Week 1 · Unit 1 Matter and Energy, opener; Lesson 1 States of Matter: solids, liquids and gases and their properties, the particle model, plasma · 3 lessons
3
A fourth state: plasma, and models of matter
Science Prep 2 Term 1, Unit 1: pp. 1–9
GoalDescribe plasma as the fourth state, and say what a model can and cannot show.
New words
plasma · a gas whose atoms are split into positive ions and free electronsion · an atom that has lost or gained electrons, so it has a chargemodel · a simple picture or object that helps us explain something we cannot see
Explanation
Plasma is the fourth state of matter. It is a gas in which the atoms have been split into positive ions and free electrons. Because it contains charged particles, plasma conducts electricity. Plasma is not just a hot gas: its particles are charged and it behaves differently from an ordinary gas. Most of the visible matter in space is plasma: the Sun and the stars are made of it. On Earth we see plasma in lightning and in the aurora, a glow seen mostly near the poles when charged particles from the Sun meet the upper air. Some devices claim to use ions to clean room air, but the claims are limited. Our particle drawings are a model: it shows spheres, but real atoms are not hard balls, and the model cannot show everything. A good model explains much and we stay aware of what it leaves out.
a gas→atoms split into ions and free electrons→plasma
Worked examples
Lightning is plasma for a very short time. Air along the path of the flash is split into ions and electrons, which is why the flash conducts electricity and glows.
State
Particles
Conducts electricity?
solid, liquid, gas
neutral atoms or molecules
usually not
plasma
ions and free electrons
yes
Compare the four states in one table: solid, liquid and gas have neutral atoms or molecules, and plasma has charged ions and free electrons.
Yasmin’s tray model shows gas particles as lentils. What does it not show? The lentils stop moving when you stop shaking, but real gas particles never stop moving by themselves.
Class activity
Where is the plasma? Make a list of five things you might see or hear about (a lightning storm, a glass of water, the Sun, ice, a night-sky glow near the poles). Mark each as plasma or not plasma, and give one reason. Check with a grown-up. Then write one sentence about what a drawing of particles can show and one about what it cannot.
Practice
1
What is plasma?
A gas whose atoms are split into positive ions and free electrons.
2
Name two places where plasma is found.
For example the Sun and stars, lightning or the aurora.
3
Why does plasma conduct electricity but an ordinary gas usually does not?
Plasma has free charged particles that can move; an ordinary gas has neutral ones.
4
A friend says plasma is just a very hot gas. What is missing from this statement?
That its atoms are split into charged ions and electrons, which changes how it behaves.
