AP Biology Properties of Water

Welcome to Educator.com.0001

Today, we are going to be focusing on water, and water is essential to life. In fact, we are composed mostly of water.0003

Organisms need water to survive, and marine organisms actually are immersed in water.0011

We are going to talk about water and some of its special properties starting with the molecular structure of water.0017

Water is given by the formula H₂O, and it consists of one oxygen and two hydrogen molecules.0028

It is a polar molecule, and if you need a review of the concept of polar and non-polar molecules, then, go back and look at the previous lecture.0041

I talked about that in detail, but here, I am going to do a review.0051

Remember that the electron pair shared between a hydrogen, and an oxygen molecule is not equally shared.0057

Oxygen is more electronegative than hydrogen, and that means that it more strongly attracts electrons.0066

As a result, the electrons spend more time near the oxygen atom than it do by the hydrogen atom.0074

This ends up giving oxygen a partial negative charge or δ-.0082

And since the electron pair spends less time by the hydrogen atom, the hydrogen atom ends up with a partial positive charge or δ+.0088

Now, one important concept or theme in biology is that of emergent properties.0104

Emergent properties are properties that emerge or come into being at higher levels of organization.0109

For example, if you just took 1 heart cell and looked at it in a petri dish, it could not pump blood.0116

But, if you put many cardiac cells together, they can pump blood.0121

So, that build in the pump blood is something that emerges when you put many cells together at a more complex level of organization.0125

And we are going to see this all the way through the course, but here, if you take a single molecule of water, it is held together by covalent bonds.0132

But, if you put many molecules together, they hydrogen bond, and new properties emerge as a result of this hydrogen bonding.0140

Looking back here at this single molecule of water, water is V-shaped.0149

And because of that and the fact that the oxygen is a partial negative and the hydrogen is a partial positive charge,0152

the overall molecule is polar meaning 1 side is relatively negative and the other is relatively positive.0159

Now, let's look at multiple oxygen, multiple water molecules associated with each other.0168

Even though a particular hydrogen is covalently bonded to a certain oxygen, it also is attracted to nearby oxygen molecules.0175

Here, we have the δ-, and on the hydrogens, δ+.0185

Even though this hydrogen is covalently bound to this oxygen, it is also attracted to a nearby electronegative oxygen molecule.0197

And this attraction shown by these dotted lines is known as a hydrogen bond.0207

Recall that hydrogen bonds are weak bonds, whereas covalent bonds are strong bonds.0213

A single hydrogen bond is not going to do much, but many together can be strong; and they account for many of the special properties of water.0217

In the natural world, you will actually find water in all 3 of its forms: liquid as water vapor - oh, excuse me - liquid as liquid water.0226

We will also find the gas form, which is water vapor, and finally in the solid form, which is ice.0241

In liquid water, the hydrogen bonds are constantly breaking and reforming with each other, whereas solid water,0254

which is ice that I have been talking about in a few minutes, is in more of a set structure, and that gives ice its specific properties.0263

So, let's go on and talk about some of these special properties of water.0272

Water's special properties make it essential to life, and some of these properties are that water0278

is both strongly cohesive and adhesive, that it has a high surface tension and a high heat capacity and a high heat of evaporation.0284

Let's just take these one at a time, first of all cohesion. Cohesion is the tendency of molecules to stick together.0296

Water is highly cohesive. The reason it is highly cohesive is because of the hydrogen bonding.0313

As I mentioned, adjacent molecules of water are held together by hydrogen bonding, so they stick together.0319

Looking at how this can affect biology, when we talk about plants, we are going to talk about transpiration.0327

This is the process by which water is drawn out from the roots of a plant all the way up to the plant and then, evaporates out the leaves.0336

And this can be hundreds of leaves with the tree.0344

So, what happens is as a molecule of water is lost from the leaf, as it evaporates, the next molecule gets pulled up.0348

One after another, these molecules of water pull each other up.0358

And they do so through tubes called xylem, tubes that carry water in plants, throughout the plant.0362

Cohesion is one property of water that allows for transpirations.0377

Now, adhesion is slightly different. This is the tendency of molecules to stick to other substances.0383

Cohesion, water sticking to itself well, so it is cohesive.0394

It is also highly adhesive and sticks to other substances, tendency of molecules to stick to other substances.0398

So, not only is water cohesive, it is adhesive, and if we think again about the xylem, the water is being pulled up one after another by cohesion.0411

