moving air particles interact with them as the fragrance spreads.
- 3.Some reach the person’s nose even though individual particles cannot be seen.
One explanation across the three states
The three states can be understood by comparing particle thermal energy with attraction. In a solid, relatively low energy and strong attraction keep particles close to their positions; they vibrate. Heating increases vibration. At melting, particles gain enough freedom to leave fixed positions and move around as a liquid.
In a liquid, particles stay relatively close while moving past one another. At boiling, their movement becomes vigorous enough for vapour to form rapidly within the liquid and at its surface. In a gas, particles have enough energy to move freely in all directions; attraction is negligible in this comparison and spacing is generally greatest. These links explain the observed shape, volume, and ability to flow rather than merely listing them.
A substance that has spread throughout water or air can become too dilute to see in one place. That does not mean its particles have stopped existing.
A cleaning application
Water alone often struggles to lift oily stains from cloth. The chapter’s simple soap model shows many soap particles surrounding the oil. One end attaches to oil while another mixes with water. That helps loosen and carry the oil away when the cloth is washed. This application relies on interactions among particles, even though we observe the change at the scale of a whole stain.
Problem
Why does adding soap help remove an oily stain from fabric?
- 1.Soap particles interact with the oil, surrounding its small portions.
- 2.Their water-friendly ends allow these portions to be carried with water.
- 3.Rinsing removes more oil from the cloth than water alone in this simple model.
Quiz
What is seen first when a potassium permanganate grain is put into still water?
In otherwise comparable glasses, where should the colour spread fastest?
Why does incense fragrance reach a distant part of a room?
What particle movement best describes a liquid?
How does soap help wash off oil in the chapter’s model?
Practice Problems
- Explain the changing appearance of water from the first pink streaks to an even colour.
- Design a fair three-glass comparison of spreading in hot, room-temperature, and cold water. List what you would keep the same.
- Explain why perfume or incense can be sensed away from its source using particle movement.
- Draw and label the movement of particles in a solid, liquid, and gas. Add a short explanation of attraction in each.
- Describe how increasing thermal energy can change a solid to a liquid and then to vapour.
- Explain how soap helps water remove an oily stain using the simple particle model.
- A student says that once the whole glass turns pale pink, the potassium permanganate has vanished. Correct the explanation.
Key Takeaways
• Particles in liquids and gases move continually; colour and fragrance make their spreading observable. • In the comparison activity, colour spreads fastest in hot water because particle movement is more vigorous. • The balance of particle thermal energy and attraction helps explain the three states and changes of state. • Solid particles vibrate, liquid particles move past neighbours, and gas particles move freely. • Soap particles can help lift oil so water carries it away.