1
00:00:06,738 --> 00:00:10,329
Why does salt dissolve in water but oil doesn't?

2
00:00:10,329 --> 00:00:12,124
Well, in a word, chemistry,

3
00:00:12,124 --> 00:00:14,328
but that's not very satisfying, is it?

4
00:00:14,328 --> 00:00:17,442
Well, the reason salt dissolves and oil does not

5
00:00:17,442 --> 00:00:19,152
comes down to the two big reasons

6
00:00:19,152 --> 00:00:21,348
why anything happens at all:

7
00:00:21,348 --> 00:00:22,281
energetics

8
00:00:22,281 --> 00:00:23,489
and entropy.

9
00:00:23,489 --> 00:00:25,322
Energetics deals primarily

10
00:00:25,322 --> 00:00:28,203
with the attractive forces between things.

11
00:00:28,203 --> 00:00:31,155
When we look at oil or salt in water,

12
00:00:31,155 --> 00:00:34,152
we focus on the forces between particles

13
00:00:34,152 --> 00:00:36,779
on a very, very, very small scale,

14
00:00:36,779 --> 00:00:38,180
the molecular level.

15
00:00:38,180 --> 00:00:39,898
To give you a sense of this scale,

16
00:00:39,898 --> 00:00:41,599
in one glass of water,

17
00:00:41,599 --> 00:00:43,110
there are more molecules

18
00:00:43,110 --> 00:00:45,816
than known stars in the universe.

19
00:00:45,816 --> 00:00:49,448
Now, all of these molecules are in constant motion,

20
00:00:49,448 --> 00:00:52,460
moving, vibrating, and rotating.

21
00:00:52,460 --> 00:00:54,774
What prevents almost all of those molecules

22
00:00:54,774 --> 00:00:56,858
from just flying out of the glass

23
00:00:56,858 --> 00:01:00,270
are the attractive interactions between molecules.

24
00:01:00,270 --> 00:01:01,230
The strength of the interactions

25
00:01:01,230 --> 00:01:04,482
between water, itself, and other substances

26
00:01:04,482 --> 00:01:07,733
is what we mean when we say energetics.

27
00:01:07,733 --> 00:01:09,874
You can think of the water molecules engaging

28
00:01:09,874 --> 00:01:11,419
in a constant dance,

29
00:01:11,419 --> 00:01:13,047
sort of like a square dance

30
00:01:13,047 --> 00:01:16,501
where they constantly and randomly exchange partners.

31
00:01:16,501 --> 00:01:19,185
Put simply, the ability for substances

32
00:01:19,185 --> 00:01:20,676
to interact with water,

33
00:01:20,676 --> 00:01:22,298
balanced with how they disrupt

34
00:01:22,298 --> 00:01:24,881
how water interacts with itself,

35
00:01:24,881 --> 00:01:27,230
plays an important role in explaining

36
00:01:27,230 --> 00:01:29,899
why certain things mix well into water

37
00:01:29,899 --> 00:01:31,649
and others don't.

38
00:01:32,048 --> 00:01:33,912
Entropy basically describes

39
00:01:33,912 --> 00:01:36,631
the way things and energy can be arranged

40
00:01:36,631 --> 00:01:38,503
based on random motion.

41
00:01:38,503 --> 00:01:41,200
For example, think of the air in a room.

42
00:01:41,200 --> 00:01:44,062
Imagine all the different possible arrangements

43
00:01:44,062 --> 00:01:46,650
in space for the trillions of particles

44
00:01:46,650 --> 00:01:48,131
that make up the air.

45
00:01:48,131 --> 00:01:49,167
Some of those arrangments

46
00:01:49,167 --> 00:01:52,163
might have all the oxygen molecules over here

47
00:01:52,163 --> 00:01:54,814
and all the nitrogen molecules over there,

48
00:01:54,814 --> 00:01:55,855
separated.

49
00:01:55,855 --> 00:01:58,230
But far more of the possible arrangements

50
00:01:58,230 --> 00:02:01,230
have those molecules mixed up with one another.

51
00:02:01,230 --> 00:02:03,481
So, entropy favors mixing.

52
00:02:03,481 --> 00:02:06,334
Energetics deals with attractive forces.

53
00:02:06,334 --> 00:02:08,417
And so, if attractive forces are present,

54
00:02:08,417 --> 00:02:10,321
the probability of some arrangements

55
00:02:10,321 --> 00:02:11,500
can be enhanced,

56
00:02:11,500 --> 00:02:13,780
the ones where things are attracted to each other.

57
00:02:13,780 --> 00:02:16,294
So, it is always the balance of these two things

58
00:02:16,294 --> 00:02:18,871
that determines what happens.

59
00:02:18,871 --> 00:02:20,285
On the molecular level,

60
00:02:20,285 --> 00:02:22,664
water is comprised of water molecules,

61
00:02:22,664 --> 00:02:25,829
made up of two hydrogen atoms and an oxygen atom.

62
00:02:25,829 --> 00:02:28,497
As liquid water, these molecules are engaged

63
00:02:28,497 --> 00:02:30,831
in a constant and random square dance

64
00:02:30,831 --> 00:02:34,386
that is called the hydrogen bonding network.