It is also helped by adhesion, though, because the water molecules adhere to the sides of the xylem.0421

If you look in a glass of water, have you ever seen a drop just sticking to the side? That is an example of adhesion.0427

The water is cohering to itself, but it also adhering to the side of the glass.0434

Surface tension is a related concept. Water has a high surface tension, and this is what allows water bugs to actually walk across water.0440

High surface tension substances can often be thought of as having like a film across them.0455

And it is actually harder to break through that top layer to break through that film than it is to move around once you are in the water.0458

So, we take more energy to break through the surface, than it is to move once you are under.0465

And the reason is because the water molecules are hydrogen bonded to each other.0469

Underneath the surface of the water, each water molecule is surrounded by many, many other water molecules.0478

And it has that attraction through hydrogen bonding to the molecules around it.0484

Underneath there is molecules on all sides, and there are hydrogen bonding to each other.0491

Now, if you look at the surface layer, though, at the surface, these molecules are attracted to each other and the ones beneath.0497

But there is no water molecules above. Therefore, their attractive forces are concentrated among fewer molecules.0513

So, here, the attractive forces are dispersed among many molecules.0521

Here, since there is no water molecules above, the bonds between the adjacent molecules and the ones below would be stronger.0525

The result is that the molecules here are more cohesive than the ones underneath, and that accounts for the high surface tension of water.0535

Alright, we covered cohesion, adhesion and surface tension.0551

Now, we are going to talk about heat capacity. OK, now, we are going to talk about heat capacity and heat of evaporation.0555

Water has both a high heat capacity and a high heat of evaporation.0563

In order to understand heat capacity, we need to talk about the concept of specific heat.0569

Water has a high specific heat, and that is what gives it its high heat capacity.0578

Specific heat is the amount of heat a substance needs to absorb to increase its temperature.0594

And, being a little more exact about that, it is the amount of heat a substance0602

needs to absorb to increase the temperature of 1 gram of the substance by 1°C.0613

This is one way that we measure it.0634

So, just in general, specific heat talks about the amount of heat a substance needs to absorb in order to raise the temperature.0637

But talking about it in specific units, one way to measure it is in calories, which is a way to measure heat: calories per gram per °C.0643

This is one way to measure specific heat.0660

OK, so looking at this a little more carefully, if something has a high specific heat,0663

if this is a large number, then, that substance is going to need a lot of heat in order for the temperature to change.0668

So, something with high specific heat is going to be harder to heat up. It is going to be harder to raise the temperature.0675

It is going to require a greater input of energy.0682

Water's specific heat/specific heat of water is 1 calorie per gram per °C, and that is actually a relatively high specific heat.0687

Let's look at it in different units that are commonly used.0708

A Joule is another unit that is used to describe energy, and specific heat is often given in Joules per °C per gram.0712

So, let's compare some different substances using this measure.0728

In this measure of units, water has a specific heat of 4.18 Joules per calories per gram.0731

Compare that to, say, lead. Lead only has a specific heat of 0.13, and ethanol alcohol, 2.44.0741

That means that it would take twice as much energy almost to raise water.0753

1 gram of water by 1°C as it would for the ethanol and 40 times more to raise water by 1°C than it would to raise the lead.0760

And this is very good for life because if we were comprised of a substance that had a low specific heat,0775

then, as soon as it got a little bit hot outside, inside, our body would more easily heat up.0783

So, specific heat is measured as I have shown, and water has a high specific heat; and the result is a high heat capacity.0791

This was background to understanding the high heat capacity.0799

High heat capacity means a substance...you can think of it as a substance can absorb0809

more energy or more heat because heat is energy with a smaller change in temperature.0821

Something with a high heat capacity can absorb a lot more heat than something with a low heat capacity and still maintain its temperature.0836

Looking at this with a concrete example, if you are cooking and have a metal pan and the handle is metal as well,0845

within a few minutes, you are not going to want to pick up that pan without a pot holder because you will burn your hands.0852

That is because metal has a lower heat capacity.0857

So, as soon as you turn on that burner, and the heat gets on the metal, temperature of the metal is going to rise.0860

What you want is a handle that has a low heat capacity because that same amount of heat will be absorbed by the handle.0870

The temperature of the handle will not change or it will not change very much, and then, you can touch the handle.0877

Thinking of water that way, marine life live in the ocean, and they are a lot better off being in a substance that has a high heat capacity0882

because even if it gets very hot out, it is a hot summer day, it is 100° out, the ocean is not going to suddenly heat up.0891

