IGCSE CHEMISTRY

• Solids

  • Solids have a fixed volume and shape and they have a high density.
  • The atoms vibrate in position but can’t change location
  • The particles are packed very closely together in a fixed and regular pattern
• Liquids

  • Liquids also have a fixed volume but adopt the shape of the container
  • They are generally less dense than solids (an exception is water), but much denser than gases
  • The particles move and slide past each other which is why liquids adopt the shape of the container and also why they are able to flow freely
• Gases 

  • Gases do not have a fixed volume, and, like liquids, take up the shape of the container
  • Gases have a very low density
  • Since there is a lot of space between the particles, gases can be compressed into a much smaller volume
  • The particles are far apart and move randomly and quickly (around 500 m/s) in all directions
  • They collide with each other and with the sides of the container (this is how pressure is created inside a can of gas)
• • Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solid expands until the structure breaks and the solid melts
• • When a liquid is heated, the substance expands more and some particles at the surface gain sufficient energy to overcome the intermolecular forces and evaporate
• Melting

  • Melting is when a solid changes into a liquid
  • Requires heat energy which transforms into kinetic energy, allowing the particles to move
  • Occurs at a specific temperature known as the melting point (m.p.) 
• Boiling

  • Boiling is when a liquid changes into a gas
  • 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 within the liquid
  • Occurs at a specific temperature known as the boiling point (b.p.)
• Evaporation

  • Evaporation occurs when a liquid changes into a gas and occurs over a range of temperatures
  • Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquid's surface at low temperatures, below the b.p. of the liquid
  • The larger the surface area and the warmer the liquid surface, the more quickly a liquid can evaporate
• 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
  • Requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature 
• Condensation

  • Condensation occurs when a gas changes into a liquid on cooling and it 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

  • Sublimation occurs when a solid changes directly into a gas
  • This only happens to a few solids, such as iodine or solid carbon dioxide
  • The reverse reaction also happens and is called desublimation or deposition
• • The pressure that gas creates inside a closed container is produced by the gaseous particles hitting the inside walls of the container
• • An increase in temperature increases the kinetic energy of each particle, as the heat energy is transformed to kinetic energy, so they move faster
  • As the temperature increases, the particles in the gas move faster, impacting the container's walls more frequently
  • If the container walls are flexible and stretchy then the container will get bigger and bigger, just like the hot air balloon!
  • If the container is made smaller, then the gas particles hit the wall more frequently
  • So when there is a decrease in volume this causes an increase in gas pressure
• • Diffusion occurs much faster in gases than in liquids as gaseous particles move much quicker than liquid particles
• • Lighter gas particles can travel faster and hence further, therefore the lower its relative mass the faster a gas will diffuse
• Element

  • A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler
• Compound

  • A pure substance made up of two or more elements chemically combined 
• Mixture

  • A combination of two or more substances (elements and/or compounds) that are not chemically combined
  • Mixtures can be separated by physical methods such as filtration or evaporation
• 1. Describe the structure of the atom as a central nucleus containing neutrons and protons surrounded by electrons in shells
• 1. Define proton number / atomic number as the number of protons in the nucleus of an atom
• 1. Define mass number / nucleon number as the total number of protons and neutrons in the nucleus of an atom
• 1. Define isotopes as different atoms of the same element that have the same number of protons but different numbers of neutrons
• State that isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration
• positive ions are known as cation and negative ions are known as anion
• 1. State that an ionic bond is a strong electrostatic attraction between oppositely charged ions
• ion- a charged particle formed by either gaining or losing electron
• an ionic bond is a strong electrostatic attraction between oppositely charged ions
• ionic structure- A giant lattice structure of atoms that have a regular arrangement of alternating positive and negative ions
• • ionic compounds are usually solid at room temperature 
  • They have high melting and boiling points
  • Ionic compounds are good conductors of electricity in the molten state or in solution 
  • They are poor conductors in the solid state
• • Ionic substances have high melting and boiling points due to the presence of strong electrostatic forces acting between the oppositely charged ions
  • These forces act in all directions and a lot of energy is required to overcome them
  • The greater the charge on the ions, the stronger the electrostatic forces and the higher the melting point will be
• 1. State that a covalent bond is formed when a pair of electrons is shared between two atoms leading to noble gas electronic configurations
• • For electrical current to flow there must be freely moving charged particles such as electrons or ions present 
  • Ionic compounds are good conductors of electricity in the molten state or in solution as they have ions that can move and carry a charge
  • They are poor conductors in the solid state as the ions are in fixed positions within the lattice and are unable to move
• 1. a covalent bond is formed when a pair of electrons is shared between two atoms leading to noble gas electronic configurations
• Simple covalent bonds have low melting point due to The intermolecular forces in covalent compounds being weak ( but they have strong covalent bondings)

  
  

  
  

  they are poor conductors of electricity because there are no free electrons or ion to carry the charge
• allotropes- different atomic or molecular arrangement of the same element in the same physical state
• Diamond

  • In diamond, each carbon atom bonds with four other carbons, forming a tetrahedron
  • All the covalent bonds are identical, very strong and there are no intermolecular forces
• Graphite

  • Each carbon atom in graphite is bonded to three others forming layers of hexagons, leaving one free electron per carbon atom which becomes delocalised
  • The covalent bonds within the layers are very strong, but the layers are attracted to each other by weak intermolecular forces
• Properties of Diamond:

  • Does not conduct electricity
  • Has a very high melting point
  • Is extremely hard and dense
• All the outer shell electrons in carbon are held in the four covalent bonds around each carbon atom, so there are no freely moving charged particles to carry the current, thus diamond cannot conduct electricity
• Diamond's hardness makes it very useful for purposes where an extremely tough material is required