Chapter 14 – Liquids and Solids

14.1 Water and Its Phase ChangesPgs 492 - 497

Water’s Unique Properties

• Water is LESS dense as a solid than a liquid!– Why ice floats on a

lake• Water has a high

surface tension– Hydrogen bonding

forms a protective “skin” on the top of water

• Water has a HIGH boiling and melting point

Water and Its Phase Changes

• What happens when we heat liquid water – think KMT people!!!– The motions of the water molecules increase

as its heated• Disrupts H-bonds between water molecules• Bubbles start to form

• What happens when we reach 100oC?– Reach the BOILING POINT

• The bubbles float to the surface and burst!• The temperature will NOT CHANGE until all of the

water has been turned into vapor– Once all of the water has been converted into vapor, then the

temperature will rise (we are heating the vapor)

Water and It’s Phase Changes

• What happens when we cool water?– The temperature of the water will

decrease until it reaches 0oC• What happens here???

– Reach the FREEZING POINT!

– The temperature will remain at 0oC until all of the liquid water has changed to ice• The temperature will then drop as cooling

continues (of the ice)

Heating/Cooling Curve of Water

Amount of Heat Need Between Phase

Changes (when you heat a substance)• Use the Specific Heat equation

from 1st semester!!!– Allows us to determine how much heat

we need to reach the next phase change!!

Examples!

• How much heat is needed to heat a pot of water from 16oC to 52 oC if the water has a mass of 112g?

• How much heat is lost or required when 8.2g water vapor is cooled from 112 ˚C to 102˚C

Energy Requirements for the Changes of State

• Changing state from solids to liquid to gas requires energy – why?– You must OVERCOME the intermolecular

forces!!• The energy required to melt 1 mol of a substance is

called the molar heat of fusion– For ice = 6.02 kJ/mol

• The energy required to change 1 mol of a liquid to its vapor is called the molar heat of vaporization

– For water = 40.6 kJ/mol at 100oC

Energy Requirements for the Changes of State

The plateau seen with the vaporization of water is MUCH longer than the plateau seen with melting ice. Why?

Calculating Energy Changes

• How much heat is required to completely melt 23.5g of ice at 0oC?

• How much heat must be removed or added to completely condense 243.1g of water vapor at 100oC?

Calculating Energy Changes:Liquid to Gas

• How much heat is required to make 32g of water at 23oC into water vapor at 100.oC?

Now you try one!

• How much heat is required or released to make 62g of water at 23oC ice at -12oC?