Chapter overview 1 week In this chapter, learners are introduced to the pH scale and learn how to measure and interpret pH values. There are two activities around pH measurement in this chapter: One is a paper activity and one is an investigation using indicators. There is an introductory activity on measurement - this helps to link this content to what learners might be doing in other subjects, such as Mathematics and Technology, and also to what they already know about measurement. The investigation uses universal indicator paper and red cabbage paper to measure the pH of a selection of household products. If you do not have universal indicator paper, but have universal indicator solution, you could easily adapt the activity in the following way: Instead of using paper, learners should add 2 - 3 drops of universal indicator solution to their test solutions, note and record the resulting colours. Red cabbage solution can also be used instead of paper. It would be best to add approximately 1 ml of each test solution to 2 ml portions of the red cabbage water. To make red cabbage indicator paper, follow the instructions below:
The time indicated for this chapter is 1 week and so a suggested 1.5 hours has been allocated to each section. However, as there are only two key tasks in this chapter, you might progress through it at a faster rate than 1 week, and then move onto the next chapter dealing with the reactions of acids with bases, which requires more time. 5.1 What is the pH value? (1.5 hours)
5.2 Indicators (1.5 hours)
When we want to say something has the properties of an acid, we use the adjective acidic. When we want to say something has the properties of a base, we use the adjective basic. In Grade 7 we learnt about acids and bases. Can you remember how to distinguish between them? Here is a table that highlights the main characteristics of acids and bases.
We used the criteria in the table above to classify a number of substances as either acids, bases, or neutral substances. The table below contains some examples and shows their classification.
Finally, we learnt that there are substances that we can use that will show whether we have an acid or a base. Can you remember what they were called? Hint: They indicate, or show, whether we have an acid or base. Indicators can show us if a substance is an acid or a base. In this chapter we are going to link some important new learning to what we already know about acids and bases. The most important new idea is that we can 'measure' how acidic or basic a substance is.
This section briefly creates the link between what learners might have done in other subjects and in previous grades about measurement and scales, especially Mathematics. It is used to show that many things can be measured and is used to introduce the idea of measuring how acidic or basic a substance is, as before this we only ever classified a substance as an acid or a base and did not make reference to a scale. Let us talk briefly about 'measuring'.
What would you measure with each of the measuring instruments below? Measuring tapes and a ruler http://www.flickr.com/photos/pat00139/5983796116/The measuring tape and ruler measure… A triple beam balance. The balance measures…
Mass. Make sure learners do not say 'weight'. This is a misconception. Different size beakers. These measuring beakers measure… A thermometer. http://www.flickr.com/photos/jackson3/3769878095/The thermometer measures… A measurement always consists of two things: a number and a unit. To explain what this means, let's imagine we are measuring the length of a pencil.
What is the length of the pencil in the picture?
The unit is a very important part of the measurement because it shows the relative size of the measurement. If you said: "The pencil is 18.5 long", people would not be sure if you meant centimeters, millimeters, or even meters! What unit would you use to measure the length of your classroom?
Meter (m). Centimeter is also acceptable. What learners need to realise is that the unit needs to be made explicit. What unit would you use to measure your mass? What unit would you use to measure temperature? Can we 'measure' how acidic or basic a substance is?
The unit that we use to measure the acidity of a substance (how acidic that substance is) is called pH. We pronounce the two letters, 'p' and 'H' separately when we say pH.
The term "pH" was first described by Danish biochemist Søren Peter Lauritz Sørensen in 1909. The definite origin is disputed, but it is widely accepted that pH is an abbreviation for "power of hydrogen" where "p" is short for the German word potenz (meaning power or exponent of) and H is the element symbol for hydrogen. Have you ever heard the term pH?
Get learners to discuss this in class for a few minutes. Perhaps they have seen adverts on TV claiming that a certain brand of shampoo or skin soap is 'pH balanced'. Ask them what they think this means. Perhaps you have heard of a certain shampoo being 'pH balanced', or a skin soap that is 'neutral'. Perhaps you have heard that it is important for the water in a swimming pool to have 'the right pH'? A kit for testing swimming pool water pH. http://www.flickr.com/photos/ryochijiiwa/4542559854/The pH scale ranges between the values of 1 and 14. In science and in everyday life, we measure the acidity of substances in pH units. We could say that the 'acidity' of a specific shampoo has a pH of 5.5. pH is the unit of measurement and 5.5 is be the number indicating the relative acidity on the pH scale. It has become acceptable, however, for us to rather say: "The pH of this shampoo is 5,5." In the next activity, we are going to get to know the pH scale a little better.
