True or False: Physical and Chemical Changes 1st Section

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True or False Physical and Chemical Changes (1st
Section)

• Decide whether these are true or false. Correct
  the ones that are false.
• 1. Density is a chemical property.
• 2. Cutting a carrot in half is a physical change.
• 3. Silver tarnishing is an example of a physical
  change.

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Unit 1 Matter and Energy

• Chapter 2 Properties of Matter
• Section 2 Changes of state are
• physical changes.

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Then and Now

• BEFORE, you learned
• Substances have physical and chemical
  properties
• Physical changes do not change a substance into
  a new substance
• Chemical changes result in new substances

• NOW, you will learn
• How liquids can become solids, and solids can
  become liquids
• How liquids can become gases, and gases can
  become liquids
• How energy is related to changes of state

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Learning Goals

• Describe how liquids can become solids, and
  solids can become liquids.
• Explain how liquids can become gases, and gases
  can become liquids.
• Determine how energy is related to changes in
  state.

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Vocabulary

• melting
• melting point
• freezing
• freezing point
• evaporation
• sublimation
• boiling
• boiling point
• condensation

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Where does dew come from?

• Does dew means its rained recently?
• No
• Water vapor in the air condenses on cool grass
  and plants

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Changes of state

• Why are changes in state physical changes rather
  than chemical ones?
• Changes in state do not change the basic substance

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Solids to Liquids Melting

• Melting is the process by which a solid becomes a
  liquid
• Melting point is the lowest temperature when an
  object begins to melt

• What is a solid?
• Definite shape, definite size, molecules vibrate
  in place
• What is melting?
• Heat causes particles to vibrate faster,
  eventually breaking loose and sliding past one
  another (liquid)

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Melting Point

• Some are very specific
• Ice melts at 32º F, 0ºC
• Some are a range
• Chocolate has a RANGE of melting

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Melting vs. Dissolving

• Melting changes the state of matter
• Dissolving just makes the matter smaller

• Quick Demo
• Sugar cubes and hot water
• Are both at the same temperature?
• Will the wrapped cube become liquid?
• Melting point of sugar is 185º C

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Describe the movement of molecules in a substance
that is at itsmelting point.

• Particles move
• fast enough to slide past
• one another.

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Liquids to Solids Freezing

• Freezing is
• the process by which liquids become a solid
• Freezing Point is
• the specific temperature at which a liquid
  becomes solid
• Particles lose energy and move more slowly
• Many things are frozen at room temperature
• Table, chair, book, anything that is a solid is
  frozen

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Melting vs. Freezing

• What is the relationship between a substances
  melting point and freezing point?
• They are the same. At that temperature, both
  solid and liquid particles exist.

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Liquids to gases evaporation

• Evaporation is
• a process by which a liquid becomes a gas
• Happens at the surface of a liquid
• Particles can escape from the surface and become
  gas
• More heat makes molecules move faster
• This is why water evaporates faster in warm
  weather than cold

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Describe the movement of particles in a liquid as
it evaporates.

• Particles move past one another at different
  speeds. The particles near the surface that have
  the greatest speeds (most kinetic energy) escape
  from the liquid and form vapor.

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Solids straight to gas

• Sublimation is
• the process by which solids lose surface
  particles directly into gas form
• Example
• Dry ice

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• Where does evaporation usually occur?
• At surface of a liquid
• By what process does a solid change directly a
  gas?
• Sublimation
• Evaporation increases with heat

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Liquids to gases boiling

• Boiling is
• another process by which liquid turns to gas
• heating water produces tiny bubbles
• contain dissolved air that is escaping from the
  liquid.
• larger bubbles form and rise
• contain energetic water molecules that have
  escaped from the liquid water to form a gas.
• Boiling point
• is the temperature at which a liquid boils
• Very specific temperature for each liquid

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Boiling Point and Elevation

• Higher elevations (meters above sea level) have
  lower pressure
• Bubbles can form easier
• Boiling point is lower
• Example Leadville, Colorado elevation 3,094m.
  Water boils at 89º C. At sea level, it boils at
  100º C
• Takes longer to cook because the temps arent as
  high
• Different substances boil at different
  temperatures
• Helium, a gas, boils at 270C ( 454F).
• Aluminum boils at 2519C (4566F).

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• How things change

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On your own reading real world examples

• Two problems in a cars fuel line involve changes
  in state. Fuel-line freeze-up occurs when water
  in the fuel tank freezes during cold weather. The
  resulting ice can block the fuel line, and
  gasoline cannot reach the engine. To avoid this
  problem, car owners add materials to the gas tank
  that absorb the water in gasoline. The second
  problem, vapor lock, occurs during warmer
  weather. At a hot spot in the gasoline line,
  gasoline can vaporize, forming a pocket of gas.
  The engine stalls because the fuel pump in the
  car is designed to pump a liquid, not a gas.

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Gases to liquids condensation

• Condensation is
• the process by which gases become liquid
• On a glass of iced tea, where does the water on
  the outside come from?
• the vapor in the air
• Cooling a gas causes the molecules to lose energy
• Touching a cold glass or window
• Condenses into a liquid

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If you remove energy from a liquid,will it
become a solid or a gas?
Name two processes by which a liquid becomes a
gas.
Evaporation and boiling

• A solid

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Phase Changes

• Solid to liquid
• Melting
• Particles get more energy and slide past each
  other
• Solid to gas
• Sublimation
• Molecules skip liquid phase

• Liquid to gas
• Evaporation surface molecules escape
• Boiling increased energy (heat) causes vapor
  bubbles to form

• Gas to liquid
• Condensation
• Cooling vapor causes molecules to lose energy and
  the slow down and get closer together

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1.2.2 Questions

• KEY CONCEPTS
• 1. Describe three ways in which matter can change
  from one state to another.
• 2. Compare and contrast the processes of
  evaporation and condensation.
• 3. How does adding energy to matter by heating it
  affect the energy of its particles?
• CRITICAL THINKING
• 4. Synthesize Explain how water can exist as both
  a solid and a liquid at 0C.
• 5. Apply Explain how a pat of butter at room
  temperature can be considered to be frozen.
• CHALLENGE
• 6. Infer You know that water vapor condenses from
  air when the air temperature is lowered. Should
  it be possible to condense oxygen from air? What
  would have to happen?