ACIDS AND ALKALIS

ACIDS AND ALKALIS

Keywords
1.acid
2.acidic
3.alkali
4.alkaline
5.base
6.indicator

7.neutralisation

8.pH scale

9.universal indicator

By the end of this chapter, you will be able to:
(a) recognise that locally available material and substances are either acidic or alkaline.
(b) understand the concept of pH as a measure of the strength of acids and alkalis.
(c) understand the reaction between acids and bases/alkalis.
(d) appreciate the importance of acids, alkalis/bases and salts in everyday life.

Competency
You should be able to appreciate the properties and importance of acids, alkalis/bases, and salts in everyday life.

1.1

Introduction video, make sure you watch it up to the end.

https://www.bbc.co.uk/bitesize/topics/z242m39/articles/zcnhxbk
Introduction
Acids and alkalis have an important role in everyday life. For instance, they are widely used in fields like agriculture, in food preparation and preservation, and in medicine and industry. However, are you aware that many locally available materials and substances are either acids or alkalis? In this chapter,
you will study the importance of acids and alkalis, how acids and alkalis react, and many more interesting things that we use and see in our daily life.

1.2

Classification of substances as acid or alkali

We have seen that acids and alkalis make substances that feel and taste different.
In this section, you will find out how to further classify substances as acids or alkalis. 

Group discussion
In small groups:
1. Which of the substances tasted in Table 1.1 are:
a) acids?
b) alkalis?
2.Give reasons for your answers in 1(a) and 1(b) above.
3. Present your ideas to your teacher and then to the
class.
Task:
Table 1.2 and Table 1.3 show some common acids and alkalis
Using the knowledge from prior learning, the internet (if available), and the library, copy and complete the following tables by stating examples of sources of each acid and alkali:

Table 1.2: Some common acids

Table 1.3: Some common alkalis

When is a substance an acid or an alkali?
It is not practically safe to taste or feel every substance to find out whether it is an acid or not. Then, how do we identify an acid or alkali other than by tasting or feeling?
Indicators
There are substances specifically set aside to determine whether a substance is acidic or alkaline.

These substances are known as indicators of acids and alkalis and are generally referred to as acid-base indicators. An indicator changes its color when placed in a solution of an alkaline or acidic substance. Grapes, red rose petals, beetroots, turmeric, and red cabbages are some of the natural indicators, whereas a universal indicator is one of the manufactured indicators.

Plant extract as an acid-alkali indicator In the next activity, we will prepare an acid-alkali indicator from the plant material and test its color in an acid and an alkali.
Activity 1.2
In this activity, you will work in groups to prepare an indicator from the leaves of red cabbage.
Preparing an indicator from leaves of red cabbage

Caution:

Wear hand and nose protective gear. Alkalis
and acids are corrosive to the skin.
What you need
1.250ml beakers.
2. Hot plates or Bunsen burner/source of heat.
3. Pair of tongs.
4.2000 ml distilled water.
5. Large screw-top storage bottle.
6. Dropper bottles of 1 M hydrochloric acid, 1 M sulphuric acid and 1 M nitric acid.
7. Dropper bottles of 1 M sodium hydroxide.

8. Red cabbage.
9. Stirrer.

10. Watch glasses or test tubes in a rack

11. googgles
12. Lemon of orange juice, soap solution, vinegar, saliva, baking powder solution, common salt solution,
solution of banana peel ashes and household bleach solution among others.
What to do
First step: Extracting an indicator from red cabbage
1. Tear a few leaves of red cabbage into very small pieces with your fingers.
2. Fill a 250 ml beaker halfway with torn cabbage leaves and add water until the beaker is about two-thirds full
3.Heat the beaker and its contents as you stir with a rod for about 10 minutes.
4.You should notice that the water turns purple as the cabbage boils. When the water is very purple, remove the beaker from the heat and allow it to cool down.
5. Filter the cooled mixture in step 4 into a clean
container/glass cup/a beaker.
The purple juice you made is an acid-alkali indicator. (If the color of the solution made is weak, concentrate it by evaporating off some of the water.)
Figure 1.1: Extracting an indicator from red cabbage

Second step: Using red cabbage as an indicator
6. Use your indicator to test the solutions:

lemon or orange juices, dilute sulphuric acid, and dilute sodium
hydroxide (caustic soda), among others.

Hint: Add one drop of indicator to one of the
test solutions in a watch glass or test tube
arranged in a rack as shown in Figure 1.2 below.

Figure 1.2: Using red cabbage extract as an indicator

7. Note and record your observations in Table 1.4 of
results below.

Table 1.4: Observatioins with red cabbage, a home-made
indicator

8. Share your findings with your fellow learners. 

Results and discussion
From the activity:
1. What general conclusions can you draw from your observations?
2. Which of the solutions tested can be classified as
(i) acidic? (ii) alkaline basic? (iii) neutral?
3. Can the plant extract (red cabbage) be used to indicate whether a substance is acidic or alkaline?
4. Suggest the advantages of using a homemade indicator to test for the nature of a substance.
Guiding video

Litmus indicator as an acid-base indicator
Litmus is a natural dye, which is Figure 1.3: Lichens the most commonly used acid-base indicator.

It is normally obtained from lichens that are found growing for example on bark, leaves and mosses (Figure1.3) Litmus, when dropped in pure water, has a dark purple color.

When placed in an acidic solution, it turns red and when placed in an alkaline solution, it turns blue. It is prepared and presented as either a solution or strips of paper.