Or conversely, if it cools down, the water temperature will still not change or fluctuate as much.0899

And it is not just for organisms that live in the water, but since most organisms are mostly made up of water,0908

again, to keep our internal temperature constant, it is important that we are made out of a substance that0913

has a high heat capacity and does not just have these big fluctuations in temperature.0919

OK, so that was heat capacity, and now, let's talk about heat of evaporation.0927

The heat of evaporation, or sometimes it is called the heat of vaporization, has to do with converting water or substance from its liquid form to its gas form.0937

And this has to do with the amount of energy it would take to convert from liquid to gas.0950

If something has a high heat of evaporation, it is going to take more energy to get the0957

molecules moving quickly enough in order for them to break free of their hydrogen bonds and form a gas.0964

That unites the concept of both the heat capacity and the heat of evaporation.0977

It has to do with the speed the molecules are moving up because looking at actually the concept of temperature, temperature is actually due to kinetic energy.0981

Temperature is the average kinetic energy of the particles of a substance.0993

Heat is energy, so molecules with a greater kinetic energy that are moving faster, they will be hotter, and they will have a higher temperature.1007

Therefore, let's think of what this has to do with hydrogen bonding.1019

Water molecules are hydrogen bonded together.1023

In order to heat up water, you need to get the molecules to move faster.1026

In order to get them to move faster, you have to break the hydrogen bonds, and that takes extra energy.1027

And that is why water has a higher heat capacity and a higher heat of evaporation because if you are talking about a substance1032

that is not hydrogen bonded together, and you add some energy, those molecules are going to speed up right away.1040

Whereas with water, first you have to use some energy to break the hydrogen bonds, then you can start speeding up the molecules.1048

Therefore, to raise the heat of something or to get it to vaporize, you need to add energy.1056

And you need to add more energy when a substance is hydrogen bonded together like this.1060

Again, the special properties of water are really closely related to the hydrogen bonding that occurs.1066

Recall that one mechanism that we use to cool ourselves is sweating, and what sweating is, is the conversion of water from its liquid form to its gas form.1075

It is the evaporation of water from our skin. In that process, water absorbs heat, and that is what is going to cool us down.1083

One more property of water that is very important that is not listed here but that is related, is the fact that ice is less dense than water.1096

In other words, the solid form of water is less dense than its liquid form, and this is an unusual property.1109

In most of substances, the solid form is more dense than the liquid form.1117

And this is another property that makes water very important especially for life in the ocean.1120

Thinking about hydrogen bonding, if the molecules are moving more quickly, then, the hydrogen bonds are going to break.1129

And in liquid water, the bonds are constantly breaking and reforming.1139

And then, when those bonds break eventually, if things are moving fast enough, and enough for those bonds to break, then, water can vaporize.1142

So that is as things speed up, the hydrogen bonds break.1151

Let's look at the opposite.1152

The freezing point of water is going to be 0°C.1155

So, as water gets cooler and cooler as the temperature drops, that is going mean that the average kinetic energy is less.1163

And kinetic energy is the energy of motion, so as the temperature cools, the water molecules are going to move slowly.1174

The more slowly they move, the more chance to hydrogen bond.1183

Eventually, they are going to get to the point where they form hydrogen bonds with the 4 adjacent water molecules.1184

And this ends up forming a very stable lattice type structure, where the different water molecules would be bonded to each other.1194

And they would form a lattice.1203

This is just really schematic.1206

But if they are forming a lattice, and let's say there is a water here, a water here, a water here, a water here,1208

and they are in this regular formation, this lattice structure is going to hold the water molecules at a certain distance from each other.1212

And it holds them a bit farther apart than they would be in their liquid form, and that is why ice is less dense than water1219

because the water molecules form a lattice structure through hydrogen bonding to 4 nearby molecules.1228

As a result, ice is actually about 10% less dense than water, and ice floats.1236

Because it floats, animals, fish, living in, say, a frozen lake will be insulated.1247

That layer of ice is going to form insulation, and it is going to prevent the change in temperature in the dead of winter.1256

This formation of ice and the fact that ice floats because it is less dense than water is another property of water that makes it essential to life.1265

In the last lecture, I introduced the concept of molarity and just briefly talked about solutions.1277

Now, we are going to cover this in detail, first some terminology.1283

When we talk about solutions, first of all, we are talking about a homogeneous mixture formed by combining a solvent in a solution.1288

