Physical and Chemical Physical and Chemical ChangesChanges

Chapter 2 Section 4Chapter 2 Section 4

Energy and ChangeEnergy and Change

Two kinds of changes in matter:Two kinds of changes in matter:Physical ChangePhysical ChangeChemical ChangeChemical Change

Physical ChangePhysical Change

A physical change A physical change alters the form of a alters the form of a substance, but does not change it to substance, but does not change it to another substance.another substance.

Examples are: Examples are: tearing paper, when a nail tearing paper, when a nail bends, spinning wool into yarnbends, spinning wool into yarn..

Chemical ChangeChemical Change

When a substance undergoes a chemical When a substance undergoes a chemical change, change, it is changed into a different it is changed into a different substance with different propertiessubstance with different properties..

Examples Examples burning wood, turning sugar into burning wood, turning sugar into caramel.caramel.

Thermal and Chemical EnergyThermal and Chemical Energy

Every substance has energy from the movement Every substance has energy from the movement of its particles, called of its particles, called thermal energy.thermal energy.

The higher the temperature of a substance, The higher the temperature of a substance, the the greater its thermal energy.greater its thermal energy.

Another form of energy comes from the Another form of energy comes from the chemical chemical bondsbonds within matter. within matter.

This form of energy is called This form of energy is called chemical energychemical energy..

Changes in MatterChanges in Matter

Any substance can either Any substance can either gain energy or lose gain energy or lose energy.energy.

In either case, the substance In either case, the substance changeschanges in some in some way.way.

Matter changes whenever energy is Matter changes whenever energy is added or added or taken away.taken away.

When something is heated, When something is heated, it gains thermal it gains thermal energyenergy

When something is cooled, When something is cooled, it loses thermal it loses thermal energy.energy.

Law of Conservation of EnergyLaw of Conservation of Energy

In every physical change and chemical change, In every physical change and chemical change, the total amount of energy the total amount of energy stays the samestays the same. .

The word The word conservation conservation comes fromcomes from conserveconserve which means “which means “to protect from lossto protect from loss.” .”

Energy can change from Energy can change from one form to another but one form to another but can never be lost.can never be lost.

For example, an unlit match contains For example, an unlit match contains chemical chemical energyenergy. When it is lit, the chemical energy . When it is lit, the chemical energy changes changes to light and thermal energyto light and thermal energy..

The total amount of chemical energy that was in The total amount of chemical energy that was in the match before it was lit is the match before it was lit is equalequal to the light to the light and thermal energy after it is lit.and thermal energy after it is lit.

Changes Between Liquids and Changes Between Liquids and SolidsSolids

Under certain conditions, a substance can Under certain conditions, a substance can change from any change from any one state of matter to one state of matter to any other.any other.

Each substance changes state at Each substance changes state at temperatures temperatures typicaltypical for that substance. for that substance.

But the overall pattern for the way But the overall pattern for the way substances change state is the substances change state is the samesame..

MeltingMelting

The change in state from a solid to a liquid The change in state from a solid to a liquid is is meltingmelting..

In most pure substances, melting occurs In most pure substances, melting occurs at a temperature that is specific to the at a temperature that is specific to the substance, called the substance, called the melting pointmelting point..

The melting point of a substance depends The melting point of a substance depends on on how strongly its particles attract one how strongly its particles attract one another.another.

FreezingFreezing Freezing is the change of state from Freezing is the change of state from liquid to liquid to

solid-just the reverse of melting.solid-just the reverse of melting. When you put liquid water into a freezer, the When you put liquid water into a freezer, the

water water loses energy to the cold airloses energy to the cold air.. The water molecules move more The water molecules move more slowlyslowly.. This means that the temperature of the water is This means that the temperature of the water is

droppingdropping.. When the temperature reaches When the temperature reaches 0 degrees 0 degrees

CelsiusCelsius, the molecules are moving so slowly that , the molecules are moving so slowly that they form they form regular patternsregular patterns. These patterns are . These patterns are the the ice crystalsice crystals that form ice. that form ice.

Changes Between Liquid and GasChanges Between Liquid and Gas

The change from liquid water to water The change from liquid water to water vapor is an example of vapor is an example of vaporizationvaporization..

Vaporization occurs when a Vaporization occurs when a liquid gains liquid gains enough energy to become a gas.enough energy to become a gas.

There are two main types of vaporizationThere are two main types of vaporizationEvaporation – Evaporation – takes place at the surface of takes place at the surface of

a liquida liquidBoiling – Boiling – takes place inside a liquid as well takes place inside a liquid as well

as on the surfaceas on the surface..

EvaporationEvaporation

An example of evaporation is An example of evaporation is when a when a puddle dries up after it rains.puddle dries up after it rains.

The water in the puddle The water in the puddle gains enough gains enough energy from the ground, air, and the sun to energy from the ground, air, and the sun to change the particles on the surface of the change the particles on the surface of the liquid from a liquid to a gas.liquid from a liquid to a gas.

The energy enables the molecules The energy enables the molecules on the on the surfacesurface of the puddle to escape into the of the puddle to escape into the air.air.

BoilingBoiling

Each liquid boils at a specific temperature, Each liquid boils at a specific temperature, called its called its boiling pointboiling point..

Like the melting point of a solid, the boiling Like the melting point of a solid, the boiling point of a liquid point of a liquid depends on how strongly depends on how strongly the particles of the substance are attracted the particles of the substance are attracted to one another.to one another.

