Chemistry: Unit 1 - Principles of Chemistry: Chapter 1: States of Matter

States of Matter

Principles of Chemistry: States of Matter

• Everything around us is made of particles too small to see. This chapter discusses the arrangement and movement of particles in solids, liquids, and gases. Chapter 3 will explore the nature of different types of particles.

Key Points

• You can't walk through a brick wall due to the strong forces of attraction between particles.

• You can swim because you can push the particles in water out of the way.

• It is easy to move through a gas because there are almost no forces between the particles.

Three States of Matter

• Solids, liquids, and gases are the three states of matter.

• You cannot walk through a brick wall, but you can move through water with some resistance. Moving through air is easy.

• When most solids melt, their volume increases slightly, meaning most liquids are less dense than the solid they come from.

• If you boil about 5 cm^3 of water, the steam will fill an average bucket.

Arrangement of Particles

• The arrangement of particles in solids, liquids, and gases explains these facts.

• In a solid, particles are usually arranged regularly and packed closely together.

  • Particles can only vibrate about fixed positions and cannot move around.

  • Strong forces of attraction keep the particles together.

• In a liquid, particles are still mostly touching, but some gaps have appeared.

  • Liquids are usually less dense than solids.

  • Weaker forces between particles allow them to move around each other, arranged randomly.

• In a gas, particles move randomly at high speed in all directions.

  • Particles are much further apart with almost no forces of attraction between them.

• Particles in a solid have less kinetic energy than those in a liquid, which have less kinetic energy than those in a gas.

Interconversions Between the Three States of Matter

• Heating a solid makes its particles vibrate faster.

• Eventually, particles vibrate so fast that the forces of attraction are no longer strong enough to hold them together, and the solid melts to form a liquid.

• The temperature at which a solid melts is its melting point.

• Particles in the liquid have more kinetic energy than in the solid, so energy must be supplied to convert a solid to a liquid.

Changing State Between Solid and Liquid

• Melting occurs when a solid is heated, and energy is put in.

• Freezing occurs when a liquid is cooled, and energy is taken out.

• If the liquid is cooled, the particles move more and more slowly.

• Eventually, the forces of attraction hold them in fixed positions, and the particles pack more closely together into a solid.

• The temperature at which this occurs is the freezing point.

• The melting point and freezing point of a substance are the same temperature.

Changing State Between Liquid and Gas

• There are two ways this can happen: boiling and evaporation.

Boiling

• Boiling occurs when a liquid is heated so strongly that the particles move fast enough to overcome all forces of attraction between them.

• The stronger the forces of attraction between particles, the higher the boiling point, because more energy is needed to overcome these forces.

• Boiling requires putting energy in (heat).

• If a gas is cooled, the particles eventually move slowly enough that forces of attraction start to form and hold them together as a liquid, and the gas condenses, releasing energy.

Evaporation

• In any liquid or gas, the average speed of the particles varies with temperature.

• At each temperature, some particles move faster than average, and others move more slowly.

• Some very fast particles at the surface of the liquid have enough energy to overcome the forces of attraction between the particles and break away to form a gas. This is evaporation.

• No bubbling occurs during evaporation; the liquid slowly disappears if open to the air.

• In a closed container, evaporation and condensation occur simultaneously.

Key Point

• Evaporation occurs at any temperature, but boiling only occurs at one temperature—the boiling point of the liquid.

• Water evaporates in puddles even below 5°C, but boils at 100°C.

Changing State Between Solid and Gas: Sublimation

• A small number of substances can change directly from a solid to a gas, or from a gas to a solid, at normal pressure without involving any liquid in the process.

• Solid to gas is sublimation; gas to solid is deposition (or sometimes called 'de-sublimation' or 'sublimation').

• An example of a substance that sublimes is carbon dioxide.

• At ordinary pressures, there is no such thing as liquid carbon dioxide—it turns directly from a solid to a gas at -78.5°C.

• Solid carbon dioxide is known as dry ice.

Working Out the Physical State of a Substance at a Particular Temperature

• A substance is a solid below its melting point, a liquid between its melting and boiling points, and a gas above its boiling point.

• To determine whether a substance is a solid, liquid, or gas at room temperature, compare its melting and boiling points to room temperature.

• Room temperature is usually taken to mean between 20 and 25°C in science.

• When making comparisons of state changes near room temperature, be clear about what value is being used as room temperature.

Diffusion

• Diffusion is the spreading out of particles from an area of high concentration to an area of low concentration.

Diffusion in Gases

• If someone releases a smelly gas like ammonia in a lab, it quickly spreads due to diffusion.

• Although ammonia particles travel at about 600 meters per second at room temperature, it takes longer than expected for the smell to travel because the particles are constantly bouncing off air particles.

• Each ammonia particle may travel 30 or more kilometers as it diffuses across the lab.

