Change of state

Question 1

When does a substance melt? similarly boil?

Answer 1

• if it is a pure substance, it melts at a well-defined temperatire called its melting point

Question 2

What are the different physical properties of the different states

Answer 2

• density
  
  • gas much less dense
  • in solid and liquid are packed together an in contact with eachother
  • molecules of gas are on average seperated by relatively large distances
• flow
  
  • liquids and gases can flow, but soolids can’t
  • atoms in solid are locked together by strong force bonds, whcih are unable to break free form
  • liquid and gases bonds are not locked together because they have too much EK, and the force bonds are not strong enough to keep the molc fixed to eachother

Question 3

What is the latent heat of fusion?

Answer 3

• when a solid is heated at its melting point, atoms vibrate so much that they break free from eachother
• solid becomes liquid due to energy being supplied at the melting point
• the energy needed to melt a solid at its melting point is the latent heat of fusion

Question 4

What happens when a liquid solodifed?>

Answer 4

• latent heat is released when a liquid soloidifed
• this happens because the liquid molc slow down as the liquid cool until the temp decreases to the melting point
• at the melting point, the molc mopve slwoly enough for the force bonds to lock the molc together
• some of the latent heat released keeps the temp at the melting point until all the liquid has solidified

Question 5

Why is latent heat said to be latent?

Answer 5

• latent means hidden
• latent heat supplied to melt a solid may be though of as hidden because no temp change takes place even tho the solid is being heated

Question 6

Why is the latent heat of fusion?

Answer 6

• fusion is a word used for the melting point of a solid because the solid fuses into a liquid as it melts

Question 7

What is latent heat of vaporisation

Answer 7

• when a liquid is heated at its boiling point, the molc gain enough KE to overcome the bonds that hold them close together
• the molc therefore break away from eachother to form bubbles of vapour in the liquid
• the energy needed to vaporise a liquid is called the latent heat of vaporisation

Question 8

Why is latent heat released when a vapour condenses

Answer 8

• vapour molc slow down as the vapour is cooled
• the molc move slowly enough for the force bonds to pull the molc together to form a liquid

Question 9

What is sublimation

Answer 9

some solid vaporise directly when heated

this is sublimation

Question 10

How do the energy for fusion and vaporisation compare?

Answer 10

• energy to vaporise a substance is more than to melt it

Question 11

What is the specific latent heat of fusion?

Answer 11

• l_f
• energy needed to change the state of unit mass of a substance from solid to liquid without change of temperature

Question 12

What is the specific latent heat of vaporisation?

Answer 12

• energy needed to change the state of unit mass of the substance from liquid to vapour without change of temperature

Question 13

What does the equation Q = ml mean?

Answer 13

• Q is the energy needed change the state of mass m without change out temperature
• where l is specific latent heat of fusion or vaporisation

Question 14

What are the units of specific latent heat?

Answer 14

• J kg^-1

Question 15

What does the temperature-time graph look like if a pure solid is heated to its melting point and beyond

Answer 15

• assuming that no heat loss occurs during heating, assuming that energy is transferred to the substance at a constant rate (power supplied)

Question 16

How can the temperature rise per second be measured from the graph?

Answer 16

• P = mc (ΔT / t)
• (ΔT / t) = P / mc

Question 17

How does specific heat capacity of a substance change?

Answer 17

• usually it is lower in liquid
• temperature rise greater in liquid for a given amount of energy