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Density and the particle model
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Why does a ship float but a pebble sink? Density, and the way particles are packed, explains it.
Density
density = mass ÷ volume
Units: g/cm³ or kg/m³. Water has a density of 1 g/cm³.
An object floats in water if its density is less than 1 g/cm³, and sinks if it is more.
Solids and liquids are usually much denser than gases, because their particles are packed closely together.
Units: g/cm³ or kg/m³. Water has a density of 1 g/cm³.
An object floats in water if its density is less than 1 g/cm³, and sinks if it is more.
Solids and liquids are usually much denser than gases, because their particles are packed closely together.
Particles and temperature
When a substance is heated, its particles move faster and usually spread out a little, so it expands.
The energy stored in the moving and arranged particles is the internal energy of the material. Heating increases it.
Ice is unusual: water expands when it freezes, so ice is less dense than liquid water and floats. Most substances are densest as solids.
The energy stored in the moving and arranged particles is the internal energy of the material. Heating increases it.
Ice is unusual: water expands when it freezes, so ice is less dense than liquid water and floats. Most substances are densest as solids.
Random motion and change
Tiny specks of smoke or pollen jiggle about randomly because fast-moving air or water particles keep hitting them: Brownian motion. This is evidence that particles move randomly.
Random motion also causes diffusion: particles spread from high to low concentration, faster in gases than in liquids.
Physical changes (changes of state, dissolving) can be reversed and keep the mass the same. Chemical changes make new substances.
Random motion also causes diffusion: particles spread from high to low concentration, faster in gases than in liquids.
Physical changes (changes of state, dissolving) can be reversed and keep the mass the same. Chemical changes make new substances.
A metal block has a mass of 270 g and a volume of 100 cm³. Find its density. Will it float in water?
- density = mass ÷ volume
- 270 ÷ 100 = 2.7 g/cm³
- 2.7 is more than 1 g/cm³, the density of water.
Answer: 2.7 g/cm³; it sinks.
Density = mass ÷ volume. Less dense than water (1 g/cm³) floats. Heating makes particles move faster and spread out; internal energy rises. Ice is less dense than water. Brownian motion shows particles move randomly.
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