65
00:02:34,386 --> 00:02:35,771
Entropy favors keeping

66
00:02:35,771 --> 00:02:38,018
the square dance going at all times.

67
00:02:38,018 --> 00:02:39,314
There are always more ways

68
00:02:39,314 --> 00:02:41,142
that all the water molecules can arrange

69
00:02:41,142 --> 00:02:42,349
in a square dance,

70
00:02:42,349 --> 00:02:44,790
as compared to if the water molecules

71
00:02:44,790 --> 00:02:45,421
did a line dance.

72
00:02:45,421 --> 00:02:47,781
So, the square dance constantly goes on.

73
00:02:47,781 --> 00:02:50,750
So, what happens when you put salt in the water?

74
00:02:50,750 --> 00:02:52,748
Well, on the molecular level,

75
00:02:52,748 --> 00:02:55,584
salt is actually made up of two different ions,

76
00:02:55,584 --> 00:02:57,838
chlorine and sodium,

77
00:02:57,838 --> 00:03:00,082
that are organized like a brick wall.

78
00:03:00,082 --> 00:03:01,502
They show up to the dance

79
00:03:01,502 --> 00:03:03,386
as a big group in formation

80
00:03:03,386 --> 00:03:05,040
and sit on the side at first,

81
00:03:05,040 --> 00:03:07,431
shy and a bit reluctant to break apart

82
00:03:07,431 --> 00:03:10,272
into individual ions to join the dance.

83
00:03:10,272 --> 00:03:12,456
But secretly, those shy dancers

84
00:03:12,456 --> 00:03:15,009
just want someone to ask them to join.

85
00:03:15,009 --> 00:03:18,233
So, when a water randomly bumps into one of them

86
00:03:18,233 --> 00:03:20,817
and pulls them into the dance away from their group,

87
00:03:20,817 --> 00:03:21,983
they go.

88
00:03:21,983 --> 00:03:23,362
And once they go into the dance,

89
00:03:23,362 --> 00:03:24,948
they don't come back out.

90
00:03:24,948 --> 00:03:27,352
And in fact, the addition of the salt ions

91
00:03:27,352 --> 00:03:29,412
adds more possible dance positions

92
00:03:29,412 --> 00:03:30,790
in the square dance,

93
00:03:30,790 --> 00:03:33,973
so it is favored for them to stay dancing with water.

94
00:03:35,156 --> 00:03:37,119
Now, let's take oil.

95
00:03:37,119 --> 00:03:39,800
With oil, the molecules are sort of interested

96
00:03:39,800 --> 00:03:40,887
in dancing with water,

97
00:03:40,887 --> 00:03:43,138
so entropy favors them joining the dance.

98
00:03:43,138 --> 00:03:45,193
The problem is that oil molecules

99
00:03:45,193 --> 00:03:47,471
are wearing gigantic ballgowns,

100
00:03:47,471 --> 00:03:50,040
and they're way bigger than water molecules.

101
00:03:50,040 --> 00:03:52,474
So, when an oil molecule gets pulled in,

102
00:03:52,474 --> 00:03:55,299
their size is really disruptive to the dance

103
00:03:55,299 --> 00:03:56,919
and the random exchange of partners

104
00:03:56,919 --> 00:03:58,622
that the waters engage in,

105
00:03:58,622 --> 00:04:00,329
a very important part of the dance.

106
00:04:00,329 --> 00:04:02,885
In addition, they are not great dancers.

107
00:04:02,885 --> 00:04:04,805
The water molecules try to engage

108
00:04:04,805 --> 00:04:06,464
the oil molecules in the dance,

109
00:04:06,464 --> 00:04:08,890
but they just keep bumping into their dresses

110
00:04:08,890 --> 00:04:11,550
and taking up all the room on the dance floor.

111
00:04:11,550 --> 00:04:13,723
There are way more ways the waters can dance

112
00:04:13,723 --> 00:04:15,765
when the oil gets off the floor,

113
00:04:15,765 --> 00:04:18,214
so the waters squeeze out the oil,

114
00:04:18,214 --> 00:04:20,803
pushing it back to the bench with the others.

115
00:04:20,803 --> 00:04:22,887
Pretty soon, when a large number of oils

116
00:04:22,887 --> 00:04:24,971
have been squeezed over to the side,

117
00:04:24,971 --> 00:04:27,280
they band together to commiserate

118
00:04:27,280 --> 00:04:29,455
about how unfair the waters are being

119
00:04:29,455 --> 00:04:31,408
and stick together as a group.

120
00:04:31,408 --> 00:04:32,746
So, it is this combination

121
00:04:32,746 --> 00:04:35,082
of the interactions between molecules

122
00:04:35,082 --> 00:04:37,472
and the configurations available to them

123
00:04:37,472 --> 00:04:38,890
when they're moving randomly

124
00:04:38,890 --> 00:04:41,216
that dictates whether they mix.

125
00:04:41,216 --> 00:04:43,637
In other words, water and oil don't mix

126
00:04:43,637 --> 00:04:47,217
because they just don't make great dance partners.