Homogeneous means it is dispersed equally throughout.1298

So, if you would just take a rock and throw it in a glass of water, that is not a solution because it is not a homogeneous mixture.1300

A solvent is the liquid in which the substance is dissolved.1309

So, if I have a glass of water, and I put some sugar in it or salt, then, the water here would be the solvent.1313

And let's say I had salt, sodium chloride, that would be the solute.1325

The solute is the substance that is being dissolved. The solvent is the one that does the dissolving.1333

This mixture, altogether, is called a solution.1339

Water is a very good solvent for many substances, but recall that it is polar.1344

Water is polar, and since like dissolves like, polar substances are going to dissolve well in water. Non-polar ones will not.1352

So, polar dissolve in polar. Non-polar dissolve in non-polar.1366

Like dissolves like.1375

Looking in at an example of this, let's look at glucose.1378

This is a structure of glucose. Glucose is a polar molecule.1381

It has these hydroxyl groups that are polar centers, and on these groups, you have water.1387

You have - excuse me - oxygen with the partial negative δ- and a hydrogen with δ+.1392

Water - we already talked about - also has polar structure, δ+ and a hydrogen, δ-, so partial negative charge on the oxygen.1399

Since these are polar, these 2 are attracted to each other, and what will happen is water can dissolve polar molecules and also ions1415

because they have a positive and negative charge by bonding to them and them, and then, forming what is called a hydration shell.1423

This hydration is going to separate out the molecules in a solute, so let's say I dissolve glucose and water.1431

I just drop a lump of glucose in the water, these water molecules are going to be attracted, and glucose will be attracted to the water.1440

And eventually, it is going to separate out the glucose molecules from each other.1449

And these are going to disperse until they are evenly distributed throughout the water.1453

Non-polar molecules do not have the same attraction for water, so they are not going to dissolve well in here.1458

Like dissolves like, so if you take a teaspoon of oil, drop it in water, you are going to watch it.1466

It would separate out. Even if you stir it, it is just going to separate right back out1472

because hydrophilic molecules - you will hear this term - or water-loving are polar.1477

So, glucose or sodium chloride which disassociates to form sodium, which is charged, and chloride, these are ions.1485

So, then, the polar water molecules also partially charged. These are going to be attracted.1498

Hydrophilic molecules are polar. Hydrophobic are non-polar like lipids, and this is very important in biology.1505

We are going to see when we talk about the cell structure that the cell membrane because it contains many non-polar lipids, large polar1514

molecules cannot cross the cell membrane without help, and that is important in regulating what gets in and what does not get into the cell.1525

Polarity also accounts for certain properties of the cell.1533

When we discuss solutions, we discuss sometimes whether they are acidic or basic or acidic or alkaline.1541

Acids can be thought of as substances that increase the concentration of hydrogen ions in a solution.1551

Bases decrease the concentration of hydrogen ions in a solution.1560

And just to add to this, one common way for bases to accomplish this is bases may do so.1564

So, they might increase the hydrogen ion concentration in the solution by increasing the hydroxide ion concentration.1576

Alright, let's just look at water. Water can actually have an acid-base reaction.1587

It can disassociate into hydrogen ions plus hydroxide ions.1595

It would actually be...and if you look, it is all balanced. We have 2 hydrogen, 2 hydrogen and 1 oxygen.1605

When it dissociates in the ions, when this hydrogen pulls away, it actually leaves behind its electron.1615

This hydrogen ion leaves behind its electron and becomes positively charged.1624

You end up with a positively charged hydrogen ion, and the OH that is left behind has an extra electron, so it is negatively charged.1629

Just for a moment to be clear, though, this is actually not going to just float around by itself.1638

What is going to happen is it is going to be associated with another water molecule, so there will be another H₂O.1642

And this is actually in a form that is bound to that, and it forms to what is called a hydronium ion.1651

But for simplicity, we are just going to refer to it as a hydrogen ion.1657

It dissociates in the hydrogen ion, in hydroxide ion, but you should know that in reality, this is not floating around.1660

Usually, it is part of a hydronium ion.1669

OK, overall, though, water mostly exists in this state, H₂O, so this reaction, the equilibrium is very far to the left, and water is actually neutral.1681

However, some substances like to disassociate, and they do disassociate and they are, therefore, very acid or very basic.1690

Water mostly exists like this, not a lot of loose hydrogen ions and hydroxide ions floating around, so it is pretty much neutral.1700

