States and properties of matter, kinetic theory Notes

Why Do Scientists Use A Model To Explain The Behaviour Of Matter?

Matter can look and behave very differently, yet the differences can be explained using a single powerful idea - matter is made of tiny particles (atoms or molecules) that move and interact with each other.

Particle Interactions Explain The States Of Matter

1. A useful starting point is the particle model: each substance is made of particles that exert forces on each other over very short distances.
2. When particles are far apart, they exert negligible forces.
3. When particles are close, they attract each other.
4. If pushed too close, they repel each other.
5. Matter commonly exists as solid, liquid, or gas.
6. Each state has different properties because of:
   1. particle spacing,
   2. particle arrangement,
   3. particle motion.

Limits of the Particle Model

1. The particle model does not show individual particles directly.
2. It explains behaviour, not exact particle shapes or sizes.

Solids Have Fixed Positions, High Strength, And Low Compressibility

1. In a solid, particles are tightly packed and occupy fixed positions in an ordered arrangement.
2. They cannot move from place to place, but they can vibrate about their fixed positions.
3. Key properties explained by the particle model:
   1. Solids keep their shape because particles are held in place by strong attractions.
   2. Solids are hard to compress because the particles are already close together; pushing them closer quickly leads to repulsive forces.
   3. It takes energy to separate particles, which is why breaking apart or melting a solid requires energy input.

Liquids Flow But Remain Hard To Compress

1. When a solid is heated, its particles may gain enough energy to break out of fixed positions.
2. The substance becomes a liquid.
3. In a liquid:
   1. Particles are still close together, so liquids are hard to compress.
   2. Particles are arranged more randomly than in a solid and can move around each other.
   3. Attractive forces still matter, so liquids form a definite volume and do not automatically spread out to fill an entire container.

Gases Spread Out And Are Easily Compressed

1. If a liquid is heated further, particles may gain enough energy to overcome attractive forces between them and become a gas.
2. In a gas:
   1. Particles are far apart.
   2. Forces between particles are usually insignificant (except during collisions).
   3. Particles move freely, so a gas fills its container.
   4. Gases are easy to compress because there is lots of empty space between particles.

Changes of State Using the Particle Model

1. Heating a substance gives particles more energy.
2. Particles move faster and may break free from fixed positions.
3. Cooling a substance removes energy from particles.
4. Particles move more slowly and may become fixed in place.

Temperature Relates To Particle Motion

1. Particles are always moving (or vibrating), and that microscopic motion is linked to what we measure as temperature.
2. Temperature is linked to how fast particles are moving.
3. When the particles move or vibrate faster, they have more energy and the substance has a higher temperature.
4. This explains why heating can change state: added energy increases particle motion until the particles can rearrange (solid to liquid) or separate significantly (liquid to gas).

Mechanical Properties of Materials

1. Hardness

1. Hardness describes how resistant a material is to scratching or indentation.
2. Hard materials are difficult to scratch.

2. Strength

1. Strength describes how much force a material can withstand without breaking.
2. A strong material does not break easily when pulled, pushed, or loaded.

3. Toughness

1. Toughness describes how much energy a material can absorb before breaking.
2. Tough materials bend or deform before breaking.

4. Stiffness

1. Stiffness describes how resistant a material is to bending or stretching.
2. A stiff material does not easily change shape under force.

5. Brittleness

1. Brittle materials break suddenly when a force is applied.
2. They show little or no bending before breaking.

6. Malleability

1. Malleable materials can be hammered or pressed into thin shapes.
2. This property is important for shaping metals.

7. Ductility

1. Ductile materials can be stretched into long, thin wires.
2. This property is important for electrical wiring.

8. Electrical Conductivity

1. Electrical conductivity describes how easily electric current flows through a material.
2. Metals are generally good conductors.
3. Non-metals are usually poor conductors (insulators).

Linking Properties to Uses

1. Engineers choose materials based on required properties.
2. A single material may be chosen because it has multiple useful properties.

Kinetic Theory Models Gases As Constantly Moving Particles

1. A particularly important application of the particle model is kinetic theory for gases.
2. Kinetic theory explains these observations:
   1. Gas particles move randomly and collide with each other and the container walls.
   2. As the temperature increases, particles move faster on average.
   3. Faster particles collide more often and with greater speed, which increases the effect of those collisions (for example, stronger pushes on the container walls).
3. Even though we cannot see the particles, we can detect their average effect.

Particle Motion in Solids, Liquids, and Gases

1. In solids, particles vibrate with limited movement.
2. In liquids, particles move more freely but remain close.
3. In gases, particles move rapidly and randomly in all directions.

Brownian Motion Provides Evidence For Molecular Motion

1. One classic piece of evidence for the particle model and kinetic theory is Brownian motion.
2. In 1827, Robert Brown observed pollen grains in water moving randomly.
3. Much later, Einstein and Smoluchowski explained the effect: the pollen or smoke particles are constantly struck by much smaller molecules in the surrounding fluid.
4. These collisions happen extremely frequently, but at any instant, they are slightly uneven.
5. The imbalance produces a small net force, pushing the visible particle in one direction, then another, creating a jittery path.

Particle Explanation of Brownian Motion

1. The suspended particle is much larger than the surrounding molecules.
2. Molecules of water or air are constantly moving and colliding with the particle.
3. These collisions are uneven and random.
4. At any moment, one side of the particle may experience more collisions than the other.
5. This uneven pushing causes the particle to move in changing directions.

Plasma Is A Fourth State With Charged Particles

1. Beyond solid, liquid, and gas, there is a fourth commonly discussed state: plasma.
2. A plasma forms when a gas is heated so much that electrons are removed from atoms.
3. The result is a mixture of positive ions and negative electrons.
4. Plasma can emit light and behave differently from normal gases.