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Students often feel uncomfortable when asked to speculate about explanations of observed behaviour. This may be because they don't want to 'get it wrong' in front of their peers. At worst it can be a mindset that says 'just tell me the answer'! This is much less likely to be an issue if students are frequently asked to speculate about outcomes and explanations of reactions. A key aim for teachers is to create a culture in the class in which all student opinions are valued and in which speculation, and sometimes getting it wrong, are routine parts of learning.

Key tips for promoting student speculation include:

• Think of experiments that provide opportunities for students to develop their skills of observation and the chance to suggest cause and effect.
• Think in advance of the lesson of key questions that will probe student understanding. These should be open questions that prompt students to think about experiments rather than closed questions that have only one answer.
• Remember to give students ample time to think about their answers.
• Ask another student to comment on or build upon a previous response to keep the topic open and expand the dialogue into other areas. 
• Ask questions that help students make connections with their prior learning.

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The 'blue bottle' experiment provides opportunities for students to make observations and to speculate about what causes them in the context of reversible reactions. It also provides an opportunity for you to ask open questions.