Each group of biological molecules has its own chemical test, and each test ends in a colour change you can see. Iodine finds starch, Benedict's finds reducing sugars, biuret finds protein, the ethanol emulsion test finds fats and oils, and DCPIP finds vitamin C. The structure of DNA, two strands of paired bases coiled into a double helix, is Extended content.
Iodine solution turns from orange-brown to blue-black with starch. Benedict's solution, heated in a water bath, turns from blue to green, yellow, orange or brick red with reducing sugars. Biuret turns from blue to purple with protein. Shaking a food with ethanol and pouring it into water gives a cloudy white emulsion if fat is present. Vitamin C turns blue DCPIP colourless. Extended: DNA is two strands coiled into a double helix; each strand carries bases, bonds between pairs of bases hold the strands together, and the bases always pair A with T and C with G.
Pick a food and a test, then press Do the test. The food goes into the tube first, then the reagent is dripped in. For Benedict's the tube moves into a water bath at about 80 °C; for the emulsion test water is added. Every result is written into the table. Finish by testing mystery A and mystery B with all five tests.
| Test for | What you do | Positive result | Negative result |
|---|---|---|---|
| starch | add a few drops of iodine solution to the food | blue-black | stays orange-brown |
| reducing sugar (such as glucose) | add Benedict's solution to a solution of the food and heat in a water bath (about 80 °C) for a few minutes | green, yellow, orange or brick red, depending on how much sugar | stays blue |
| protein | add biuret solution to a solution of the food | purple (violet) | stays blue |
| fats and oils | shake the food with ethanol, then pour the ethanol into water | cloudy white emulsion | stays clear |
| vitamin C | add a solution of the food, drop by drop, to DCPIP | blue colour disappears | stays blue |
State both colours: the colour of the reagent at the start and the colour it turns. "Iodine turns blue" instead of blue-black, or a Benedict's result given without heating, are classic lost marks.
Put 1 cm3 of blue DCPIP in a tube, choose a juice, and press Add one drop again and again until the DCPIP status reads colourless. The number of drops goes into the table. Try fresh lemon, fresh orange, carton orange, boiled lemon juice and apple juice, then the 0.1% vitamin C standard.
| Juice in the sim | Drops to decolourise 1 cm3 DCPIP | Vitamin C |
|---|---|---|
| fresh lemon | 4 | most |
| 0.1% vitamin C standard | 5 | known: 0.1% |
| fresh orange | 6 | |
| carton orange | 10 | |
| boiled lemon juice | 17 | |
| apple juice | 25 | least |
Do not read the number of drops as the amount of vitamin C. More drops means less vitamin C in each drop of juice.
Extended. DNA is the molecule that carries the genetic code. Strand 1 is already built; press the base that pairs with each highlighted base to build strand 2, then press Unzip to pull the two strands apart.
Pair the bases A with T and C with G, never A with G or C with T. And it is the bonds between pairs of bases that hold the two strands together.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
On the Food tests bench, mystery B turns Benedict's brick red and gives a cloudy white emulsion. Iodine and biuret give no change. What does mystery B contain?
In the sim, what has to happen before the Benedict's colour change appears?
In the Vitamin C tab, fresh lemon decolourises the DCPIP after 4 drops and boiled lemon juice after 17 drops. What does this show?
Extended: in the DNA tab the next base on strand 1 is G. Which button builds the correct partner?
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which reagent and colour change show that a food contains protein?
A food sample stays orange-brown with iodine, turns orange when heated with Benedict's and turns purple with biuret. The sample contains:
Juice P decolourises 1 cm3 of DCPIP after 12 drops; juice Q after 4 drops. Which statement is correct?
Extended: one strand of a DNA molecule has the bases A C G T T. What is the sequence on the other strand?
This skill is now recorded on your profile. Keep the chain going.