INSTRUCTIONS:
QUESTIONS:
Which of the substances in the table at the start of this chapter can you find on the pH scale above? Write their names and approximate pH values in the table below.
Circle the names of all the acids in the table above with a red pen or koki.
Learners should circle orange juice, lemon juice and gastric acid in red.
Write the lowest and highest pH values of these acids. This represents the pH range of the acids on our list.
Lowest value: 1 (gastric acid); Highest value: 3 (orange juice)
Does this range lie below or above pH 7?
Circle the names of all the bases in the table above with a blue pen or koki on the pH scale above.
Learners should circle bicarbonate of soda, soapy water, bleach and ammonia solution in blue.
Write the lowest and highest pH values of these bases below. This represents the pH range of the bases on our list.
Lowest value: 9 (baking soda); Highest value: 13 (bleach)
Does this range lie below or above pH 7?
Find water on the scale and circle it with a green pen or koki. Is water an acid or a base? Or is it perhaps something else?
Water is neither an acid nor a base. Water is a neutral substance.
Learners may need help recalling this information.
Water has a pH equal to 7.
Which do you think is more acidic: orange juice or lemon juice? If you are not sure, ask yourself this question: Which one is more sour?
Lemon juice is more acidic.
Which one has the lower pH: orange juice or lemon juice?
Lemon juice has a lower pH. In the above activity we learnt a number of important things:
This information has been summarised visually in the following diagram.
We saw in the activity that lemon juice, which is more sour than orange juice, has a lower pH than orange juice. Does that mean that the relative pH of a substance will tell us how acidic or basic it is?
When we compared orange juice and lemon juice earlier, we learnt something important: The lower the pH of a substance, the more acidic it is. For bases we can state the folllowing: The higher the pH of a substance, the more basic it is. Here is a summary:
We have learnt that the pH value of a substance tells us if it is an acid or a base. But how do we measure pH? One way to measure pH is with the help of acid-base indicators. Can you remember what they are? The next section will refresh your memory.
We know that some substances change colour when they react with an acid or a base. These substances are called acid-base indicators, which can show us if a substance is an acid or a base. Different indicators change colour at different pH values. The table below shows a selection of acid-base indicators and the colours they will have at different pH values.
In Grade 7 we made and indicator from red cabbage and even made some red cabbage indicator paper. Can you find red cabbage water on the table above? In acids, the red cabbage water will turn red or pink. In neutral solutions it will be purple or violet. Which colours will the red cabbage indicator be when it is mixed with a base? If red cabbage indicator is mixed with something that is only slightly basic, it will turn blue. If it is mixed with something that is strongly basic, it will turn yellow. When you look at the table above and you compare the information given for red cabbage water with the picture below, the colour changes you observed in the red cabbage water (in Grade 7) will make sense! Red cabbage water mixed with base (left) and with acid (right). The blue drop at the top is the juice in a neutral solution (water).You may recall that we also learnt about litmus, the most widely used of all acid-base indicators. Can you find litmus on the table of indicators? Litmus does not change colour in the presence of a neutral substance, but responds to acids and bases in the following way:
Litmus can be bought as a solution or as litmus paper, although the paper is more commonly used. Blue and red litmus paper. By changing to different colours in the presence of an acid or a base, indicators can show us if a substance is an acid or a base. In the next section we are going to learn about a special indicator that is so sensitive that it not only tells us whether a substance is an acid or a base, but also what its approximate pH is!
Video showing universal indicator Unlike litmus, universal indicator can show us much more accurately how acidic or basic a solution is. Can you find universal indicator on the previous table of indicators? Universal indicator can change into a whole range of colours, depending on the pH of the solution. In the following picture, solutions of increasing pH were mixed with universal indicator to show its full range of colours. Universal indicator can have many different colours, from red for strong acids to dark purple for strong bases. The liquid inside the middle test tube is neutral (pH = 7) and this is shown by the green colour of the indicator.
Universal indicator solution can display so many colours is because it is actually a mixture of several different indicators. Like litmus, universal indicator also comes in paper form, with the pH colour range of the indicator printed on the packaging.
In the next investigation we will test a number of household substances with red cabbage indicator paper and with universal indicator paper.
The purpose of this investigation is to determine whether universal indicator and red cabbage can be used to show whether one substance is more acidic or basic than another. INVESTIGATIVE QUESTION: What question are we trying to answer with this investigation?
Learners can formulate their own question, but it should be something along the following lines: Can universal indicator paper and red cabbage indicator paper show us whether one substance is more acidic or basic than another? HYPOTHESIS: What do you think the answer to the investigative question is? You should try to make a prediction.