  *At room temperature, ammonia particles travel at speeds of about 600 metres per second so they should be able to travel from one end of a lab to the other in less than 1/100s (0.01 s). This would be the case if they travelled in a straight line without bumping into anything else. However, each particle is bouncing off air particles on its way. In the time that it takes for the smell to reach all corners of the lab, each ammonia particle may have travelled 30 or more kilometres!

Demonstration of Diffusion in Gases

• Diffusion in gases can be shown by using gas jars, one containing bromine gas and the other air.

• When the lids are removed, the brown color of the bromine diffuses upwards until both gas jars are uniformly brown.

• The gases mix randomly to give an even mixture of bromine and air particles.

• The same experiment can be carried out with hydrogen and air; identical explosions result from both jars showing diffusion has occurred in each.

Different Speeds of Gases

• Ammonia (NH3) and hydrogen chloride (HCl) gases react to give white solid ammonium chloride (NH4Cl):

  NH3(g) + HCl(g) -> NH4Cl(s)

• Pieces of cotton wool soaked in concentrated ammonia solution and concentrated hydrochloric acid are placed at the ends of a long glass tube.

• Ammonia and hydrogen chloride particles diffuse along the tube and form a white ring of solid ammonium chloride where they meet.

• The ring forms closer to the hydrochloric acid end because ammonia particles are lighter and move faster.

Diffusion in Liquids

• Diffusion through a liquid is very slow if the liquid is completely still.

• If a small jar of strongly colored solution (such as potassium manganate(VII) solution) is placed in a gas jar of water, it can take days for the color to diffuse throughout all the water.

• This occurs because particles in a liquid move more slowly and are closer together than in gases.

The Dilution of Colored Solutions

• If 0.01 g of potassium manganate(VII) is dissolved in 1 cm^3 of water, it forms a deep purple solution. If we take the volume of 1 drop as 0.05 cm^3, there are 20 drops in 1 cm^3, and each drop contains 0.0005 g of potassium manganate(VII).

• If you dilute this solution by adding water until the total volume is 10000 cm^3, you should still just be able to see the purple color.

Solutes, Solvents, and Solutions

• When a solid dissolves in a liquid:

  • The substance that dissolves is the solute.

  • The liquid it dissolves in is called the solvent.

  • The liquid formed is a solution.

• When a solution is made, the attractive forces between the particles in the solute (solid) are broken and new attractive forces are formed between the solvent and solute particles.

• Whether a particular solid is soluble in any solvent depends on whether the new attractive forces are strong enough to overcome the old ones.

The Solubility of Solids

• The solubility of a solid in a solvent at a particular temperature is usually defined as the mass of solute which must dissolve in 100 g of solvent at that temperature to form a saturated solution. In other words, it is the maximum mass of solute that dissolves in 100 g of solvent at a particular temperature.

• A saturated solution is a solution which contains as much dissolved solid as possible at a particular temperature. There must be some undissolved solute present.

• For example, the solubility of sodium chloride (common salt) in water at 25°C is about 36 g per 100 g of water.

Measuring Solubility

• To measure the solubility of potassium nitrate in water at 40°C:

  1. Weigh an evaporating basin.

  2. Heat a boiling tube of water to just above 40°C.

  3. Add potassium nitrate to the water in the boiling tube and stir rapidly until no more of it will dissolve and there is undissolved solid left over.

  4. Allow the solution to cool to exactly 40°C.

  5. Pour off some of the solution into the evaporating basin (it is important that you only pour off solution and no solid). You do not have to pour off all the solution.

  6. Weigh the evaporating basin and contents.

  7. Heat the evaporating basin and contents gently to evaporate off all the water.

  8. When it looks as if all the water has evaporated, weigh the evaporating basin and contents.

  9. Heat the evaporating basin and contents again and then re-weigh. This is to make sure that all the water has, indeed, evaporated and is called heating to constant mass.

• We heat the solution gently to make sure that none spits out. If some did spit out, we would record a lower mass of solid, and the solubility would appear to be lower than the actual value.

• Wear eye protection and heat gently to avoid burns from hot solid spitting out of the basin.

Solubility Curves

• The solubility of solids changes with temperature, and this can be plotted on a solubility curve.

• Most solids have solubility curves where their solubility increases with temperature.

• You can use solubility curves to work out what mass of crystals you would get if you cooled a saturated solution.

• Consider the solubility curve for potassium nitrate (KNO3). At 90°C, 200 g of potassium nitrate dissolves in 100 g water. At 30°C, only 50 g will dissolve.

• Therefore, if we have a solution containing 200 g of potassium nitrate in 100 g of water and let it cool down from 90°C to 30°C, 150 g of potassium nitrate must be released from the solution, which it does as crystals. We say that potassium nitrate crystallises out of the solution or precipitates out of the solution.

