# Wikijunior:Particles/Summary

 Wikijunior:Particles Final quiz Summary

## Introduction

• Properties of matter
• Matter must have mass
• Matter must take up space

## Matter

• Different kinds of matter
• Solids' shapes cannot be altered, and nor can its volume
• Liquids' shapes can be altered, but not its volume
• Gases' shapes can be altered, and so can its volume
• Change of states
• Solid → Liquid: Melting
• Happens at a fixed temperature
• Liquid → Solid: Freezing
• Happens at a fixed temperature
• Gas → Liquid: Condensation
• Does not happen at a fixed temperature
• Liquid → Gas: Boiling
• Happens at a fixed temperature
• Evaporation does not happen at a fixed temperature

## Particle theory

• Matter is made of particles;
• Particles are infinitesimal, and current scientific instruments cannot see them;
• Different kinds of particles have different sizes;
• There must be gaps between particles
• Particles are always in motion.

## Diffusion

• A common phenomenon observed in particle movement
• Particles always go to a place of lower density

## Brownian motion

• When particles collide, they move in random directions
• Observed in smoke cells
• Discovered by Robert Brown
• In 1827
• While observing pollen grains

## Particle model

• A model that mimics the movement of particles
• Can explain:
• The three states of matter
• Gas pressure
• Density
• Thermal expansion and contraction
• Heat makes particles move more vigorously

## Three states of matter

• Solid
• Particles cannot move freely
• Particles only vibrate in fixed positions
• Particles take on a fixed shape
• Liquid
• Particles do not take on a fixed shape; their shape depends on that of their container
• Particles have a fixed volume
• Particles can slide over one another
• Particles do not move as freely as those in gas
• Gas
• Particles move freely in all directions
• The shape and volume depend on that of the container

## Gas pressure

• Refers to the pressure exerted on a surface by gas
• Measured
• in Pascals, or 'Pa'
• With a Bourdon gauge or pressure sensor
• Appears in air, in which case it's called air or atmospheric pressure
• Vacuum
• Absence of air
• Magdeburg hemispheres
• Two hollow hemispheres form an empty sphere
• The hemispheres cannot be separated
• The air pressure on the outside greatly exceeds that of the inside

## Density

• $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$
• If an object is submerged in a liquid or gas, it:
• Floats if its density is less than that of the liquid or gas
• Sinks if its density is more than that of the liquid or gas
• Density of water = 1
• Hot-air balloons
• There is an engine in the balloon
• As the engine is heated, the particles inside the balloon move more and are therefore further apart
• As density of the air inside the balloon deceases, it floats.
• Ships
• The overall density of ships are under that of the water because there is a lot of air inside
• Submarines
• Whether it is submerged or surfaced depends on the ballast tank
• When the ballast tank is filled with seawater, it sinks
• When the water is pumped out, it surfaces

## Thermal expansion and contraction

• Particles move more vigorously when heated, and are further apart as a result
• This makes the object larger, or expands
• When they are cooled, the particles move less vigorously and are closer together
• The object is made smaller, or contracts
• Bimetallic strips
• Made of two metal strips composed of different metals
• The metals do not expand to the same extent
• The strip bends when heated as a result
• Used in
• Thermostats
• The connection is cut off when an appliance is too hot
• The bimetallic strip in the appliance expands and therefore no longer completes the circuit
• Fire alarm
• The circuit is closed when the fire alarm is too hot
• This makes the alarm ring because the built-in bimetallic strip completes the circuit