qualitative and quantitative testing Flashcards by Areeba Batool Year 12

what is a hypothesis

your predcition

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what is validity

how well your experiment tests your hypothesis (includes all control variables, accurate and reliable results)

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what is reliability

achieved by repeating the experiment and having reproducible data

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what is accuracy

a measurement close to the true value - improved by taking a narrower range of readings within a set range

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what is precision

all results are close together

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what is an independant variable

the variable you change in an experiment

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what is a dependant variable

the variable that is measured

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what is a controlled variable

the part of an experiment in which the independant variable is not applied

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what does qualititative testing help identify

the presence or absense of an ion or compound in solution

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what does qualititative testing rely on

the biological molecules in the sample passing into the solution

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how do we test for biological molecules

you will need to grind and squash the food samples and mix them with a small volume of water (lipids in alcohol)
wear eye protection when carrying out all of these tests

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what is a positive control

positive sample, used to check that all reagents are working. allows comparison with test samples

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what is a negative control

has same conditions as the test samples without the independant variable. used to check that the independant variable is causing the change

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how do we test for starch

add iodine solution (in potassium iodide) to a sample

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what is the positive result for starch

colour change from yellow/brown/orange to blue-black

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what causes the colour change in the iodine test

when dissolved in iodide, the iodine forms a triiodide ion which slips into the middle of the amylose helix. this causes the colour change

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what is the difference between a reducing sugar and a non reducing sugar

reducing sugars act as reducing agents in chemical reactions (donates electrons to other molecules). non reducing agents do not act as reducing agents

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which sugars are reducing

all monosaccharides and some disaccharides such as maltose and lactose

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what makes a sugar reducing

they can give electrons to other molecules

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what happens if you heat a reducing sugar with Benedicts solution

there is a colour change from blue to green to yellow to orange red

what does Benedicts solution contain

copper (II) ions - Cu2+

what forms the orange red colour

benedicts solution is a blue colour. when it reacts with the sugar it donates an electron and is reduced to Cu+ forming a orange red copper (I) oxide - Cu2O

what is the orange red mixture

a precipitate because it comes out of solution and forms a solid, suspended in the reaction mixture

what happens if you use Benedicts solution in excess

the intensity of the red colour is proportional to the concentration of the sugar. its green if little precipitate is formed, orange red if lots of precipitate is formed

how else can you test for reducing sugars

commercially manufactured test strips. you simply dip the strip into the test solution and compare the colour with the calibration card supplied. this tells you whether the reducing sugar is present or absent

what are non reducing sugars

they dont act as reducing agents in chemical reactions.

which sugars are non reducing

most disaccharides (sucrose) and some simple polysaccharides

how do we test for non reducing sugars

- we have to hydrolyse the bond bond first (to free up the reducing groups) - heat with hydrochloric acid to break the glycosidic bond and expose the reducing groups - then heat it at 60°C with benedicts reagent - cool solution and use sodium hydrogen carbonate to neutralise it - test for reducing sugars again

what does a positive test in the 2nd reducing sugars suggest

that there was non reducing sugars in the first place

what can happen if a sample contains reducing and non reducing sugars

if you have a positive test for reducing sugars from your first sample, you can go test for non reducing sugars in an equal sized second sample. if its present the precipitate from this second sample will have more mass then the precipitate from the first sample. you can extract the precipitate from the mixture by filtration

how do we test for lipids

emulsion test - take a sample and mix it through with ethanol. any lipid will go into solution in the ethanol (lipids are insoluble in water) - filter - pour the solution into water in a clean test tube - a cloudy white emulsion indicates the presence of lipids - this is made of tiny droplets that come out of solution when mixed with water

how do we test for proteins

the biuret test - if protein is present, the colour changes from blue to lilac.

what causes the colour change in the biuret test

colour formed by a complex between the nitrogen atoms in a peptide chain and copper 2+ ions

what does the biuret test actually test

the presence of peptide bonds

what is the quantitative test for carbohydrates

colorimetry

what happens if more sugar is present

- amount of precipitate will increase - amount of copper (II) ions remaining in solution will decrease

how does a colorimeter work

- by shining light through a sample. - we would use a centrifuge to separate the precipitate and any excess benedicts solution (supernatant) - using a pipette we can take the supernatant and place it in a cuvette which is placed in the colorimeter

what could affect the transmission of light

leaving greasy fingerprints on the cuvette

what is transmission

how much light gets through

what is absorption

how much light has been absorbed

what are colour filters used for

greater accuracy

what should you do when using a colorimeter

zero the device between each reading by placing an appropiate 'blank' sample to reset the 100% transmission/absorption

the more reducing sugar there is...

....the more Cu2+ is converted to Cu+ to form a precipitate. results in more red Cu+ and less Cu2+

what happens to the red precipitate

its filtered off

the more precipitate filtered and removed...

...the paler blue the solution is

the higher the glucose concentration...

....the lower the absorbance and the higher the transmission when using a colorimeter

why is the colorimeter classes as 'semi quantitative'

you can compare how much sugar was contained in different samples

how would you know which filter to use for absorption

depending on the colour of the solution you would use the opposite colour in the spectrum

steps for using colorimeter

1) zero colorimeter 2) using blank 3) use red filter 4) use known concentration of lactose 5) serial dilution 6) construct calibration curve 7) test unknown sample (using the same method) 8) read from calibration curve to determine the unknown concentration