Let's look at what is called a strong acid. A strong acid is one that is going to really have a reaction where it completely wants to dissociate.1711

Hydrochloric acid/HCl - this is hydrochloric acid - is a strong acid. It dissociates to form hydrogen ions and chloride ions.1725

In the reaction, equilibrium is to the right, so if I took a cup of water, and I threw some hydrochloric acid in it1734

this is going to dissociate, and there is going to be a lot more hydrogen ions floating around.1743

And that is going to increase the concentration of the hydrogen ions in the solution.1750

Since this is increasing the concentration of hydrogen ions in the solution that I make, it is an acid.1755

And it is a strong acid because it dissociates almost completely.1763

Let's look at a base. Let's look at a strong base to start with.1768

KOH/potassium hydroxide, this is going to dissociate to form potassium ion plus hydroxide ions.1780

Now, as I said, bases may increase or may decrease the concentration of hydroxide ions indirectly through the formation of hydroxide.1789

A base dissociates to KOH, then, the hydroxide combines with hydrogen ions that are floating around to form water.1802

The result is going to be a decrease in the concentration of hydrogen ions.1815

So, again, a base decreases the concentration of hydrogen ions in a solution.1820

It can do so directly or indirectly. Here, it is happening indirectly.1826

But we recognize that when we see something dissociating from a hydroxide ion, the end result is going to be decreasing the hydrogen ions.1831

Acids raise the hydrogen ion concentration, bases decrease.1841

Sometimes we call these acids, hydrogen ion donors because they increase the level.1846

And sometimes we call bases, hydrogen ion acceptors because they will accept a hydrogen ion, and thus, decrease the concentration of hydrogen ions.1857

So, you can think of this as base is accepting. The base is accepting a hydrogen ion, and acid is donating a hydrogen ion.1868

Another example is ammonia, and let's look at what happens with this.1882

This is a base. Now, it does not form hydroxide ions, in fact, it actually lowers the concentration of the hydrogen ions directly.1892

It is a hydrogen ion acceptor.1901

Ammonia will combine with hydrogen ion to form ammonium. In doing so, it directly lowers the hydrogen ion concentration in a solution.1905

For example, if I had water, and I threw some ammonia in it, that ammonia would bind with these hydrogen ions and form ammonium.1930

And I would end up with a solution that is more basic because it is lower in hydrogen ion concentration.1942

Just some properties of acids that you might hear about, acids turn litmus paper red.1951

We are going to talk about pH and pH paper and measuring pH, which is a way of measuring how acidic or basic a substance is.1958

Acids turn litmus or pH paper red, and they tend to taste sour.1966

An example would be lemon. Lemons are acidic.1974

Bases turn litmus paper blue, pH paper, they turn it blue. They feel slippery.1977

They taste bitter. An example would be soap.1990

I said that bases feel slippery just like if you think about what soap feels like. It is very slippery.1996

Buffers are very important in organisms because buffers resist the change in pH.2003

If you added hydrochloric acid to a solution, the solution is going to become more acidic.2015

If you add hydrochloric acid plus a buffer, the acidity will change less than if you just added the hydrochloric acid alone.2023

So, what these do is they minimize changes in the concentration of hydrogen ions, and they do this by accepting.2033

They accept hydrogen ions when an acid is added, and they donate hydrogen ions when a base is added, so they resist change.2048

Our blood has a pH of around 7.4, and if the pH goes out of that range, it can be very detrimental to life.2072

So, in order to prevent that change, we have buffers.2080

A very important buffer in our blood is carbonic acid. H₂CO₃, this is carbonic acid, and this is a weak acid.2085

And its corresponding base, it dissociates to form H⁺ plus bicarbonate/HCO₃^-.2096

Most buffers are weak acids and a corresponding base or weak bases and a corresponding acid.2109

And I am actually going to - with weak acids and bases - we show the reaction going in both directions.2115

Because unlike with strong acids that just want to completely disassociate, these do not- the reaction is going back and forth.2121

So, let's think about this.2129

If the blood becomes too acidic, that means that the concentrations of hydrogen ions is too high.2131

What bicarbonate will do is it will bind to these hydrogen ions, and these will actually move to the left; and carbonic acid will be formed.2137

So, you can think of it as absorbing the excess hydrogen ions.2145

Now, let's say a base is added to our blood. The base is going to decrease the concentration of hydrogen ions.2148