Very often, it is presented as red litmus paper and blue litmus paper, bound in small booklets as shown in Figure 1.4.

Figure 1.4: Red and blue litmus papers

Figure 1.5: Litmus solution

In the next activity, you will use a litmus indicator to decide
whether a given solution is an acid or alkali. 

Activity 1.3
Testing substances using litmus indicator
Caution:

Wear hand and nose protective gear.
Bleaches and acids are corrosive to the skin.
In this activity, you will work in groups to test substances
using a litmus indicator.
What you need
1. Test solution: Lemon juice, vinegar, diluted bleach,
diluted liquid soap, tea, cola, tap water, distilled water,
and carbonated water.
2. Watch glasses or glass beakers (100 ml or 250 ml).
Figure 1.6: Litmus paper test

What to do
1. Place some lemon juice in a glass beaker or a watch glass and test it with a piece of red litmus paper, as
shown in Figure 1.6. Is there any color change?
2.Repeat step1 using a piece of blue litmus paper.
3. Record your observations as in Table 1.5.
4. Repeat steps 1-3 with the rest of the test solutions.
5. Share your findings with other members of class.

Table 1.5: Observations with litmus indicator

Results and discussion

Results and discussion
1.Are there any substances that do not change the color of litmus at all? so, what are they?
2.What do you think is the term used to refer that do not change the color of litmus at all?
3.Can you identify these substances?
4.What general conclusions can you draw from your observations?

1.3 How strong is an acid or alkali solution?
We now know how acid-base indicators can be used to distinguish between an acid and an alkali It is apparent that acids and alkalis are corrosive depending on the concentrations or strength However, are we able to know how much acidity a particular solution has? That is to say can we tell how strong the acid
or alkali solution is? We are to solve this problem by use of the universal indicator The universal indicator is a mixture of several acid-base indicators. The universal indicator is capable of changing color over a wide range of acidity strength or concentration of a solution.
Corrosive

Acids and bases are corrosive and. thus, can wear away materials and even cause chemical burns when in
contact with the skin.
Before looking at how to use the universal indicator, let’s look at what pH means.
A pH scale has been designed to measure the acidity concentration of a solution.

The p in pH comes from the German word ‘potenz’, which means power. The H stands for hydrogen because all acids contain the element hydrogen in a reasonable amount.

The pH should simply be taken as a number that indicates the acidic or alkaline nature of a solution. Hence, the pH runs from 0 (for more acidic) to 14 (for more alkaline).

See Figure 1.7. The universal indicator is available both as a solution or as strips of universal indicator paper.

Figure 1.7 The pH scale

On the pH scale, values 5s than represent an acidic color depending on the pH of the solution you place it in See Figure 18
A universal indicator will change from green to a different

Figure 1.8: The pH scale with its colours

Activity 1.4 Testing substances using Universal Indicator
Caution: Wear hand and nose protective gear. bleaches and acids are corrosive to the skin
In this activity, you work in groups to test substances using he universal indicator.

 What you need
1.Test solution: Lemon juice, vinegar, diluted bleach diluted liquid soap, tea, cola, tap water and carbonates water.
2.Droppers or glass rods.
3.Watch glasses or test tubes.
4.Universal indicator (solution or strips).

What to do
1. Place 2-3 drops of lemon juice in a watch glass or test tube followed by a drop of the universal indicator. Is there any color change?
Figure 1.9: Using universal indicator

Video demonstrating how to use a universal indicator

2. Note the pH value corresponding to the observed color. a strip of universal indicator can be dipped into the test solution
3. Record your observations as in Table 1.6.
4. Repeat steps 1-3 with the rest of the test solutions.
5. Share your findings with the other members of the class.

Table 1.6: Testing with pH scale

Results and discussion.
1.Comment on the nature of each solution/liquid.
2. What conclusions can you draw from this activity about colours obtained practically and those on the charts?
3.Can you tell which solutions are:
a) weak and strong acids?
b) weak and strong bases/alkalis?
Solutions have different degrees of acidity and alkalinity (i.e. pH value). Thus, acids and bases can be put into classes depending on their strength.
The pH values of some common solutions are shown in Figure 1.10.

Figure 1.10: pH of some substances in everyday life

The pH value of a given substance/solution can be directly measured using a pH meter. Figure 1.11 shows a pH meter.
Figure 1.11: Measuring the pH of a solution using a pH meter

Chapter Summary

In this chapter, the learner has learned that;

1. an acid tastes sour, has a pH below 7, turns blue litmus paper red, and dissolves in water to produce hydrogen ions as the only positively charged ions.
2. some acids are found naturally occurring in plants and animals.
3. bases have a bitter taste, are slippery, turn red litmus blue and a have pH above 7.
4. alkalis are soluble bases and produce hydroxide ions when dissolved in water.
5. an indicator that distinguishes between acids and bases.
6. indicators can be prepared from plant extracts like flower petals, cabbage leaves, etc.
7. commonly used for commercial indicators include methyl orange, litmus paper, and phenolphthalein indicator.
8. the pH scale is used to show the degree of acidity or basicity of a solution and it ranges from pH 0 to pH 14.
9. the universal indicator shows the exact pH of a solution by use of colors and numbers on the pH scale.
10. acids react with bases to form salt and water only and the process is called neutralization.
11. hydrogen gas explodes with a pop sound when treated with a burning splint.
12. carbon dioxide turns lime water milky.
13. hydrochloric acid and nitric acid are very strong mineral acids.

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