Module 1 Building Blocks of Matter

Unit 1.1 Particles and Properties

Starter check

What you need to know before you start the contents of this unit.

You will need to have an understanding of the general scientific vocabulary, including the meaning of particles, property, solid, liquid, gas, melting, boiling, diffusion.

Aims By the end of this unit you should understand that:

* Different substances have different sets of properties.

* There are two sorts of properties, physical and chemical.

* Matter can exist in different states.

* All matter is made up of particles.

* The type, arrangement and movement of the particles affects the properties of matter, including evaporation, condensation, dissolving, diffusion and the effect of pressure on gases.

Diagnostic test

Try this test at the start of the unit. If you score more than 8O%, then use this unit as a revision for yourself and scan through the text. If you score less than 80% then work through the text and re-test yourself at the end by using this same test.

The answers are at the end of the unit.

1 List four physical properties of water. (4)

2 What is there between the particles of the air we breathe? (1)

3 a) In which would you expect the particles to be closer together, a solid or a liquid? (1)

b)In which would you expect the particles to be closer together, a gas or a liquid? (1)

c) In which would you expect the particles to be moving more quickly, a solid or a liquid? (1)

d) In which would you expect the particles to be moving more quickly, a gas or a liquid? (1)

4 Complete the following sentences, by using any of the following words. They might be used more than once or not at all.

Repulsion, attractions, mloving, vibrating, static, disintegrates, breaks up, remains unchanged, whole, surface, inside, slowest, fastest, upwards, middle, downwards, gas, vapour, fixed, melting, boiling, solid, liquid, quicker, more difficult, easier.

a) When a solid melts, the forces of _____ between the particles can no longer hold them together because they are _____ too much, so the structure _____ and the particles become free to move throughout the _____ of the liquid. (4)

b) When a liquid evaporates, the _____ moving particles escape from the _____ of the liquid. They can escape, because at the surface of the liquid the nett force of attraction on a particle is _____ into the liquid. (3)

c) When a liquid boils, bubbles are formed in the liquid. These bubbles contain _____. Boiling happens at a _____ temperature called the _____ point. (3)

d) When a solid dissolves in a liquid, particles of the _____ are knocked off by collision with _____ particles and they diffuse into and mix with the _____ particles. The hotter the liquid, the _____ its particles are moving, so the _____ it is for them to knock particles off and dissolve the solid. (5)

5 Why do gases diffuse more quickly than liquids? (2)

6 For each of the following substances, say whether it would be a solid, a liquid or a gas at room temperature (room temperature is taken as being 20 °C):

Substance A – melting point 25 °C, boiling point 150 °C (1)

Substance B – freezing point – 25 °C, boiling point 100 °C (1)

Substance C – melting point 15 °C, boiling point 110 °C (1)

Substance D – freezing point – 253 °C, boiling point – 150 °C (1)

Substance E – melting point 1250 °C, boiling point 1700 °C (1)

31 Marks (80% = 25)

1.1.1 Properties

Chemistry is the study of matter. There are lots of words that are used to mean 'a particular type of matter'. Two of the most common are 'substance' and 'material'. The word 'matter' covers everything the Universe is made of, whether it is solid, liquid or gas. The words 'material' and 'substance' simply mean one particular sort of matter, like water or soil or air or monosodium glutamate.

Look at the paper on which this is printed. It looks white. It is smooth to the touch. Two of the properties of this paper are that it is white and smooth. Properties are simply what you can detect or measure about a material or what it does. If you were to put a match to this paper, for example, it would burn. This property is summarised by saying that it is combustible.

Some of the properties of materials are ones that they have of themselves, without any other material being involved. Gold, for example, is shiny and melts at 1065 °C. Properties like colour, melting point, boiling point, electrical conductivity, do not depend on any other substance, and are called the physical properties of gold.

Some properties of materials describe how they behave with other materials or chemicals. Gold is not attacked by dilute acids, for example, whereas lots of other metals are. Properties like this are called chemical properties because a chemical reaction occurs and the material is permanently changed.

Properties are really just descriptions of materials – what they look like, feel like, how they behave, etc. Notice that some are negative – 'gold is not attacked by dilute acid'. In science, negative information is often just as important as positive.

Each substance has its own unique set of properties.

If you look at a crowd of people: lots of them have dark hair; a few of them may be 2 m tall; lots of them may have a mass of 70 kg; lots of them may have green eyes; lots of. them may have eyebrows that meet in the middle, but there is probably only one individual in the crowd who has all of these characteristics.