The buffer will minimize that change or mitigate it by dissociation and thus, donating hydrogen ions.2154

Buffers resist change in pH by donating hydrogen ions if need or accepting them if needed.2164

They are going to go the opposite way of whatever.2172

If you have added an acid, the acid increases the hydrogen ions. The buffer will take those up.2175

If you add a base, the hydrogen ion concentration decreases. The base will donate.2180

It does the opposite to keep things stable.2186

And I have mentioned pH, which you have probably heard of before, but let's go into some detail about this.2190

pH is a scale that we use to describe either how acidic or alkaline a solution is, and it is on a scale of 1 to 14.2196

Neutral is right here at 7.2207

Distilled water would be here at 7.2211

So, pH, it is often paper and you can dip it into the solution, and then, look at the color change.2217

And like I said, acids are on the red end, and then, bases are on the blue end.2224

If something is more basic - OK - or alkaline equals a higher pH, which equals a lower concentration of hydrogen ions,2232

hydrogen ions move in the opposite direction as the scale.2247

Less hydrogen ion concentration, more basic higher pH, more acidic.2253

Remember that if something is an acid, it donates hydrogen ions, so more acidic equals a lower pH.2259

And that is going to equal a higher concentration of hydrogen ions.2268

So, the scale moves the opposite way as the concentration of hydrogen ions.2275

Just to give you some examples, stomach acid is around here in the 2 to 4 range.2281

Most of our body, the pH is around 7.4, so blood is right here at around 7.4.2289

Distilled water is at 7.2297

Stomach contents, stomach acid is in the 2 to 4 range, so it is acidic, and the enzymes in our stomach work very well at that low pH.2301

Vinegar is an acid. Vinegar is around 3.2311

OK, vinegar is around 3. It is acidic.2323

A base, an example would be baking soda. That is a base, and that has a pH of about 9.2328

It is important to know that this is a logarithmic scale.2337

That means that if I am looking at, say, vinegar with a pH of 3, and then, I measure some stomach acid, and it has a pH of 4,2344

the difference between these is actually pretty large. It is actually ten times.2354

So, a pH of 4 is ten times more basic than a pH of 3, or you could say that pH of 3 is ten times more acidic than the pH of 4.2360

If I went from 3 to 5, it would be 10 x 10 or 100, so each unit is a tenfold difference.2374

And the reason we measure it this way is that there is a very wide range of how acidic or basic a solution can be.2382

And in order to just have a nice, condensed, compressed scale that is not going to be humongous, we use logs just to condense it.2388

And then, we can show a big range on just a small scale.2395

What pH actually is, is the negative log of the hydrogen ion concentration.2400

At a pH of 7, what you have is 1 x 10^-7 moles per liter of hydrogen ions.2410

This is hydrogen ion concentration, and right here at 7, we would have 1 x 10^-7.2428

If we went up or if we went down one, then, it would be increase in the hydrogen ion amount by a factor of 10.2434

OK, again, pH, important points: low numbers are acidic. High on the pH scale is basic.2447

Acidic represents a high concentration of hydrogen ions. Basic represents a lower concentration of hydrogen ions.2454

Water is neutral at around 7.2462

This is a logarithmic scale, so moving one unit translates to a tenfold difference in how acidic or basic or in a hydrogen ion concentration.2466

Alright, starting with example one: how is the shape of water molecule related to many of the special properties of water?2480

Recall that water is V-shaped.2487

Many of the special properties of water are due to the fact that it is polar.2494

This polarity allows for hydrogen bonding with nearby oxygen molecules.2500

Because water is V-shaped, one side of the molecule ends up being relatively negative and the other, relatively positive.2507

Oxygen is electronegative. It attracts the electron pair more strongly than the hydrogen, has a partial negative charge.2517

The hydrogen ions - excuse me - the hydrogen atoms have a partial positive charge. Therefore, it is polar.2525

This end of the molecule is relatively negative. This is relatively positive.2532

If it were a linear molecule, it would not have this polar nature. Because of this polar nature, hydrogen bonding occurs.2537

Hydrogen atoms that are covalently bound to one oxygen, are still attracted2550

to other nearby oxygens because these are also partially negatively charged, and they form hydrogen bonds.2558

The polarity of water allows for hydrogen bonding, and it is this hydrogen bonding that accounts for many of the special properties of water.2565

List five special properties of water.2575

So, we talked about why these properties exist, so what are they?2579

Well, first of all, water has a high heat capacity.2583

It also has a high heat of vaporization or you could call it heat of evaporation.2592

