Chemistry paper 1

• The pH scale is logarithmic, meaning that the difference between two values on the scale is not equal to their actual differences.
• Acids are substances which release H+ ions when dissolved in water
• A solution with a pH value of 7 has an acidity equivalent to one part hydrogen ion (H+) per million parts water
• A solution with a pH value of 7 has an acidity or alkalinity equivalent to pure water.
• Acidic solutions have lower pH values than neutral ones (less than 7).
• Bases are substances which release OH- ions when dissolved in water
• A solution with a pH value less than 7 is more acidic than pure water.
• Neutralisation reactions occur when an acid reacts with a base to form salt and water
• Alkalis are bases which have been dissolved in water
• A neutral solution has a pH value of 7
• An indicator is a substance whose colour changes depending on whether it's in contact with an acid or base.
• Neutralisation reactions occur when acids react with alkalis or bases
• Strong acids completely dissociate into H+ ions and anions in aqueous solutions.
• Weak acids only partially dissociate into H+ ions and anions in aqueous solutions.
• An indicator changes colour depending on whether it's in an acidic or basic environment
• Base + Acid → Salt + Water
• The strength of an acid depends on the number of hydrogen atoms that can be replaced by metal ions, as well as its ability to donate protons (hydrogen ions).
• The reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base) produces sodium chloride (a soluble ionic compound) and water.
• Acidic solutions contain hydrogen ions (H+) as the main cation, while basic solutions contain hydroxide ions (OH-) as the main anion.
• Indicators change colour because they contain weak acids that can be protonated or deprotonated depending on the pH level of their environment
• Acids react with metals to produce salts and hydrogen gas.
• Bases react with acids to produce salts and water.
• A neutralisation reaction is when an acid reacts with a base to form salt and water.
• A neutral solution has equal numbers of hydrogen ions and hydroxide ions.
• Stronger acids release more H+ ions than weaker ones
• Solutions containing more than one type of solute may have different boiling points due to differences in intermolecular forces.
• In a titration experiment, a known volume of a standard solution is added dropwise to another solution until the desired endpoint is reached.
• Atomic theory proposed by Democritus from ancient Greece around 500 BC:

  • Everything is made up of tiny particles that cannot be broken down further
  • Particles are separated by empty space
• John Dalton's atomic theory in the 1800s:

  • Atoms described as solid spheres
  • Different types of spheres make up different elements
• J.J. Thompson's Plum Pudding Model in 1897:

  • Atoms cannot be solid spheres
  • Atoms contain negatively charged particles known as electrons
  • Proposed atom as a general ball of positive charge with discrete electrons stuck in it
• Ernest Rutherford's Gold Foil Experiment in 1909:

  • Positively charged alpha particles fired at a thin sheet of gold
  • Some alpha particles deflected to the side, some even deflected back
  • Disproved J.J. Thompson's Plum Pudding Model
  • Proposed the nuclear model with a compact nucleus containing all the positive charge and a cloud of negative electrons around it
• Niels Bohr's model in 1913:

  • Electrons orbit the nucleus similar to planets orbiting the sun
  • Electrons are held in shells
  • Prevents the atom from collapsing
• Rutherford's discovery of protons in the nucleus:

  • Positive charge in the nucleus made up of small discrete particles known as protons
• James Chadwick's discovery of neutrons in the nucleus:

  • Provided evidence for neutral particles in the nucleus known as neutrons
• Techniques for separating soluble and insoluble solids from liquids include filtration, evaporation, and crystallization
• If a solid doesn't dissolve in a liquid, it forms a mixture of a liquid and an insoluble solid (e.g., sand in water)
• If a solid dissolves in a liquid, it forms a solution with the solid being the solute and the liquid being the solvent (e.g., sodium chloride salt in water)
• Filtration is used to separate insoluble solids from liquids by using filter paper with tiny holes that allow water to pass through but not solids
• Evaporation is used to separate soluble solids from liquids by heating the solution in an evaporating dish or crucible to evaporate the solvent and leave dry crystals of the solid
• Some solids may decompose when heated (thermal decomposition), so for these, a slower technique called crystallization is used