Science

• The three states of matter are solid, liquid and gas. Melting and freezing take place at the melting point, boiling and condensing take place at the boiling point.
• Main features of the solid in a particle model

  In a solid, the particles are fixed in position and only vibrate – they can't flow around. 
• Main features in a liquid in a particle model

  In a liquid, the particles are still very close together but they can flow past each other.
• Main features of a gas in a particle model

  In a gas, the particles move randomly and there is empty space between them.
• Define density.


  Density describes how closely packed the particles are in a solid, liquid or gas.
• Density in solids and liquids.

  In solids and liquids, the molecules are tightly packed together. The difference is, in a liquid, the molecules have enough energy to push past each other.

  As a result of this, the density of solids and liquids are roughly the same
• Density in gas

  • In a gas, the molecules are widely separated
  • As a result of this, gases have significantly lower densities than solids or liquids
• Gas pressure

  The higher the temperature, the greater the average kinetic energy and so the faster the average speed of the molecules.
• Gas pressure

  . The temperature of the gas is related to the average kinetic energy of the molecules.
• Internal energy


  Energy is stored inside a system by the particles (atoms and molecules) that make up the system.
• Heating and changes of state

  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 of the substance.
• Heating and changes of state.

  . The stronger the forces between the particles the higher the melting point and boiling point of the substance.
• When a material is heated or cooled, two changes may happen to the particles within the material:

  • Chemical bonds between the particles may form, break or stretch. There is a change in the chemical potential store of energy in the material.
  • The material will heat up or cool down as the particles within it gain or lose speed. There is a change in the thermal store of energy within the material.
• Heating and changes of state

   If bonds are stronger, more internal energy will be needed to break them and so the temperature for melting or boiling will be higher.
• Specific Heat Capacity


  The specific heat capacity of a material is the energy required to raise one kilogram (kg) of the material by one degree Celsius (°C).
• Specific Latent Heat


  Specific latent heat is the amount of energy required to change the state of 1 kilogram (kg) of a material without changing its temperature.
• The purity of a a substance


  a pure substance is a single element or compound, not mixed with any other substance
• 'everyday' definition of purity

  , a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state.
• development of the atomic model

  it was found that the mass of electrons, which had recently been discovered, was very much less than the mass of atoms so they must be sub-atomic particles, then an experiment showed that most of the alpha particles directed at thin gold foil passed through but a few bounced back, suggesting the positive charge was concentrated at the center of each gold atom. and finally discovery of neutrons in the nucleus which explained why the mass of atoms was greater than could be accounted for by the mass of the protons.
• size of an atom


  Atoms are very small, having a radius of about 0.1 nm (1 x 10–10 m).
• size of a molecule


  The radius of a small molecule is about 0.5 nm (5 x 10–10 m).
• the atom is a positively charged nucleus surrounded by negatively charged electrons, with the nuclear radius much smaller than that of the atom and with almost all of the mass in the nucleus.
• relative mass and charges for a proton


  mass = 1 charge= +1
• relative mass and charge of a neutron


  mass= 1 charge= 0
• relative mass of an electron


  mass= very small charge= -1
• The sum of the protons and neutrons in an atom is its mass number
• isotopes


  Atoms of the same element can have different numbers of neutrons
• An electron microscope has a much higher resolving power than a light microscope so it can be used to study cells in much finer detail.
• Plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and a nucleus containing the genetic material.
• Bacterial cells (prokaryotic cells) are much smaller in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and may have one or more small rings of DNA called plasmids
• Most animal cells have the following parts: a nucleus , cytoplasm, a cell membrane, mitochondria, ribosomes.
• In addition to the parts found in animal cells, plant cells often have: chloroplasts, a permanent vacuole filled with cell sap.
• diffusion


  the net movement of particles from an area of high concentration to an area of lower concentration
• Factors that affect the rate of diffusion across a membrane are: the difference in concentration, the temperature, the surface area of the membrane
• osmosis


  water diffuses from where it is more concentrated through a partially permeable membrane to where water is less concentrated
• active transport


  the movement of a dissolved substance from a region where it is less concentrated to a region where it is more concentrated. This requires energy from respiration.
• Mitosis


  The process of cell division in which a single cell divides into two or more daughter cells that are genetically identical to the parent cell
• Cell cycle


  The series of stages a cell goes through, including growth, DNA replication, and cell division
• Chromosome carries a large number of genes
• Reason for cell division

  • Organisms can grow during development of multicellular organisms
  • Repair damaged tissues