Identify the mystery substances

Identify the mystery substances

Pencil and paper
Overview
Using this Resource
Connecting to the Curriculum
Marking Student Responses
This task is about testing with different solutions to identify four solid substances.
Part A
Set up a spotting tile with a small amount of each solid as indicated by the key.
Row A
 
Row B
 
Row C
      
Add 3 drops of the following solutions to the solids in each row and then write down the changes you see.
i) 
ii) 
iii)
To row A add dilute hydrochloric acid.
To row B add iodine solution.
To row C add universal indicator solution.
 
a) Record your observations in the table below.
 
  Bicarbonate of soda Cornflour Sherbet Sugar
Hydrochloric acid        
Iodine solution        
Universal indicator        

Part B

  • Use your observations from part A to identify the unknown solids labelled W, X, Y, and Z. The solids could be bicarbonate of soda, cornflour, sherbet, sugar, or a mixture of two of these solids.
  • Set up your spotting tile as before, with unknown solids W, X, Y, and, Z in each column.
  • Add 3 drops of dilute hydrochloric acid to Row A, 3 drops of iodine solution to row B, and 3 drops of universal indicator solution to Row C.
a) Record your observations in the table below.
 
  W X Y Z
Hydrochloric acid        
Iodine solution        
Universal indicator        
 
b)
 
 
On the basis of your tests, identify the solids in the substances W, X, Y, and Z.
 

 
Substance W: ______________________________
 
Substance X: ______________________________
 
Substance Y: ______________________________
 
Substance Z: ______________________________
Task administration: 
This task can be completed with pencil and paper and other equipment.
 
Equipment (Part A):
Spotting tile; small amounts of bicarbonate of soda, cornflour, sugar, sherbet; dilute hydrochloric acid; iodine solution; universal indicator; eye-droppers; spatula.
 
Equipment (Part B):
Spotting tile; dilute hydrochloric acid; iodine solution; universal indicator solution; eyedropper; spatulas; unknown solids labelled as W, X, and Z – these could be W = sugar, X = bicarbonate of soda, Y = equal amounts of sherbet and cornflour mixed, Z = cornflour.

  • Students may need to be warned about contamination when doing this investigation.
  • You may want to correct errors in part A before students begin part B.
  • Sherbet can be made up from bicarbonate of soda, citric acid, and castor sugar in the ratio 1:2:6. *

* NOTE: Normally icing sugar would be used, but this contains cornflour which affects the identification of substance Y. The castor sugar could be ground up more finely.

Level:
5
Curriculum info: 
Science, Investigating in science, Material World
Key Competencies: 
Thinking
Keywords: 
chemical change, pattern seeking, investigation, interpretation, practical
Description of task: 
This practical task consists of two parts. Students test various substances with hydrochloric acid, iodine solution, and universal indicator, and then complete a table with their observations. The second part requires students to identify four unknown solids using their tests and results from part one.
Curriculum Links: 
Science capabilities
The capabilities focus is brought about by the conversations you have and the questions you ask.
Capability: Gather and interpret data
This resource provides opportunities to discuss looking for patterns in observational data to guide inferences.
Science capability: Gather and interpret data (TKI)
 
Answers/responses: 

Part A
 

 a)
  Bicardonate of Soda Cornflour Sherbet Sugar

Hydrochloric
acid

 Bubbles of gas/fizzes None Fizzes None

Iodine
solution

 No change (brown) Blue/black None No change (brown)

Universal
indicator

 Blue No change (green) OR orange* Red No change (green)

Part B
 

 a)
  W X Y Z

Hydrochloric
acid

 None Bubbles of gas/fizzes Fizzes None

Iodine
solution

 No change (brown) No change (brown) Blue/black Blue/black

Universal
indicator

 No change (green) Blue Red No change (green) OR orange*


 

 b)

W = Sugar
X = Bicarbonate of soda
Y = Sherbet and cornflour
Z = Cornflour
*The first drop of universal indicator in cornflour went orange in our trials, if more drops were added the colour became greener.