chemistry

• The three states of matter are solids, liquids and gases

• A substance can usually exist in all three states, dependent on temperature (and pressure)

• State changes occur at the melting point (solid to liquid, liquid to solid) and at the boiling point (liquid to gas and gas to liquid)

  • Melting and freezing occur at the melting point

  • Boiling and condensing take place at the boiling point

• 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 sphere

    Summary of the properties of the three states of matter

    	Solid	Liquid	Gas
    Diagram
    Arrangement of particles	Regular arrangement	Randomly arranged	Randomly arranged
    Movement of particles	Vibrate about a fixed position	Move around each other	Move quickly in all directions
    Closeness of particles	Very close	Close	Far apart

Changing 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 between the particles, the more energy that is needed to overcome them 

• Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance

• Changing states is a physical change

  • The particles themselves remain the same, it is just the forces between the particles which change 

Melting

• Melting is when a solid changes into a liquid

• Heat / thermal energy absorbed by the particles is transformed into kinetic energy

• This causes the particles to vibrate more and start to move / flow

• Melting happens at a specific temperature, known as the melting point (m.p.) 

Boiling

• Boiling and evaporation are both when a liquid changes into a gas

  • However, there is a key difference between boiling and evaporation

• In boiling, heat / thermal energy causes bubbles of gas to form inside the liquid, allowing for liquid particles to escape from the surface and within the liquid

• Boiling happens at a specific temperature, known as the boiling point (b.p.)

Freezing

• Freezing is when a liquid changes into a solid

• This is the reverse of melting and occurs at the same temperature as melting

  • So, the melting point and freezing point of a pure substance are the same

  • For example, water freezes and melts at 0 ºC

• Freezing needs a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature 

Evaporation

• Evaporation occurs over a range of temperatures

  • It can happen at temperatures below the boiling point of the liquid

• Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquid's surface at low temperatures

• The larger the surface area and the warmer the liquid surface, the more quickly a liquid can evaporate

Condensation

• Condensation occurs when a gas changes into a liquid on cooling and takes place over a range of temperatures

• When a gas is cooled its particles lose energy and when they bump into each other they lack the energy to bounce away again, instead, they group 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

Diffusion and 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
  

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

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 solute 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