Portfolio #9 Feedback

Generally very good summaries.

Well done

A) Don't forget the risk assessment for both HCl and NaOH and don't forget you need to mention the dangers of the chemical AND how the risks involved in using them will be minimised.

B) For this experiemnt - if you can use colour just write the name of the colour.

C) According to the syllabus "Indicators were identified with the observation that the colour of some flowers depends on soil composition"Thus, it would be wise to use this as a starting point when talking about indicator discovery - ie mention litmus if you want but note that it was a lichen extract - but even still this is not a FLOWER. You should mention hydrangeas... (look up their colour in acidic/basic soil).

D) When assessing their effectiveness you need to specify that to distinguish acid/base/neutral there must be different colours for pH <7, pH 7 & pH >7 AND you should state the colours for each. Eg if the indicator was always red except for pH 8 it would not distinguish between acid base or neutral (as red could mean all three) it could only distinguish between 'pH 8' or 'not pH 8'. If a indicator was green for pH1-6, red for pH 7, blue for pH 8-9, then green for pH 10+ could only be said to distinguish neutral  and weakly basic (as green could mean acid or strong base).
Also - indicators can never determine the exact pH of a sample. Each colour appears over a small range - so indicators can give an approximate pH (or pH range) only. Only probes can give an exact pH.

E) You only need to assess the plant extracts (beetroot, strawberry, blueberry & red cabbage) not UI, LS, BB, MO or PP

F) Re the 'hypothesis':
"Best" is subjective. Hypotheses can't be subjective or non-specific. It would have ben better to say - the indicator with the most distinct colour differences in acidic/neutral/basic solutions will be...
But even that is not really a hypothesis as a scientific hypothesis should relate the independent and dependent variables together based on some scientific basis.
Eg HYPOTHESIS : "If Alkenes are more reactive than alkanes, then alkenes will change the colour of bromine water faster than alkanes" - this is clear and relates the IV (type of bonding) to the DV (rate of colour change) - hence it is a hypothesis.
The one on the sheet is more of a 'Guess' than a hypothesis

G) When writing the conclusion make sure you consider the points in (D) and (F) above  - be careful not to say that your indicator could determine exact pH's etc. You do need to say whether the aim was achieved and whether the hypothesis was supported - but be careful to phrase it on the line of 'most suitable at distinguishing a range of pH's, or 'had colours that distinctly differentiated between acidic basic and neutral' rather than saying 'best'.Remember that a hypothesis cannot be 'proven' it can only be 'supported'.

H) You must assess EACH fruit/vegetable separately. You must specify what each can/can't do with respect to distinguishing acidic/basic/neutral

I) Non destructive testing is when a sample can be analysed without causing damage to the sample OR changing the chemistry of the sample.Eg using gamma radiation to investigate a weld in a pipe is non destructive testing (NDT) as the gamma rays do not damage or change the pipe (they pass through and expose photographic film which Is like an ‘X-ray’ of the pipe) but cutting the pipe open to inspect the weld is destructive testing (DT).
Using an indicator is considered destructive testing as once the indicator chemical is added the chemistry of the sample is changed.
So what can be done? Use indicators on a small sample of the substance being tested – the small sample is ‘changed’ but the remaining substance is unaffected. Use indicator paper which has (effectively) no impact on the substance being tested. Use a pH probe to determine the pH – this is a non-destructive way of measuring pH (NB a pH probe is just a galvanic cell without an electrolyte, once in the solution the acid (or base) acts as the electrolyte, a current flows in the circuit and the pH is determined. It is able measure pH as there is a special glass ‘salt bridge’ around one electrode that only allows H+ ions to pass through – so the current flow in the probe is dependent on the H+ concentration of the solution (and as pH is a measure of H+ concentration) it can calculate the pH for you.