💥 Chap 1 Priciples of Chemistry - OL Chemistry Revision Notes

1.1 States of Matter
   1.1.1 The Three States of Matter    1.1.2 Diffusion & Dilution    1.1.3 Solutions    1.1.4 Solubility    1.1.5 Practical: Investigate the Solubility of a Solid in Water at a Specific Temperature

The Three States of Matter

• The three states of matter are solids, liquids and gases
• A substance can usually exist in all three states, dependent on temperature (and pressure)
• Different state changes occur at the melting point and at the boiling point depending on whether the substance is heating up or cooling down
  • At the melting point
    • Melting (solid → liquid) when heating up
    • Freezing (liquid → solid) when cooling down
  • At the boiling point
    • Boiling (liquid → gas) when heating up
    • Condensing (gas → liquid) when cooling down

• Individual atoms themselves do not share the same properties as bulk matter
• The three states of matter can be represented by a simple model
  • In this model, the particles are represented by small solid spheres

Summary of the Properties of Solids, Liquids and Gases

Interconversion Between the States of Matter

• The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles
  • The stronger the forces of attraction, the more energy that is needed to overcome them for a state change to occur
  • Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance

• When matter changes from one state to another due to changes in temperature or pressure, the change is called an interconversion of state
• It is a physical change involving changes in the forces between the particles of the substances, the particles themselves remain the same, as do the chemical properties of the substance
• Physical changes are relatively easy to reverse as no new substance is formed during interconversions of state
• The interconversions have specific terms to describe them:

A Summary of State Changes 

Melting

• Melting is when a solid changes into a liquid
• The process requires heat energy which transforms into kinetic energy, allowing the particles to move
• It occurs at a specific temperature known as the melting point which is unique to each pure solid 

Boiling

• Boiling is when a liquid changes into a gas
• This requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and from within the liquid
• It occurs at a specific temperature known as the boiling point which is unique to each pure liquid 

Freezing

• Freezing is when a liquid changes into a solid
• This is the reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the same
  • Water for example freezes and melts at 0 ºC

• It requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature which is unique for each pure substance 

Evaporation

• When a liquid changes into a gas
• Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquids surface at low temperatures, below the boiling point of the liquid
• The larger the surface area and the warmer the liquid/surface, the more quickly a liquid can evaporate
• Evaporation occurs over a range of temperatures, but heating will speed up the process as particles need energy to escape from the surface

Condensation

• When a gas changes into a liquid, usually on cooling
• When a gas is cooled its particles lose energy and when they bump into each other, they lack energy to bounce away again, instead grouping together to form a liquid

Sublimation

• When a solid changes directly into a gas
• This happens to only a few solids, such as iodine or solid carbon dioxide
• The reverse reaction also happens and is called desublimation or deposition

Interconversion between the three states of matter

 

Diffusion & Dilution

• Diffusion and dilution experiments support a theory that all matter (solids, liquids and gases) is made up of tiny, moving particles

Diffusion in gases

Diffusion of red-brown bromine gas

Description:

• Here, we see the diffusion of bromine gas from one gas jar to another
• After 5 minutes the bromine gas has diffused from the bottom jar to the top jar

Explanation: 

• The air and bromine particles are moving randomly and there are large gaps between particles
• The particles can therefore easily mix together

Diffusion in liquids

Diffusion of potassium manganate(VII) in water over time

Description:

• When potassium manganate (VII) crystals are dissolved in water, a purple solution is formed
• A small number of crystals produce a highly intense colour

Explanation: 

• The water and potassium manganate (VII) particles are moving randomly and the particles can slide over each other
• The particles can therefore easily mix together
• Diffusion in liquids is slower than in gases because the particles in a liquid are closely packed together and move more slowly

Dilution

Dissolving potassium manganate (VII) in water

Description:

• When potassium magnate (VII) crystals are dissolved in water, the solution can be diluted several times
• The colour fades but does not disappear until a lot of dilutions have been done

Explanation:

• This indicates that there are a lot of particles in a small amount of potassium manganate (VII) and therefore the particles must be very small

Solutions Terminology

• You need to know all the following terms used when describing solutions:

Terminology About Solutions Table

Solubility

• Solubility is a measurement of how much of a substance will dissolve in a given volume of a liquid
  • The liquid is called the solvent
  • The solubility of a gas depends on pressure and temperature

• Different substances have different solubilities
• Solubility can be expressed in g per 100 g of solvent
• Solubility of solids is affected by temperature
  • As temperature increases, solids usually become more soluble

• Solubility of gases is affected by temperature and pressure; in general:
  • As pressure increases, gases become more soluble
  • As temperature increases, gases become less soluble

Solubility Curves

• Solubility graphs or curves represent solubility in g per 100 g of water plotted against temperature
• To plot a solubility curve, the maximum mass of solvent that can be dissolved in 100 g of water before a saturated solution is formed, is determined at a series of different temperatures

Solubility curve for three salts. While the solubility of most salts increases with temperature, sodium chloride, or common salt, hardly changes at all

Worked Example

Use the solubility curve to answer these questions:

1. Determine how much potassium nitrate will dissolve in 20 g of water at 40 °C?
2. 200 cm3 of saturated lead(II)nitrate solution was prepared at a temperature of 90 °C. What mass of lead(II)nitrate crystals form if the solution was cooled to 20 °C?

Answers

Problem 1

At 40 °C the solubility is 68 g per 100 g of water

So scaling, 68 x (20 / 100) = 13.6 g of potassium nitrate will dissolve in 20 g of water

Problem 2

Solubility of lead(II) nitrate at 90 oC is 118 g / 100 g water, and 64 g / 100 g water at 20 °C.

Therefore for mass of crystals formed = 118 – 64 = 54 g (for 100 cm3 of solution).

However, 200 cm3 of solution was prepared,

So total mass of lead(II) nitrate crystallised = 2 x 54 = 108 g

Practical: Investigate the Solubility of a Solid in Water at a Specific Temperature

Aim:

To measure the solubility of a salt at different temperatures

Method:

1. Prepare a two beakers, one as a hot water bath and one as an ice bath
2. Using a small measuring cylinder, measure out 4 cm3 of distilled water into a boiling tube.
3. On a balance weigh out 2.6 g of ammonium chloride and add it to the boiling tube
4. Place the boiling tube into the hot water bath and stir until the solid dissolves
5. Transfer the boiling tube to the ice bath and allow it to cool while stirring
6. Note the temperature at which crystals first appear and record it in a table of results
7. Add 1 cm3 of distilled water then warm the solution again to dissolve the crystals
8. Repeat the cooling process again noting the temperature at which crystals first appear.
9. Continue the steps until a total of 10 cm3 of water has been added

Apparatus for investigating the solubility of a salt with temperature

Results:

Table of Results for Recording Solubility

 

Graph:

Use the results to plot a solubility curve for ammonium chloride at different temperatures. Solubility is on the y-axis and temperature is on the x-axisConclusion:

The shape of the graph will allow to state how the solubility varies with temperature