week 7

Question 1

cohesion

Answer 1

the attractive forces between substances that are alike

Question 2

adhesion

Answer 2

the attractive forces between unlike substances

Question 3

cohesive forces are responsible for

Answer 3

surface tension

Question 4

surface tension

Answer 4

a property of liquid surfaces resulting from intermolecular bonding which causes the liquid to minimize its surface & resist its deformation

Question 5

surface tension acts to

Answer 5

reduce the surface area of a body of liquid

Question 6

the size of the surface tension can be measured by determining

Answer 6

the force required to hold in place a wire, being used to stretch a film of liquid

Question 7

surface tension equation

Answer 7

γ = F / L

Question 8

we usually multiply the length by 2 when solving for surface tension

Answer 8

the length over which the force is being applied in the diagram shown is twice the length of the wire, as the liquid film has two surfaces

Question 9

surface tension γ has units

Answer 9

N . m^-1

Question 10

in order to prevent surface tension from collapsing a bubble…

Answer 10

the pressure inside the bubble must be greater than the outside pressure

Question 11

when two bubbles collide

Answer 11

air always flows from the smaller to the larger bubble

(this is because pressure is greater in the smaller bubble)

Question 12

to calculate gauge pressure

Answer 12

ΔP = 4γ / r

Question 13

surfactant

Answer 13

a substance that, when added to a liquid, reduces the liquid’s surface tension

Question 14

surfactant molecules tend to

Answer 14

concentrate near the surface

Question 15

what makes a substance a surfactant?

Answer 15

having one hydrophilic and one hydrophobic side

Question 16

surfactants are of major importance to

Answer 16

lung function

Question 17

interfacial tension

Answer 17

the tension at the interface (a point where 2 systems meet/interact)

Question 18

what determines interfacial tension?

Answer 18

the size of the adhesive forces

(usually between the liquid and walls of the container)

Question 19

what are two types of interfacial tension?

Answer 19

1. beading
2. wetting

Question 20

water beading an oily/waxy surface

Answer 20

• the cohesive intermolecular forces between water molecules are larger than the adhesive forces between water molecules and molecules on the surface
• this results in a compact droplet with a large contact angle, θ.

Question 21

water wetting a hydrophilic surface (ex: glass)

Answer 21

• the adhesive forces between the water and the surface are stronger than the cohesive forces between the water molecules
• the droplet spreads out, and the contact angle is low

Question 22

_____ ‘wets’ the surface of glass, while _____ does not ‘wet’ the surface of glass

Answer 22

water, mercury

Question 23

contact angle

Answer 23

• a quantitative measure of the tendency to bead
• the angle that the edge of the liquid-air surface makes with the liquid-solid surface

Question 24

contact angles more than 90˚are indicative of

Answer 24

beading

Question 25

contact angles less than 90˚are indicative of

Answer 25

wetting

Question 26

if the cohesive forces are weak in comparison to the adhesive forces for a fluid in a vertical tube, the contact angle will be _____ giving a _____ meniscus

Answer 26

small, negative

Question 27

if the cohesive forces are strong in comparison to the adhesive forces for a fluid in a vertical tube, the contact angle will be _____ giving a _____ meniscus

Answer 27

large, positive

Question 28

capillary action

Answer 28

when a thin glass tube is placed in a liquid such as water, the liquid often rises up the tube

Question 29

why does more liquid rise the smaller the tube is?

Answer 29

a smaller radius means more contact with the surface for a particular volume of liquid, and hence a greater mass of liquid that can be supported by the contact force

Question 30

h = 2𝛾𝑐𝑜𝑠𝜃 / 𝜌𝑔𝑟

Answer 30

h - height the liquid travels up the tube above the level of surrounding liquid γ - surface tension 𝜃 - contact angle (angle liquid surface makes with tube surface) 𝜌 - density r - tube radius g - acceleration due to gravity