Acids, Alkalis & Titrations (Edexcel IGCSE Chemistry)

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  • What pH range is classed as a strong acid?

    The pH range for a strong acid is 0-3.

  • What pH range is classed as a weak acid?

    The pH range for weak acids is 4-6.

  • What pH is classed as neutral?

    pH 7 is classed as neutral.

  • What pH range is classed as a weakly alkaline?

    The pH range for a weak base is 8-10.

  • What pH range is classed as a strongly alkaline?

    The pH range for a strong base is 11-14.

  • What colour is litmus in acidic and alkaline conditions?

    Litmus is red in acid and blue in alkaline conditions.

  • What colour is phenolphthalein when added to an acid?

    Phenolphthalein is colourless in acid.

  • What colour is methyl orange in acidic and alkaline conditions?

    Methyl orange is red in acid and yellow in alkaline conditions.

  • Describe how to use universal indicator to test the pH of a substance.

    To measure the pH of a substance with universal indicator:

    • Add a few drops of universal indicator solution to the substance.

    • Check the colour against a colour chart to determine the pH value.

    (Universal indicator paper can also be used).

  • Why is universal indicator not used measure pH during titrations?

    Universal indicator is not used for titrations as it does not give a sharp colour change which is required to identify the end-point.

  • The pH scale starts at pH 0 and goes up to what pH?

    The pH scale goes from pH 0 - 14.

  • Which ion causes a substance to become acidic when added to water?

    The ion that causes a substance to be acidic is a hydrogen ion, H+.

    (Proton is also an acceptable answer)

  • Which ion causes a substance to become alkaline when added to water?

    The ion that causes a substance to become alkaline is a hydroxide ion, OH-.

  • What substances are produced when an acid and an alkali react?

    When an acid and alkali react the products are water, H2O, and a salt.

  • State the net ionic equation for neutralisation.

    The net ionic equation for neutralisation is:

    H+ (aq) + OH- (aq) ⟶ H2O (l)

  • What type of reaction occurs when an acid and an alkali react?

    The type of reaction that occurs when an acid and alkali react is neutralisation.

  • True or False?

    All reactions of acids are neutralisations.

    False.

    Not all reactions of acids are neutralisations. For example, when a metal reacts with an acid, although a salt is produced, there is no water formed, so it does not fit the definition of neutralisation.

  • True or False?

    The lower the pH, the more acidic the solution.

    True.

    The lower the pH, the more acidic the solution is.

  • True or False?

    All alkalis have a pH value above 7.

    True.

    All alkalis have a pH value above 7.

  • State what is meant by concordant results.

    Concordant results are results that are within 0.1 cm3 of each other.

  • What is an acid-base titration used for?

    An acid-base titration is a method used to determine the concentration of an unknown acid or alkali.

  • What is the purpose of the indicator in a titration?

    The indicator is used to identify the endpoint of the titration by changing colour sharply when the reaction is complete.

  • True or False?

    Leaving a funnel in the burette during a titration is acceptable.

    False.

    The funnel should be removed from the burette before starting the titration, as it can drip liquid into the burette, making the initial reading false.

  • Why is it important to repeat a titration?

    Repeating a titration and achieving concordant results increases the accuracy of the measurement.

  • State what is meant by the term endpoint.

    The point at which an indicator changes colour sharply during a titration, signalling the completion of the reaction.

  • What is the purpose of swirling the flask during a titration?

    Swirling the flask during a titration ensures thorough mixing of the solutions and helps to identify the endpoint more accurately.