Science

• Temperature


  Measure of how hot an object is
• SI unit of temperature

  Kelvin (K)
• Expansion and Contraction


  • Take place in solids, liquids and gases
  • Object gains heat, expands
  • Object loses heat, contracts
• Applications of expansion and contraction


  • Opening a jar (lid expands when placed in hot water, making it easier to unscrew)
  • Bimetallic strips (made of 2 metals that expand at different rates, used in thermostats and bimetallic thermometers to keep appliances at a constant temperature)
• Ways heat can be transferred from one object to another

  • Conduction
  • Convection
  • Radiation
• Conduction


  The transfer of heat energy without the physical movement of the medium
• Conduction


  • A metal ladle inside a pot of hot soup - heat is transferred from the heated end of the ladle which is in contact with the heat source, to the other end of the ladle
• Solids

  • Usually better conductors of heat than fluids
  • Particles are arranged more neatly and closely packed to one another than those in gas and liquids
  • Close arrangement of particles enables faster and quicker transfer of heat
• Convection


  The transfer of heat from one place to another by the movement of a medium
• Fluids (liquids and gases) mainly transfer heat through convection
• How convection works in liquids

  1. Water is heated up from the bottom

  2. As water is heated, it expands and its volume increases

  3. The mass stays constant and the increase in volume results in a decrease in density

  4. The hotter and less dense water rises to the top

  5. The cooler and denser water sinks to the bottom

  6. Convection currents are formed and heat energy is evenly distributed
• Radiation


  The transfer of heat energy from a hotter body to a cooler body without the need for a medium
• Factors affecting rate of radiation

  • Temperature of an object
  • Surface area of an object
  • Dark coloured objects absorb heat better than light coloured surfaces
  • Dull, rough surfaces are also better absorbers of radiation
• Applications of heat transfer

  • Convection - Air-con, heater, kettle, sea breeze, land breeze
  • Radiation - Buildings, clothes, kettles, emergency blanket, solar collector
  • Conduction - cooking
• A change in which no new substances are formed is called a physical change. Freezing and boiling are examples of physical changes
• A change in which new substances are formed is called a chemical change. When a chemical change occurs, it means that a chemical reaction has taken place.

  Substances that react with each other are known as reactants. The new substances formed are known as products.
• A word equation includes the reactants (which are on the left) used and the products (right) formed in a chemical reaction.
• Example of word equations:

  methane + oxygen —> carbon dioxide + water
• When magnesium reacts with oxygen gas to form magnesium oxide, we can write this as a word equation: Magnesium + Oxygen → Magnesium Oxide
• During a chemical reaction, atoms of the reactants rearrange themselves and combine chemically to form products. The atoms of the products are always obtained from the reactants.
• The total

  number of atoms before and after a chemical reaction is

  balanced. This means that the number of each type of atom remains the same during the reaction.
• Different types of chemical changes:

  • Combustion
  • Thermal decomposition
  • Oxidation
  • Neutralisation
• Combustion is a chemical reaction in which a substance is heated in the presence of oxygen to form one or more substances. Heat and light are usually given out.
• Thermal decomposition is the process in which a substance breaks down into two or more simpler substances upon heating. One example is: calcium carbonate —> calcium oxide + carbon dioxide
• Oxidation is a chemical change that a substance undergoes when it gains oxygen. One example is Rusting, it occurs when iron reacts with oxygen and water to form rust.
• Acids

  • Have a sour taste
  • May be dangerous and can burn the skin
  • turns blue litmus paper red
  • reacts with alkalis, metals and carbonates
  • Examples: citric acid, sulphuric acid, nitric acid, hydrochloride acid
• Alkalis (Or Bases)

  • have a bitter taste
  • have a soapy feel
  • can be dangerous and can burn the skin
  • turns red litmus paper blue
  • reacts with acids in neutralisation reactions
  • Examples : sodium hydroxide, calcium hydroxide, baking soda, ammonia
• Chemical reactions between Acids and Alkalis:

  1. Acid + Alkali = Salt + Water (A + A = S + H2O)
  2. Metal + Acid = Salt + Hydrogen (M + A = S + H2)
  3. Carbonate + Acid = Salt + Hydrogen + Carbon Dioxide (C + A = S + H + C02)
• We can use the pH scale to measure how acidic or alkali a solution is. Solutions with pH levels less than 7 are acidic. Solutions with pH levels more than 7 are alkaline. Solutions which are very acidic or very alkaline can burn our skin.
• Applications of neutralisation

  When an alkali is added to an acid, the pH of the solution formed increases as the alkali neutralises the acid. When an acid is added to an alkali, the pH of the solution formed decreases. Upon complete neutralisation, the solution formed has a pH level of 7 and is neither acidic nor alkaline.

  Some examples of neutralisation reactions in daily life are: Antacid, toothpaste, substances for soil
• What causes chemical changes to occur?

  • mixing (e.g. reactions of acids with alkalis, metals and carbonates)
  • heating (e.g. carbon undergoes complete combustion to form carbon dioxide gas)
  • exposure to light (e.g. photosynthesis)
  • interaction with oxygen (e.g. rusting, cellular respiration)
  • using an electric current (e.g. electroplating)
• small area


  more force exerted
• The smaller the contact area


  The higher the pressure exerted
• The unit of pressure is newton per square metre, also known as the pascal (Pa)
• Liquid pressure at a point within the liquid


  Depends on the depth of the point
• As there are more water particles at the bottom of the container/water, the gravitational force on the water particles pushing down on the bottom of the container applies a larger force. Hence, there is higher pressure exerted at the bottom of the container/water
• The layer of air also exerts pressure on all objects and beings on earth
• Atmospheric pressure at a point within the air

  Depends on the altitude and height of the point
• Low Atmospheric Pressure at Elevated Heights


  Atmospheric pressure on Earth depends on the height of the object. As Earth is surrounded by a thick layer of air that have mass, the layer of air particles pushes down on Earth's surface due to Earth's gravitational force, exerting a pressure of approximately 100 000 Pa at sea level. When we are at a height above the sea level (e.g., on a mountain), fewer air particles push down on us. Hence, the atmospheric pressure exerted on us decreases.
• Humidity is the concentration of water vapour in the air