Grade 8
The concept of pH value
Name: _________________
Date: ____________________
In the previous lesson we have learnt what is pH and what each pH range mean.
Short review question:
1. What is pH? _______________________________ [2]
2. What does i) pH 1-7
ii) pH 7
iii) pH 7-14
mean? [3]
3. Why do you think that rainwater has a much lower pH than pure water?
Safety Precautions:
Universal indicator solution is an alcohol-based solution and is a flammable liquid. Some
household chemicals are toxic by ingestion or inhalation and are skin and eye irritants. Avoid
contact of all solutions with eyes and skin and clean up spills immediately. Do not taste or
ingest any material in the lab and do not remove any remaining food items after they have
been used in the lab. Wear goggles, chemical-resistant gloves, and a chemical-resistant
apron. Wash hands thoroughly with soap and water before leaving the laboratory.
(https://www.flinnsci.com/api/library/Download/8f8e74fc20424293b525903b665cda76)
Question 1:
How are we going to determine the pH of a substance?
Today we are going to use chemical indicators (universal indicator) to tell us whether a
substance is an acid, base or neutral. The universal indicator will display color for you to
determine its pH.
DIATILE AND JUDY |
Objective
Learners will be able to determine the pH of household substances using universal indicator.
Apparatus and materials
-
Universal indicators
-
beakers
-
Stirring rods
-
Household items: rooibos tea, orange juice, coffee, milk, aspirin, coke, washing
powder, dish washer, soap, shampoo, shaving cream, tomato sauce, domestos.
-
Distilled water (to dilute or dissolve the substances before testing its pH)
Instruction
1. Make groups of 10 learners each
2. Take out the items that you have brought from home.
3. Put each item in a separate beaker. If the item is solid, dissolve it with distilled water.
If the color of liquid is too dark, use distilled water to dilute it.
4. In groups, each of you will take turns to show the items you have brought. All
members of the group will have to first guess the pH and color of the item on the
indicator.
5. Then, use universal indicator to find out the color and pH of the item. Also indicate
whether the item is basic/neutral/acidic. Use table 1 to write down your observations.
6. Arrange the items from most acidic to most basic.
DIATILE AND JUDY |
Name of products Acid/ Neutral/
Base
Color
pH
Other observations
Eg: smell/taste
(only for edible
items!), texture etc
1.
2.
3.
4.
5.
7.
8
9
10.
Most acidic
1.
2.
3.
4.
5.
6.
7.
8.
9.
Most basic
10.
[30]
Question 2
Pepsin is an enzyme that is released in the stomach and is responsible for breaking food proteins
down.
Use the graph showing the function of pepsin at various pH-levels to answer the following
questions:
Enzyme
function
1
2
3
4
5
pH
DIATILE AND JUDY |
2.1
2.2
2.3
2.4
Name the independent variable.
Name the dependent variable.
What pH would you estimate stomach acid is in order for pepsin to function most
efficiently? (You can give a range.)
If somebody is suffering from indigestion or acid reflux, what would you
recommend they take? How effective do you think drinking milk will be?
[2]
[2]
[2]
[2]
Total: 45
DIATILE AND JUDY |