So, it takes a lot of heat, a relatively high amount of heat to change the temperature of water.2602

And it takes a relatively high amount of heat to change water from its liquid form to its gas form.2609

Water is strongly cohesive. Water molecules stick together.2617

Water is strongly adhesive. Water molecules also stick to other substances pretty well, very well.2626

It has a high surface tension.2635

Because of the hydrogen bonding at that surface layer, it is a little bit harder to break through the surface layer.2640

And that forms essentially a film. It can be thought of as a film across the water.2647

In addition, the solid form, which is ice, is less dense than the liquid form of water.2655

This is because of the stable lattice structure that is formed by hydrogen bonding at low temperatures.2667

The result is that ice floats. The solid form of water floats on the liquid form.2674

These are six properties. It has only asked us to name five, but any five of these would have sufficed.2681

Lemon juice has a pH of 2. Milk has a pH of 6.2689

How many times more basic is milk than lemon juice?2693

OK, pH of 6, that is milk, so we could say 6, 5, 4, 3, 2, and over here, at a pH of 2 is lemon juice.2696

Lemon juice is more acidic than milk, and milk is relatively basic.2714

It is basic. It is more basic than lemon juice.2721

So, this is asking how much more basic or you could rephrase it the other way: "how much more acidic is the lemon juice?".2725

Because this is a logarithmic scale, each unit represents a tenfold change.2731

So, going from 2 to 3 is tenfold, ten times. 3 to 4 is a tenfold change, 4 to 5 and then 5 to 6.2736

So, going from 2 to 6, I am going to be changing, increasing tenfold times another 10, times another 10, times another 10.2748

That gives me 100,000, 10,000. That is a very large range, and this shows you why we need to use a logarithmic scale2758

because it would not be very practical to try to fit these wide ranges onto a scale that was just a linear scale.2771

Which should define our acids, which are bases and why?2781

Remember that acids increase the hydrogen ion concentration. Bases decrease the hydrogen ion concentration.2786

Let's look at this first one. This is showing the dissociation of a substance into...this should actually be 2 H⁺s + SO₄^2-.2800

Since this is dissociating to form hydrogen ions, it is increasing the concentration of hydrogen ions to the solvent it is added to.2817

Therefore, this is an acid, and this is actually sulfuric acid; and it dissociates to form sulfates plus hydrogen ions.2829

HNO₃ dissociates to form H⁺ + NO₃^-.2842

Again, since this is dissociating to form hydrogen ions, it is increasing the concentration of hydrogen ions, and therefore, it is an acid.2850

This is a strong acid, nitric acid, and it is dissociating to form hydrogen ions plus nitrate ions.2858

OK, here, we have NaOH dissociating to form Na⁺ + OH^-.2875

Recall that another way to look at bases is that they increase concentration of hydroxide ions.2882

And then, the hydroxide ions bind the hydrogen ions to form water, and that, in turn, decreases the hydrogen ion concentration.2892

So, you can think of a base either as just something that decreases hydrogen ion concentration or that increases hydroxide ions.2902

Since this is increase in the concentration of OH^-, it is a base, and this is a strong base, sodium hydroxide.2910

Sodium hydroxide disassociates into sodium and hydroxide.2920

Here, I have the arrows going both ways because these are weaker bases, so the reaction is not so far to the right.2928

Now, let's look at what is happening with this one.2937

It is dissociating to form...these are weaker acids or bases, we have to figure out2939

which one this is dissociating to form a hydrogen ion plus this other compound.2944

Increase hydrogen ion, I have an acid. This is a weak acid, it is acetic acid.2949

Acetic acid dissociates to hydrogen ions and compound called acetate.2955

Finally, Na₂CO₃ plus water dissociates to form two sodium ions plus this is bicarbonate and hydroxide.2962

So, I can see with this increase in the hydroxide, what I have is a base, and this is sodium bicarbonate, which is a weak base.2972

It dissociates to form sodium bicarbonate and a hydroxide.2987

Again, here we have an acid, sulfuric acid, another acid, nitric acid, and a base.2996

It is a strong base, sodium hydroxide; a weak acid, acetic acid; and a weak base, sodium bicarbonate.3003

And we define which are acids, which are bases and why in terms of what they do to the hydrogen ion concentration.3011

So, that concludes this lecture on the properties of water, and I will see you again soon.3018