Boiling Point and Air PressureBoiling Point and Air Pressure Boiling point also depends on the Boiling point also depends on the pressure of pressure of

the air above a liquid.the air above a liquid. The The lower the air pressurelower the air pressure above the liquid, the above the liquid, the

less energy the liquid molecule need to escape less energy the liquid molecule need to escape into the air.into the air.

As you go up in elevation, As you go up in elevation, air pressure air pressure decreases.decreases.

At the air pressure near sea level, the boiling At the air pressure near sea level, the boiling point is point is 100 degrees Celsius100 degrees Celsius..

In the mountains, air pressure is In the mountains, air pressure is lowerlower and so is and so is the boiling point of water.the boiling point of water.

CondensationCondensation

The opposite of vaporization is called The opposite of vaporization is called condensationcondensation..

Condensation occurs when a Condensation occurs when a gas loses enough gas loses enough thermal energy to become a liquidthermal energy to become a liquid..

Examples are Examples are clouds form when water vapor in clouds form when water vapor in the atmosphere condenses into liquid dropletsthe atmosphere condenses into liquid droplets..

When the droplets get heavy enough, When the droplets get heavy enough, they fall to they fall to the ground as rain.the ground as rain.

Condensation cont.Condensation cont.

When you observe condensation by breathing onto When you observe condensation by breathing onto a mirror, you can see the warm water vapor in a mirror, you can see the warm water vapor in your breath condense on the cooler surface of your breath condense on the cooler surface of the mirror.the mirror.

The droplets then evaporate in water vapor again.The droplets then evaporate in water vapor again.

You cannot see water vapor in the air. When you You cannot see water vapor in the air. When you see steam from water boiling it is tiny droplets of see steam from water boiling it is tiny droplets of liquid water suspended in the air.liquid water suspended in the air.

Changes Between Solid and GasChanges Between Solid and Gas

SublimationSublimation occurs when the surface occurs when the surface particles of a solid gain enough energy to particles of a solid gain enough energy to become a gas.become a gas.

Particles do not pass through the Particles do not pass through the liquid liquid statestate at all. at all.

An example of sublimation An example of sublimation is the change is the change that dry ice undergoes.that dry ice undergoes.

Dry ice is the common name for Dry ice is the common name for solid solid carbon dioxide.carbon dioxide.

Why do we use Dry Ice?Why do we use Dry Ice?

As dry ice changes from a solid to a liquid, As dry ice changes from a solid to a liquid, it absorbs energyit absorbs energy, so materials near it stay , so materials near it stay cold and drycold and dry..

For this reason using dry ice is an For this reason using dry ice is an excellent way to excellent way to keep temperatures lowkeep temperatures low when a refrigerator is unavailable.when a refrigerator is unavailable.

When dry ice becomes a gas, When dry ice becomes a gas, it cools it cools water vapor in the air, which form fogwater vapor in the air, which form fog..

Chemical ChangesChemical Changes

Chemical Reaction is another name for Chemical Reaction is another name for chemical chemical change.change.

In some chemical reactions, In some chemical reactions, one substance one substance breaks down into two or more other substancesbreaks down into two or more other substances..

In other reactions, In other reactions, two or more substances two or more substances combinecombine, forming one or more new substances., forming one or more new substances.

All chemical reactions have one thing in All chemical reactions have one thing in common. common. They all produce new substancesThey all produce new substances..

Energy and Chemical ReactionsEnergy and Chemical Reactions

Like all changes in matter, chemical reactions Like all changes in matter, chemical reactions occur when substances occur when substances gain or lose energygain or lose energy..

All chemical reactions either All chemical reactions either absorb energy or absorb energy or release energy.release energy.

What is the source of energy that a plant uses to What is the source of energy that a plant uses to grow?grow?

A plant changes the A plant changes the sun’s light energysun’s light energy into the into the chemical energychemical energy that it needs to make it grow. that it needs to make it grow.

Stored Chemical EnergyStored Chemical Energy

Chemical energy that has been absorbed and Chemical energy that has been absorbed and stored can be stored can be later releasedlater released..

An example is An example is burning wood in a woodstoveburning wood in a woodstove.. Wood combines with Wood combines with oxygenoxygen in the air, in the air,

producing substances such as producing substances such as carbon dioxide, carbon dioxide, water, and soot.water, and soot.

Chemical energy is transformed into Chemical energy is transformed into thermal and thermal and electromagnetic energy during this processelectromagnetic energy during this process..

Your body Your body releases chemical energyreleases chemical energy that you that you obtain from foods that you eat.obtain from foods that you eat.

Controlling Chemical EnergyControlling Chemical Energy How can you make a chemical reaction occur How can you make a chemical reaction occur

faster or slower?faster or slower? You just have to You just have to add energy or take energy add energy or take energy

away.away. If you are baking a cake, and the kitchen is If you are baking a cake, and the kitchen is

warm, warm, reactions in the cake batter happen reactions in the cake batter happen faster.faster.

The hotter the oven the The hotter the oven the fasterfaster the cake bakes. the cake bakes. You You discontinuediscontinue the chemical reaction by taking the chemical reaction by taking

the cake the cake out of the ovenout of the oven.. Putting the cake in the Putting the cake in the refrigeratorrefrigerator, slows down , slows down

the the chemical reactionschemical reactions that make the cake spoil. that make the cake spoil.