chemical changes

• Chemical reactions involve the breaking and forming of chemical bonds between atoms.
• an indicator is a dye that changes colour
• titrations allow you to find the concentration of a solution by measuring the volume of acid or base needed to neutralise a given volume of solution
• Use the pipette and pipette filler to add a measured volume of sodium hydroxide solution to a clean conical flask
• Add a few drops of indicator and put the conical flask on a white tile
• Fill the burette with hydrochloric acid and note the starting volume
• Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix
• Stop adding the acid when the end-point is reached (when the indicator first permanently changes colour). Note the final volume reading
• Repeat steps 1 to 5 until concordant titres are obtained
• More accurate results are obtained if acid is added drop by drop near to the end-point
• to increase the accuracy of your titration you need several consistent readings
• repeat titration to increase accuracy
• acids ionise in aqueous solutions to produce hydrogen ions (H+)
• the strength of an acid depends on how many H+ it produces per molecule of acid
• strong acids completely dissociate into their constituent ions, whereas weak acids only partially dissociate
• a strong base will react with all the H+ produced from a given amount of acid
• HCl ----> H+ + Cl-
• weak acids do not fully ionise solution. only a small proportion of acid particles dissociate to release H+ ions
• neutralisation reactions involve the transfer of protons between substances
• the ph of a solution is a measure of the concentration of H+ ions in the solution.
• for every decrease of 1 on the pH scale, the concentration of hydrogen ions increases by 10.
• sulfuric acid is an example of strong acid
• acids form H+ ions in water
• alkalis form OH- ions in water and are bases
• metal oxides and metal hydroxides are bases
• acid + medal oxide = salt + water
• acid + medal hydroxide = salt + water
• metal + water = metal hydroxide + hydrogen
• hydrochloric acid + copper oxide = copper chloride +water
• acid + metal carbonate = salt + water + carbon dioxide
• metal carbonates are bases
• acids react with metals to form salts and hydrogen gas
• you can make salts using an insoluble base (practical)
• 1. gently warm the dilute acid using a bunsen burner, then turn off the bunsen burner.
  2. add the insoluble base to the acid a bit at a time, until you see excess.
  3. filter out the excess solid using filter paper to get salt solution
  4. gently heat the solution using a water bath to evaporate some water.
• a reaction that separates a metal from its oxide is called reduction
• in reduction, the oxygen is removed and carbon gain oxygen and is oxideised. e.g, iron oxide + carbon = iron + carbon dioxide
• the positive ions are called cations and will move towards the cathode and gain electrons (they're reduced)
• negative ions move towards the anode and lose electrons (they're oxidised)
• metals form positive ions, so they're attracted to the negative electrode
• the cathode is negative