Aseel Samaro

Explaining the density of solids and liquids

The particle model is very useful for explaining what density is.

By understanding how to change density, we make it possible to explore the deepest ocean trenches and to float massive tankers on our seas.

Introduction

Density is defined by:

The units of density are g/cm3.

What is density ?

Solids are usually more dense than liquids and gases

There are more particles (and hence more mass) packed in the same volume.

However, some liquids are more dense than some solids because of how the particles are arranged in a solid.

Substance Solid or liquid Density (g/cm3)

brick solid 1.4

paper solid 0.7

ice solid 0.9

iron solid 7.9

alcohol liquid 0.79

baby oil liquid 0.8

water liquid 1.0

syrup liquid 1.3

honey liquid 1.4

The density of a selection of substances

Use the previous table to explain what would happen if you put the following materials together

a. Paper and syrup: the paper floats on top of the syrupb. Iron and alcohol: the iron sinks in the alcoholc. Brick and honey: the brick will float within the liquid; and not on

top; they have the same densityd. Baby oil and water: the baby oil floats on top of the water; they are

immiscible liquids

Much smaller amounts of sweeteners are needed in soft drinks than sugar. How does this explain why a can of sugar-free drink is less

dense than a can of normal drink.

there are fewer particles dissolved using sweeteners; so it is less dense

Temperature: As a solid or liquid is heated, the particles move about more because

they have more kinetic energy, and can move further apart.

The volume of the solid or liquid increases.

The mass of the particles remains unchanged.

The density of a hot liquid will, therefore, be lower than that of a cooler one, enabling it to float on top of the colder one.

Factors affecting density

Dissolved salts: If some salt is dissolved in water, the volume change is very small.

This is because the salt particles fill the spaces between the liquid particles, so the overall volume does not change very much.

In this way, more mass is added with little change in volume, so the density increases.

Floating in the Dead Sea

When a saucepan of liquid is heated, the hotter liquid at the bottom rises. Use ideas about density to explain why this happens.

The particles of the hot liquid have more energy; and so move further apart.This means the particles are less closely packed; and the hot liquid is less dense than the colder liquid; causing the hotter liquid to rise.

The Dead Sea has a salt concentration ten times greater than in normal seas. It is possible for people to just lie and float on top of it,

without effort. Explain why this is possible.There are more dissolved salts in the Dead Sea water; so it is more dense than a human body; so the body floats

This figure shows the density of water at different temperatures. Ice and water are not typical in the way the density changes with

temperature.

Density anomalies: ice and water

Ice, instead of being more dense than cold water, is in fact less dense.

Ice is made of water molecules.

The bonds between the atoms of the molecules are very strong.

In ice, the hydrogen bonds hold the water molecules apart in an open lattice structure. When the ice melts, the rigid hydrogen bonds break and the water molecules come closer together, making it more dense than ice

Look at the graph in Figure 2.3.11c.At what temperature does water have its maximum density?

At what temperature does ice have its minimum density?

How is it possible for fish to survive at the bottom of a frozen lake?Ice is less dense than cold water; and so floats on top. This slows down the freezing process for the water at the bottom; and it remains liquid until it is so cold that the whole body of water freezes. Fish can therefore survive at the bottom of the lake

while the water remains as a liquid.

The world’s largest floating iceberg is 295 km long and 37 km wide; it is larger than the island of Jamaica.

Did you know…?

Thank you