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Making bubbles II

This task is about critiquing the variables in a fair test.

Question 1Change answer

Mereani planned an investigation to find out which was the best washing-up liquid for making bubbles. This was her plan:

Add a few drops of washing-up liquid to 20 ml water, at room temperature.
Use a loop of wire, put it in the mixture, take it out, blow, and count the bubbles.
Add a few drops of a different washing-up liquid to another 20 ml container of water at room temperature.
Use another loop of wire, put it in the mixture, take the loop out, blow, and measure how long the bubbles last.

Mereani planned an investigation to find out which was the best washing-up liquid for making bubbles. This was her plan:

Add a few drops of washing-up liquid to 20 ml water, at room temperature.
Use a loop of wire, put it in the mixture, take it out, blow, and count the bubbles.
Add a few drops of a different washing-up liquid to another 20 ml container of water at room temperature.
Use another loop of wire, put it in the mixture, take the loop out, blow, and measure how long the bubbles last.

There are several problems with Mereani's plan. What are four things she should change in her plan so that the investigation will be a "fair test"?