Unit 10 Common Acids and Alkalis

Topic

Can acidity be changed?

Curriculum Link

10.2 Indicators for testing acids and alkalis: pH scale as an indication of relative acidity, measurement of pH value

Estimated Lesson Time

80 minutes

Introduction

The quality or taste of some drinks differs when they are placed in a different environment. Take red wine as an example, some wine tasters pay much attention to how to hold the wineglass before they drink.

Figure 1 Figure 2

Comparing with the way of holding the wineglass in Figure 1, the way of the holding the wineglass in Figure 2 can warm the wine slightly according to some wine tasters. This might change the taste of some wine a little bit. Obviously, the warmth of the hand affects the temperature of the wine, which in turn causes a change of the acidity of the wine and makes its taste different.

Therefore, in order to enjoy the best of a drink, it is worth investigating whether temperature affects the acidity of a drink, and how if it does.

Key Question

The taste of some drinks is related to acidity. How is acidity affected, if any, by temperature?

Learning Objectives

In this activity, the students should be able to

  1. recognize that solutions are classified into acidic and alkaline;
  2. explore how the pH value of a solution is affected by temperature (or other factors);
  3. acquire planning and designing skills in designing an experiment to measure pH value against temperature and proposing hypothesis before carrying out the experiment (SP3);
  4. acquire measuring skills  and experimenting skills in measuring temperature and pH value (SP1 & SP4);
  5. acquire comparing and classifying skills  by identifying the trend of change of the pH value for acidic and alkaline solutions (SP2);
  6. acquire interpreting data skills in analyzing the data collected (SP5).

Teaching Plan

Task (Time) Brief Description Materials Objectives

Engagement
(10 min)
  • Teacher shows two different ways of holding wineglass and arouses students' interest by asking them which way is better or if there is no difference at all. If students' response is that there is a difference, teacher further elicits students' response on the factor(s) that are involved and then prompts students to investigate the relationship between acidity and temperature.
  • Teacher demonstrates how to use a pH meter correctly to measure pH value of common acids and alkalis.
  • Teacher prompts students to search on the Internet for information about the causes of acidity and students propose hypothesis before carrying out the investigation.
  • pH meter
  • Solutions of different acidity
 (1) & (3)

Exploration
(30 min)

Students design set-up(s) to measure pH value against temperature and carry out the experiment

  • in a systematic way
  • in compliance with all necessary safety precautions
  • with necessary measure(s) to increase the accuracy of the readings (e.g. rinse the probe of the pH meter with distilled water after taking readings of one kind of solution)
(2), (3) & (4)

Evaluation
(10 min)

Students analyze the set(s) of data to

  • identify the trend of the change of pH value against temperature
  • compare the results of acidic and alkaline solutions to check whether they exhibit the same trend of change
   (5) & (6)

Elaboration & Explanation
(10 min)
  • Students revisit the website to propose hypothesis and attempt to describe/explain the results of the experiment.
  • Students predict the trend of the change of pH value in environments of temperature lower than room temperature and suggest how to achieve it.
   (6)

Exploration
(20 min)
  • If time allows, students repeat the experiment with lower temperatures.
   (3) & (4)

*Apparatus and materials required: