Can someone please explain validity in experiments to me?? I am so confused
and
For heat of combustion of alkaniols experiment - if this was done at school would this automatically be considered invalid if so why??
Any help would be great thanks
Validity is mostly concerned with maintaining all the variables the same to ensure the experiment, as implied by the name, valid. If you changed what you were measuring in the middle of an experiment then your experiment wouldn't have valid results as your results are not specific to the dependent variable anymore as you've introduced another one.
The heat of combustion experiment wouldn't be considered invalid if you kept your controlled variables controlled, measured the change in mass and changed the fuels accordingly. If you've kept these all as they should be, then you have yourself a valid experiment. But, what I think you're hinting at in your question is actually
accuracy rather than validity. Accuracy is concerned with how close your values are to the accepted ones and the way in which you recorded your results. Because the latter is far less significant (only really concerned with parallax error in observing thermometer), we don't really need to focus on it as much. This is because our results do not drastically change when small readings on the thermometer are taken incorrectly.
Now on to what really shakes up our results:
Heat loss is by far, the most significant contributor to inaccurate results; i.e. results which deviate from the accepted values for the heat of combustion of your independent fuels.
Your heat of combustion experiment depends on the mass changes which is a result of the fuel having evaporated. This is what we are measuring. We obtain these mass changes by heating the fuel. If all the heat from the flame was conserved and directed straight into the alkanol, (i.e. through use of a bomb calorimeter) then we wouldn't see large discrepancies. But, because the experiment wasn't performed in a closed system, heat inevitably escaped - dissipated into the surroundings. By not having the most efficient heat transfer, our results are no longer reliable as such a big portion of what our results depended on was unreliable.
I hope that makes sense. Essentially you need to know that the heat losses to the surroundings significantly decreases the reliability of the experiment. Let me know if you want me to clarify the above further.