a physical property of a substance

the amount of heat required to raise the temperature of 1 kg of a substance by 1 K

SI unit J kg^-1 K^-1

Equation:

Q = cm∆T

Q = Heat added

C = specific heat

M = mass

∆ = change in temp

H₂ O  4.18 KJ

Blood 3.6 KJ

q = m x C_g x (T_f - T_i)
m = 250g
C_g = 4.18 J ^oC^-1 g^-1
T_f = 56^oC
T_i = 20^oC

q = 250 x 4.18 x (56 - 20)
q = 250 x 4.18 x 36
q = 37 620 J = 38 kJ

Q = mL

Q =amount of energy released or absorbed during the change of phase of the substance (in joules

m =mass of the substance

L =specific latent heat for a particular substance (J kg^-1 )

Specific latent heat is found when energy is divided by mass.

is defined as the amount of heat required to raise the temperature of a given object by 1 Kelvin

SI unit of heat capacity J K^-1

Can be applied to an object or substance as a whole.

Is the product of the specific heat capacity and the mass.

(Specific heat capacity of tissues =3.5 kj kg^-1 ºC^-1)

TOTAL HEAT CAPACITY = SPECIFIC HEAT CAPACITY x MASS

For a 70kg patient the toal heat capacity would by 245kJ ºC^-1

ARE LOW DUE TO GASES' LOW DENSITY

This is important in anesthesia.

The specific heat of gas is measured in terms of a volume, it is the heat required to raise 1 liter of gas through 1K (1C).

Extremely small quantity of heat is required or lost when the temperature changes in a small volume of a gas.

the heat required to convert 1 kg of a substance from one phase to another at a given temp

SI unit of specific latent heat J kg^-1

The lower the temp the more latent heat is needed to vaporize a substance.

The temperature at which the latent heat of vaporization becomes ZERO corresponds to its critical temperature of 36.5 ºC.

At this temp, N₂O changes spontaneously from liquid to vapour without the supply of any external energy.

Above this critical temp, N₂O cannot exist as a liquid

Stored as a liquid under pressure in glass ampules with a tap so that a fine jet of the substance can be directed onto the skin.  

Vaporization causes pronounced cooling of the skin, thus impairing conduction in the sensory nerves and providing enough analgesia for minor surgical procedures.

This principle is important to the design of vaporizers.

As gas passes over liquid anesthetic in a vaporizer, the liquid vaporizes, taking latent heat of vaporization from the remaining fluid and from the surrounding vaporizer walls. Thus, the temp of the remaining anesthetic agent and of the vaporizer  walls falls.  But a fall in temperature of the anasthetic in the vaporizer renders it less volatile, lowering its saturated vapour pressure and so reducing the amount of anesthetic vaporized.

If a nitrous oxide cylinder (stored as a liquid) is allowed to empty rapidly. Latent heat is required to convert liquid to gas. Thus temp of the cylinder falls and water vapour from the air may condense or freeze on the outside of the cylinder.  Vapor pressure falls rapidly inside the cylinder, and pressure gauge gives a low reading.  It recovers after cylinder is turned off.

It is more economical to store oxygen as a liquid. Stored at -160 ºC

Critical temp is -119 ºC, it cannot exist as a liquid above this temp.

Stored at -160 C. So it has to pass through a superheated coil. If O2 flows at a fast rate, temp falls due to the removal of latent heat and its vapour pressure falls. Supplementary heat is needed and provided by a pressure raising vaporizer.  A control valve controls the flow of liquid O2 to the pressure-rasking vaporizor. In the vaporizer, the oxygen is warmed and vaporized to the pipeline pressure.

In anesthesia heat is lost by the dry, inspired gases.

This heat loss may contribute to the general problem of hypothermia, esp. in young children. 

Heat loss can be avoided by humidifying inspired gases and is reduced when an anesthetic circle system with soda lime is used.