• The dashed lines marked on the graph come from answers to the next part of the question. In this case, the solubility curve is virtually a straight line - this will not always be the case. If you are asked to draw a line of best fit, it can be either a straight line or a curve.

Calculations using Solubility

• The results for this experiment could be:

  • Mass of evaporating basing: 25.72 g

  • Mass of evaporating basin + solution: 58.00 g

  • Mass of evaporating basin + dry crystals: 38.00 g

• We need to calculate the mass of the solid and also the mass of water evaporated from the solution:

  • mass of crystals = 38.00 - 25.72 = 12.28g

  • mass of water = 58.00 - 38.00g = 20.00g

• 12.28 g of solid is the maximum mass that dissolves in 20.00 g of water, therefore 5 times as much would dissolve in 100 g of water. That works out at 61.4 g. The solubility of potassium nitrate at 40°C is therefore 61.4 g per 100 g of water.

• More generally, we can calculate the solubility of a substance in 100 g of solvent using the equation:

  solubility (g/100 g) = \frac{mass \space of \space solute}{mass \space of \space solvent} * 100

Chapter Questions

1. What name is given to each of the following changes of state?

   • a. Solid to liquid - Melting

   • b. Liquid to solid - Freezing

   • c. Solid to gas - Sublimation

   • d. Gas to solid - Deposition

2. a. Draw diagrams to show the arrangement of the particles in a solid, a liquid, and a gas.
   b. Describe the difference between the movement of particles in a solid and a liquid.

   • i. The movement of particles in a solid is limited to vibration about fixed positions, whereas in a liquid, particles can move around each other.

3. The change of state from a liquid to a gas can be either evaporation or boiling. Explain the difference between evaporation and boiling.

   • Evaporation occurs at any temperature and only at the surface of a liquid, whereas boiling occurs at a specific temperature (boiling point) and throughout the liquid.

4. The questions refer to the substances in the table.

a. Write down the physical states of each compound at

*   i. 30°C: A - gas, B - liquid, C - solid, D - solid
*   ii. -100°C: A - liquid, B - solid, C - solid, D - solid
*   iii. 80°C: A - gas, B - gas, C - solid, D - solid

b. Which substance has the greatest distance between its particles at 25°C? Explain your answer.

*   Substance A has the greatest distance between its particles because it is a gas at 25°C, and in gases, particles are much further apart.

c. Why is no boiling point given for substance C?

*   No boiling point is given for substance C because it sublimes, meaning it goes directly from a solid to a gas without becoming a liquid.

d. Which liquid substance would evaporate most quickly in the open air at 25°C? Explain your answer.

*   Substance B would evaporate most quickly because it has the lowest boiling point (34.5°C), indicating weaker interparticle forces compared to substance D.

5. The table shows the solubility of potassium chloride at various temperatures.

6. Refer to Figure 1.14 on page 7 showing the diffusion experiment.

a. Explain why the ring takes a few minutes to form.

*   The ring takes a few minutes to form because the gas particles have to diffuse through the air in the tube, colliding with other particles along the way, which slows down their movement.

b. i. If you heat a gas, what effect will this have on the movement of the particles?

*   i. Heating a gas will increase the kinetic energy of the particles, causing them to move faster.

ii. In the light of your answer to i, what difference would you find if you did this experiment outside on a day when the temperature was 2°C instead of in a warm lab at 25°C? Explain your answer.

*   ii. If the experiment were done at 2°C instead of 25°C, the particles would move more slowly, and the ring would take longer to form because the particles have less kinetic energy at lower temperatures.

c. Explain why the ring was formed nearer the hydrochloric acid end of the tube.

*   c. The ring formed nearer the hydrochloric acid end of the tube because ammonia particles are lighter and move faster than hydrogen chloride particles, so they travel a greater distance in the same amount of time.

d. i. Suppose you replaced the concentrated hydrochloric acid with concentrated hydrobromic acid. This releases the gas hydrogen bromide (HBr). Hydrogen bromide also reacts with ammonia to form a white ring. Suggest a name for the white ring in this case.

*   i. The white ring would be ammonium bromide (NH_4Br).

ii. Hydrogen bromide particles are about twice as heavy as hydrogen chloride particles. What effects do you think this would have on the experiment?

*   ii. If hydrogen bromide particles are about twice as heavy as hydrogen chloride particles, they would diffuse more slowly, and the ring would form even closer to the hydrobromic acid end of the tube.

7. Use the words given below to complete the following paragraph. Each word may be used once, more than once, or not at all.
   boils solution solute saturated evaporates solvent condenses

   • Sodium chloride dissolves in water to form a solution, and the sodium chloride is the solute. The water is called solvent. If the solution is heated to 50°C, some of the water evaporates until the solution becomes saturated and sodium chloride crystals start to form.