Learners should make their own prediction/hypothesis. IDENTIFY VARIABLES:
What will you be changing in this investigation? What is this variable called?
We will change the substances that we are testing. The independent variable is the one that is changed.
What will you be measuring in this investigation? What is this variable called?
We will measure the pH. This is the dependent variable.
What will you keep the same? What is this variable called?
The controlled variable is the type of indicator that we are using in the measurement, namely the universal indicator paper or the red cabbage paper. MATERIALS AND APPARATUS:
METHOD:
RESULTS AND OBSERVATIONS: Record your observations in the table.
Sequence the substances that you tested according to the colour change of the universal indicator, from the most acidic (darkest red) to the most basic (purple).
The answer here depends on the results and the substances used. QUESTIONS:
Which of the test substances are acids?
Learners should list all the substances that gave pH measurements below 7.
Which of the test substances are bases?
Learners should list all the substances that gave pH measurements above 7.
Which of the test substances are neutral substances?
Learners should list all the substances that gave pH measurements equal to 7.
Which substance is the strongest acid?
Activity-dependent answer.
Which substance is the strongest base?
Activity-dependent answer.
Count all the different colours that were possible with the red cabbage.
Activityr-dependent answer.
What colour(s) did the red cabbage paper turn in the test substances that were acids?
The red cabbage paper should turn a red-pink colour in an acid.
What colour(s) did the red cabbage paper turn in the test substances that were bases?
The red cabbage paper should turn a blue-green colour in a base.
What colour(s) did the red cabbage paper turn in the test substances that were neutral?
The red cabbage paper should turn (or remain) purple with neutral substances.
Do you think red cabbage indicator can be used to actually measure pH? Why or why not?
Learner-dependent answer. Allow learners to express an opinion here, backed by one or two sentences as motivation. CONCLUSIONS:
What is your conclusion(s)? (Here you should answer the investigative question.)
Learner-dependent answer. They should note that universal indicator allows them to measure the pH of individual substances. By arranging the substances in order of increasing pH, they can rank the substances from most acidic to most basic. They should note that red cabbage indicator does give a range of colours with varying pH, but that it is not as effective as universal indicator for measuring pH. Something to think about: Extension question
What could we do to make red cabbage indicator suitable for measuring pH?
Learners could discuss this question in class if there is time. The important thing here is that each of the colours of the red cabbage indicator needs to be standardised/linked to a specific pH value or range of values. Hence, if we had a range of samples of known pH, we could 'calibrate' the red cabbage solution, by mixing it with each of the pH standards and carefully noting the colour. This would allow us to produce a colour chart, that we could then use in the same way as the one that is available for universal indicator and other commercial indicators. In the last investigation we explored whether or not universal indicator paper or red cabbage indicator paper could tell us whether a substance is more acidic or basic than another. The advantage of using universal indicator over other indicators is that universal indicator can give us more accurate pH measurements. This is because it has different colours for different pH values. Most other indicators change colour only once or twice over the entire pH range. Many other colourful foods can be used to make acid-base indicators. Check out the diagram below for some examples. You could even try out a few of them at home! pH indicators made from edible substances. Measuring pH with indicator solutions or paper is easy, economical and convenient if we have only a few measurements to make. If we have many pH measurements to make, tearing and dipping paper strips and matching them up with a colour chart can become quite tedious and time-consuming.
Universal indicators give a range of colours that can be used to determine the pH of a solution. Litmus paper can only indicate whether a solution is acidic, neutral or basic. What other quick and easy ways are there to measure pH?
Scientists use a pH meter to quickly and accurately measure the pH of a substance. While they are much more expensive to purchase than indicator paper or solution, they are a worthwhile investment for a laboratory that has to make many pH measurements daily and need these measurements to be done quickly. A portable pH meter.http://commons.wikimedia.org/wiki/File:Alkaline_pH.jpg A pH meter is an electronic instrument with a special sensor at the end that is sensitive to acids and bases. This is more accurate than the universal indicator. Help the scientist to read the pH of the solutions in the photos and classify them as acidic, neutral or basic!
The solution on the left has a pH of 7.053 so it is neutral. The solution on the right has a pH of 10.33 so it is basic. pH meters work as follows: When the sensor is dipped into the test solution, it sends a signal to the electronic circuitry of the pH meter, which is converted to a pH reading on a small LCD screen. In this chapter we have learnt about the pH scale. We have also learnt how to make pH measurements and how to interpret pH values.
Concept map What can you use to determine whether a substance is an acid, base or neutral? Fill this in on the concept map. Finally, complete it by completing the information for the universal indicator. Fill in acid, base or neutral, depending on the colours listed.
This is the completed concept map.
Total [30 marks] |