5.1 Liquids and Gases

Fluid – a form of matter that flows when any force is applied, no matter how small. Liquids AND gases are fluids.

Liquid – a phase of matter that can flow and change shape, but has constant volume

Gas – phase of matter with high energy molecules that can expand to fill a container

Pressure – a distributed force per unit area that acts within a fluid. A force applied to a fluid creates pressure

Pressure acts in all directions and is caused by the collisions of many, many atoms.

Intermolecular Forces (IMF) - forces between separate atoms and molecules that are attractive at a distance, but repulsive at close range. That way they don’t overlap.

2 types of forces on atoms:

Strongest forces are between atoms bonded together into molecules and compounds.

A weaker force acts between molecules, or between atoms NOT bonded together. Example: Intermolecular forces hold water together in liquid water and in ice.

The Role of Thermal Energy:

The phases of matter (solid, liquid, and gas) exist because of competition between thermal energy and molecular forces.

Intermolecular forces try to bring molecules close. Thermal energy causes molecules to vibrate and spread apart.

High Thermal Energy (High Temperature) IMF is broken and molecules spread apart in a gas.

Medium Thermal Energy – in a liquid, molecules partially break apart IMF and move around, but not enough to escape.

Low Thermal Energy – fixed in a solid

Food coloring in hot and cold water:

https://www.youtube.com/watch?v=YbhwXSv9rk4

https://www.youtube.com/watch?v=bN7E6FCuMbY

Melting and Boiling

Melting point – the temperature at which a substance changes from a solid to a liquid

Melting point of water is 0*C

Different substances have different melting points because IMF have different strengths.

Iron melts at 1,500*C

IMF in iron are STRONGER than IMF in water

Material Melting Point *C

Boiling Point *C

Tungsten 3,422 5,555Iron 1,538 2,861Copper 1,085 2,562Aluminum 660 2,519Lead 327 1,749Hard Plastic 240 300Candle Wax 50 400Water 0 100Alcohol -108 78Nitrogen -210 -196Oxygen -219 -183Helium none -269

Fire Ant Colony with liquid aluminum:

https://www.youtube.com/watch?v=IGJ2jMZ-gaI

Boiling – when enough Thermal Energy is added, IMF are completely broken and liquid becomes gas. The temp at which liquid becomes gas is called the boiling point.

Changes in phase require energy

Heat is added to ice, temp increases until it reaches 0*C

Then temp stops increasing! When all ice is melted, temp starts to increase again

We need some materials to be solid at room temp, others to be liquid, and others to be a gas.

Most materials have a higher density as a solid than as a liquid

EXCEPT WATER!

Solid water is a 6-sided crystal structure (snowflake) and molecules spread out! Water expands! ICE floats because it is less dense than water.

Density of ice = 0.92 g/cm3

Density of water 1.0 g/cm3

Ice is less dense than liquid water, so it floats on the surface of lakes and ponds when they freeze in winter, which prevents the water below from reaching freezing temperatures.

Oxygen and Nitrogen are gases at room temp. If the temp gets low enough, they will become liquid and even solid.

Liquid Nitrogen -196*C can rapidly freeze materials

Liquid Oxygen is used in rockets, because in outer space, there is no oxygen to burn fuel

Making ice cream with liquid nitrogen:

https://www.youtube.com/watch?v=utjdLDojbCw

Liquid oxygen:

https://www.youtube.com/watch?v=7NXfyCezUFk

Evaporation and Condensation:

Evaporation – change from a liquid to a gas at a temperature below the boiling point. Evaporation of sweat from your skin on a hot day cools your body by carrying away energy

Condensation – change from gas to a liquid at a temp below the boiling point. Water vapor molecules stick to a cool surface and form drops of liquid water

Relative humidity – ordinary air contains water vapor. Evaporation adds water; condensation removes water. You feel hot and sticky on a humid day

Convection – transfer of heat through the motion of fluids such as air and water

Natural convection – fluids expand when they heat up. Hot water or air rises up, cooler air or water sinks to the bottom.

Forced convection uses a pump or fan to blow air or water around

Atmosphere – air feels light because it is 1000 times less dense than water

Air:

76.6% Nitrogen

20.6 % Oxygen

1% Water Vapor

1.8 % Argon, Carbon Dioxide, and other gases

Weather: Earth’s weather is created by gigantic convection currents. Energy from the Sun warms the ground. Air near the ground warms up and rises.

Rain – at high altitudes, temperature of air drops and water vapor condenses

Two gas laws:

Boyle’s Law: For a fixed amount of a gas at a fixed temperature, pressure and volume are inversely proportional

Charles Law: Gases expand when heated: When pressure on a gas is held constant, temperature and volume are directly related