Chemistry gcse topic 3

• Solutions can be classified as acidic, alkaline or neutral depending on their pH.
• The pH scale measures how acidic or alkaline a solution is and ranges from 0 to 14.
• Anything that forms a solution with a pH of less than 7 is an acid.
• The lower the pH, the more acidic the solution.
• Anything that forms a solution with a pH of greater than 7 is an alkali.
• The higher the pH, the more alkaline the substance is.
• Neutral substances are neither acidic nor alkaline and have a pH of exactly 7.
• Pure water is an example of a neutral substance.
• Acids release hydrogen ions (H*) when they are in an aqueous solution.
• A base is a substance that reacts with an acid to produce a salt and water.
• An alkali is a base that is soluble in water.
• Alkalis form OH- ions (otherwise known as hydroxide ions) in water.
• A neutral solution has a pH of 7, and that acidic solutions have lower pH values and alkaline solutions have higher pH values.
• Acids in solution are sources of hydrogen ions.
• A base is any substance that reacts with an acid to form a salt and water only.
• Alkalis are soluble bases.
• Alkalis in solution are sources of hydroxide ions.
• The higher the concentration of hydrogen ions in a solution, the more acidic it is, so the lower its pH will be.
• As the concentration of hydrogen ions increases, the pH decreases.
• In alkaline solutions, the higher the concentration of OH- ions, the higher the ph.
• pH can be measured using an indicator, which is a dye that changes colour depending on whether it's above or below a certain pH.
• Indicators are simple to use - add a few drops to the solution you're testing then compare the colour the solution goes to a pH chart for that indicator.
• An example of an indicator is Universal indicator.
• Litmus, methyl orange and phenolphthalein are three indicators that change colour in certain solutions.
• The colour that each indicator changes in acidic solutions is different from the colour it changes in alkaline solutions.
• Three or four drops of indicator is more than enough when testing solutions.
• Acidic solutions have a pH less than 7 and alkaline solutions have a pH greater than 7.
• Acids react with bases in neutralisation reactions.
• An acid will react with a base to form a salt and water - this is called a neutralisation reaction.
• The general equation for a neutralisation reaction is shown below.
• Neutralisation reactions between acids and bases can also be shown as an ionic equation in terms of H* and OH- ions.
• During neutralisation reactions, hydrogen ions (H+*) from the acid react with hydroxide ions (OH) from the base to produce water.
• The equation for this reaction is: H+ (ag) +OH(ag)- H0.
• When an acid neutralises a base (or vice versa), the solution that's formed is neutral - it has a pH of 7.
• At pH 7, the concentration of hydrogen ions is equal to the concentration of hydroxide ions.
• An indicator can be used to show that a neutralisation reaction is over.
• Calcium oxide is a base that reacts with hydrochloric acid to give calcium chloride (a salt) and water.
• The equation for the reaction between calcium oxide and hydrochloric acid is: 2HCI + CaO -> CaCh, + H,O.
• To investigate how the pH of a solution of dilute hydrochloric acid changes on addition of calcium oxide, measure out 150 cm° of dilute hydrochloric acid into a conical flask, use a pipette or a measuring cylinder for this, measure out 0.5 g of calcium oxide using a mass balance, carefully add the calcium oxide to the hydrochloric acid, wait for the base to completely react, then record the pH of the solution, using either a pH probe or Universal indicator paper.
• Repeat steps 2 to 4 until all the acid has reacted.