In the same way: lots of substances are colourless; lots of substances are odourless; lots of substances are liquids at room temperature, but if you came across a beaker full of a colourless, odourless liquid, particularly in a chemistry lab, you might hesitate to drink it until you'd checked that it wasn't acidic or alkaline and that it boiled at 100 "C. In other words, water, like all other substances, has a unique set of properties.

1.1.2 Change of State

Melting point, boiling point, and freezing point are all properties of substances. The melting point is the temperature at which a substance changes from a solid to a liquid. The freezing point is the temperature at which a substance changes from a liquid to a solid. It is important to understand that these two temperatures are numerically the same – it just depends on whether you are heating it or cooling it, as to what you call them (Figure 1.1.1).

The same is true for the boiling point and condensing point. Boiling is when the pressure of the vapour trying to escape from the surface of the liquid is just above the pressure of the atmosphere trying to keep it in. So at the boiling point a lot of the vapour escapes and bubbles of vapour can be seen; at temperatures well below this, only a little of it evaporates as only a few particles have enough energy to break through the surface (Figure 1.1.2).

Putting these together, you get a diagram shown in Figure 1.1.3:

[ILLUSTRATION OMITTED]

Suppose you know the melting and boiling points of a substance, you can decide whether it will be a solid, a liquid or a gas at any temperature by using the diagram. For example, if you had a substance that has a melting point of 10 °C and a boiling point of 98 °C, what state will it be in at 56 °C?

As you can see from Figure 1.1.4, 56 °C is between the melting and the boiling point, so the substance will be a liquid at that temperature because the temperature of its boiling point has not yet been reached. But even at temperatures below its boiling point, some of the liquid can slowly evaporate away.

1.1.3 Particles

It is sometimes difficult to believe that everything is made up of particles, because they are so small that you can't see them. In several places around the country, e.g. Wiltshire and Oxfordshire, there are figures carved into the chalk hillside.

The carvings are made up of individual little bits of chalk, but when we are a long way from it they are so small that we cannot see them – all we see is the smooth horse-shaped figure. In the same way the edge of this paper looks perfectly smooth, but if you look at it with a powerful magnifying glass or microscope, you will see that it is actually quite ragged. It's just that the indentations are so small they cannot be seen with the naked eye.

Even though it hasn't been possible to see such particles until fairly recently, scientists have believed for a long time that all matter is made up of particles. Understanding how these particles behave explains some of the properties of matter.

Before we look at these explanations, it is important to establish one thing. When we say that all matter is made up of particles, it raises the question 'what is there in between the particles?' In a recent survey conducted in the fictitious town of West Framlingham, we asked people what they thought there was between the particles of air. It was noted that when asked the question, one hundred percent of those polled said nothing (in fact their mouths fell open and a blank stare appeared in their eyes). And they were absolutely right to say 'nothing' because there is 'nothing' at all in between the particles. (Actually, one of the respondents was pressed to speak and said 'there was even smaller bits of air' and another said, 'glue keeps them together'.) We explained that 'all matter' includes 'air and glue' and that these must also be made up of particles. We asked again what was between their particles, and they soon got the point and now said NOTHING. Right again.

So what properties of materials does the existence of particles explain?

STATE AND CHANGE OF STATE

What are the properties that distinguish between solids, liquids and gases? Firstly, solids have a fixed shape, whereas liquids and gases change shape spontaneously – they 'flow' and so must be kept in a container.

Secondly, solids and liquids have a fixed volume, whereas gases expand spontaneously to take up all the space that is available to them.

Thirdly, solids are almost impossible to compress or squeeze, liquids are very difficult, even in a sealed syringe, but gases are easy to compress and squash (think of a bicycle pump where gas is squashed into a tyre).

How can we explain these properties in terms of particles?

In a solid, the particles are fixed in place in some kind of order. They can move, but only by vibrating backwards and forwards around fixed positions, You could imagine it as being like a scrum in a. game of rugby where the players are locked together, tugging and heaving but not letting go of each other (Figure 1.1.5).

Of course, in a solid the particles aren't held together by putting their arms round each other. What holds them in place are forces of attraction between the particles.

