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**Chapter 2 Energy and Matter**

2.7 Changes of State

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**Melting and Freezing A substance**

is melting when it changes from a solid to a liquid is freezing when it changes from a liquid to a solid such as water has a freezing (melting) point of 0 °C

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**Heat of Fusion The heat of fusion**

is the amount of heat released when 1 gram of liquid freezes (at its freezing point) is the amount of heat needed to melt 1 gram of a solid (at its melting point) for water (at 0 °C) is 334 J or 80 cal 1 g of water

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**Guide to Calculations Using Heat of Fusion (or Vaporization)**

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**Calculations Using Heat of Fusion**

How much heat in calories is needed to melt 15.0 g of ice at 0 °C ? STEP 1 Given: 15.0 g of water(s) change of state: melting at 0 °C STEP 2 Plan: g of water(s) g of water(l) STEP 3 Write conversion factors: 1 g of water = cal 1 g of water and cal 80 cal g of water

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**Calculations Using Heat of Fusion (continued)**

STEP 4 Set up the problem to calculate calories: 15.0 g water x cal = cal 1 g water

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**Learning Check How many joules are released when 25.0 g of water**

at 0 °C freezes? 1) 334 J 2) J 3) J

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**Solution 3) 8350 J STEP 1 Given: 25.0 g of water(l)**

change of state: freezing at 0 °C STEP 2 Plan: g of water g of water STEP 3 Write conversion factors: 1 g of water = 334 J 1 g of water and __334 J___ 334 cal 1 g of water

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**Solution (continued) STEP 4 Set up the problem to calculate joules:**

25.0 g water x J = J (3) 1 g water

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**Sublimation Sublimation occurs when a solid changes directly to a gas**

is typical of dry ice, which sublimes at 78 C takes place in frost-free refrigerators is used to prepare freeze-dried foods for long-term storage

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**Evaporation and Condensation**

Water evaporates when molecules on the surface gain sufficient energy to form a gas condenses when gas molecules lose energy and form a liquid

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**Boiling At boiling, water molecules acquire the energy to form a gas**

bubbles appear throughout the liquid

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**Heat of Vaporization The heat of vaporization is the amount of heat**

absorbed to vaporize 1 g of a liquid to gas at the boiling point released when 1 g of a gas condenses to liquid at the boiling point Boiling Point of Water = 100 °C Heat of Vaporization (water) = J or 540 cal 1 g of water

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**Comparing Heat of Fusion and Heat of Vaporization**

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**Learning Check How many kilocalories (kcal) are released when 50.0 g**

of water(g) as steam from a volcano condenses at 100 °C? 1) 27 kcal 2) 540 kcal 3) kcal

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**Solution 1) 27 kcal STEP 1 50.0 g of water(g) = 50.0 g of water(l)**

Change of state: water condensing at 100 °C STEP 2 Plan: g of water(g) g of water(l) STEP 3 Write conversion factors: 1 g of water = 540 cal 1 g of water and cal 540 cal g of water 1 kcal = cal 1 kcal and cal 1000 cal kcal

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Solution (continued) STEP 4 Set up the problem to calculate kilocalories: 50.0 g water x cal x 1 kcal = 27 kcal 1 g water cal

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**Summary of Changes of State**

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**Heating Curve A heating curve**

illustrates the changes of state as a solid is heated uses sloped lines to show an increase in temperature uses plateaus (horizontal lines) to indicate a change of state

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Learning Check A. A plateau (horizontal line) on a heating curve represents 1) a temperature change 2) a constant temperature 3) a change of state B. A sloped line on a heating curve represents

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**Solution A. A plateau (horizontal line) on a heating curve represents**

2) a constant temperature 3) a change of state B. A sloped line on a heating curve represents 1) a temperature change

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**Cooling Curve A cooling curve**

illustrates the changes of state as a gas is cooled uses sloped lines to indicate a decrease in temperature uses plateaus (horizontal lines) to indicate a change of state

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**Learning Check Use the cooling curve for water to answer each.**

A. Water condenses at a temperature of 1) 0 °C 2) 50 °C 3) 100 °C B. At a temperature of 0 °C, liquid water 1) freezes 2) melts 3) changes to a gas C. At 40 °C, water is a 1) solid 2) liquid 3) gas D. When water freezes, heat is 1) removed 2) added

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**Solution Use the cooling curve for water to answer each.**

A. Water condenses at a temperature of 3) 100 °C B. At a temperature of 0 °C, liquid water 1) freezes C. At 40 °C, water is a 2) liquid D. When water freezes, heat is 1) removed

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**Combined Heat Calculations**

To reduce a fever, an infant is packed in 250 g of ice. If the ice (at 0 °C) melts and warms to body temperature (37.0 °C), how many calories are removed? STEP 1 Given: 250 g ice changes to water at 37.0 °C Need: calories to melt at 0 °C and warm to 37.0 °C STEP 2 T = 37.0 °C – 0 °C = 37.0 °C 37.0 °C 0 °C

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**Combined Heat Calculations (continued)**

STEP 3 Conversion factors: 1 g of water (0 °C) = 80 cal 1 g of water (0 °C) and cal 80 cal g of water (0 °C) 1 g of water(l) = cal/g °C 1 g of water(l) and cal/g °C 1.00 cal/g °C g of water(l)

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**Combined Heat Calculations (continued)**

STEP 4 Set up problem: (1) Heat to melt ice (fusion) 250 g ice x cal = cal 1 g ice (2) Heat the water(l) from 0 °C to 37.0 °C 250 g x °C x cal = cal g °C Total: cal cal = cal (rounded)

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