Still thinking about the scrum, think about what happens when the ball is thrown into it. The players start to move more vigorously (not to mention violently) as they jostle for position and possession. As the movement increases in violence, players start to lose their grip on each other. They become free to move independently. It is the same with the particles in a solid. If you put energy into the solid, the particles start to vibrate more vigorously.

The forces of attraction cannot hold them together any longer and the particles become free to move independently. They move in a straight line until they collide with something and bounce off. The solid melts (Figure 1.1.6).

The particles in the liquid aren't much further apart than they were in the solid, so you can compress liquids a bit (a little like a loose maul in rugby), but not much because the particles are quite close together already.

To go back to rugby: once the ball is free from the scrum, the players who were in the scrum move more widely apart and play speeds up. This is just like what happens when you heat a liquid. Its particles start to move so quickly that the forces of attraction are completely overcome, the particles are no longer restrained at all; they move as far apart as they can and the liquid evaporates or boils, and the gas spreads out to occupy all the space available in the room or wider afield.

Suppose you had two teams playing, one made up of really massive players and one of smaller players. Which ones would you have to apply more force to, to get them moving about in the scrum? Obviously the answer is the more massive ones. It is the same with substances. Other things being equal, substances with more massive particles will have higher melting and boiling points, because you need to put in more heat energy to get their particles moving fast enough to break up the solid structure or escape from the liquid.

Suppose you had two teams playing whose players were equally massive, but one team had tremendous arm strength and could lock together really well. Which ones would you have to apply more force to now to break them apart from the scrum? Obviously it's the ones with the strongest grip on each other. It is the same with substances. Other things being equal, the particles that have the strongest forces of attraction between them will have the highest melting and boiling points, because you need to put in more heat energy to get their particles moving fast enough to break up the solid structure or escape from the liquid.

1.1.4 Evaporation

There are two ways that liquids can change to gases: boiling, and evaporation. What are the differences between them?

When a liquid boils, you can see bubbles in the body of the liquid. These are not full of air. They are full of the vapour of the substance that is boiling. Boiling happens in the body of, or inside the liquid, but with much turbulence and sufficient energy to push out a lot of particles through the surface into the: vapour. When a liquid evaporates, you don't see anything in the inside of the liquid. Evaporation happens at the surface of the liquid. Even solids evaporate; for example if you put a solid block of 'air freshener' in a room the smell can be noticed from some distance away, eventually. That is because some of the particles, usually the loosely-held and smaller ones, are evaporating from the block, pass into the air and eventually reach your nose. It shows that some solids can evaporate. Some solids do riot evaporate, because their particles are held strongly together and might anyway be too large to evaporate. The change from a solid directly to a gas, or from a gas to a solid, without going through the liquid state, is called sublimation.

Liquids can evaporate slowly or they can be made to boil. The boiling point of a liquid is the temperature at which boiling occurs. If water is heated, its temperature will start to go up until it reaches 100 "C, and it will stay at that temperature while the liquid boils. Boiling happens at a fixed temperature called the boiling point. At this temperature there are loads of water particles breaking through the surface of the water as they overcome the pressure of the air trying to keep them in. At the boiling point the vapour pressure of the water escaping from the liquid equals the atmospheric pressure. Evaporation, on the other hand, happens at any temperature. Even water left outside at only 1 °C would evaporate slowly from its surface, but it evaporates even more quickly at 30 "C. How do we explain evaporation in terms of particles?

Imagine a particle in the middle of a liquid. It is surrounded on every side by other particles. They are not much further apart than they were in the solid, so the forces of attraction are still fairly strong. It is attracted by all of the surrounding molecules. There are, as Tennyson almost put it, 'Attractions to the right of it, attractions to the left of it, Attractions in front of it, attractions in rear of it, Attractions above it, attractions below it.' All the attractions cancel each other out, so the particle goes on travelling in a straight line at the same speed until it collides with something.

Now think about a particle in the surface layer.

There are attractions to each side and attractions below, but above it there is air, in which the particles are very widely spaced compared with the liquid. There is hardly any upward attraction upon the liquid particles. The overall effect is that particles in the surface layer experience a downward attraction (Figure 1.1.7).

The particles in liquids, just as in gases, are moving in random directions with a random range of speeds. Some of them must be moving upwards. Some of these will reach the surface. There they experience a downward attraction, but some of them will be moving fast enough to escape. The liquid evaporates.

If you increase the temperature, the particles will start to move faster and more of them will be moving quickly enough to escape when they reach the surface. The liquid will evaporate faster at higher temperatures.