Food tests — final check key

1. BK1 · Final check

Model answer. Egg white contains nitrogen and cooking oil does not. The protein in egg white is made from amino acids, and every amino acid contains nitrogen. Oil is made of molecules of one glycerol and three fatty acids, which contain only carbon, hydrogen and oxygen.

A full answer includes

• Egg white contains nitrogen; cooking oil does not.
• The protein in egg white is made from amino acids, which contain nitrogen.
• Cooking oil is made from fatty acids and glycerol, which contain only carbon, hydrogen and oxygen.

Also accept

• egg white has N; protein from amino acids (contain N); oil from fatty acids and glycerol (C, H, O only)

Watch for

• “Both contain nitrogen, because both are food.” — Being food is not the test: ask what each is built from. Egg-white protein comes from amino acids, which contain nitrogen; oil comes from fatty acids and glycerol, which contain only carbon, hydrogen and oxygen.

2. BK2 · Final check

Model answer. A starch molecule is a long chain of thousands of glucose units joined together: one kind of small unit repeated many times. A fat molecule is small by comparison and is not a chain: it is always one glycerol joined to three fatty acids.

A full answer includes

• Starch: a long chain of thousands of glucose units, one kind of unit repeated.
• Fat: one glycerol joined to three fatty acids; not a chain.

Also accept

• starch: chain of many glucose units; fat: one glycerol + three fatty acids, not a chain

Watch for

• “A fat is a long chain of fatty acids.” — A fat molecule is one glycerol with three fatty acids.

3. FK1 · Final check

Model answer. Put 2 cm³ of the fruit drink in a test tube and 2 cm³ of water in a second tube as the control. Add 2 cm³ of Benedict's solution to each. Wearing eye protection and using a test-tube holder, stand both tubes in a water bath at about 80 °C for 5 minutes. If the drink contains a reducing sugar, its tube turns green, yellow, orange or brick-red and a precipitate forms; the more sugar, the further along the series towards brick-red. If it stays blue, like the control, no reducing sugar was detected. Other volumes and a hotter (even boiling) water bath for a few minutes also work, as long as both tubes are treated the same way.

A full answer includes

• Benedict's solution added to the drink in one tube (for example 2 cm³ of each) and to water in a control tube.
• Both tubes heated in a hot water bath (for example about 80 °C for 5 minutes, or a boiling water bath for a few minutes), held with a holder, with eye protection.
• Positive: green, yellow, orange or brick-red with a precipitate (more sugar towards brick-red); negative: stays blue like the control.

Also accept

• Benedict's with sample and with a water control; heated in a hot or boiling water bath for a few minutes (e.g. 80 °C, 5 min); colour series with precipitate = positive, blue = negative
• any sensible equal volumes and any hot water bath heating both tubes the same way

Watch for

• “Benedict's works without heating.” — It must be heated in a water bath.
• “No control is needed.” — The control shows the negative colour under the same conditions.

4. FK2 · Final check

Model answer. (a) Iodine turns blue-black (starch). Benedict's solution, heated in the water bath, turns green, yellow, orange or brick-red with a precipitate (reducing sugar). Biuret solution turns purple (protein). The emulsion test gives an even cloudy white (fat). DCPIP stays blue after 20 drops, like the control. (b) The DCPIP test would be negative. "Not detected" is better than "none" because a very small amount might be below what the test can show.

A full answer includes

• (a) Iodine: blue-black. Benedict's (heated): green to brick-red with a precipitate. Biuret: purple. Emulsion: evenly cloudy white. DCPIP: still blue after 20 drops.
• (b) DCPIP (vitamin C); a test can miss an amount too small for it to show.

Also accept

• (a) iodine blue-black; Benedict's green to brick-red with precipitate; biuret purple; emulsion cloudy white; DCPIP stays blue; (b) DCPIP negative; a small amount could be missed

Watch for

• “A test names a food.” — Each test detects a substance; this one food gives four positive results.
• “DCPIP goes colourless because there is no vitamin C.” — With no vitamin C, DCPIP stays blue.

5. FK3 · Final check

Model answer. Shake a small amount of the food with 2 cm³ of ethanol in a dry test tube, so any fat dissolves. Let solids settle, then pour the clear ethanol into a second tube containing 2 cm³ of water. Do the same with water instead of food as the control. If the food contains fat or oil, the liquid turns evenly cloudy white, because the fat forms an emulsion of tiny droplets spread through the water; if not, it stays clear. Ethanol is highly flammable, so keep it well away from the hotplate, hot water and any flame.

A full answer includes

• Shake the food with 2 cm³ of ethanol in a dry tube; pour the clear ethanol into a tube of 2 cm³ water.
• Positive: the liquid turns evenly cloudy white (an emulsion of tiny fat droplets); negative: stays clear like the water control.
• Ethanol is flammable: keep it away from the hotplate, hot water and any flame.

Also accept

• ethanol then pour into water; evenly cloudy white = positive; clear = negative; water control; ethanol away from heat

Watch for

• “A positive emulsion test is a white solid at the bottom, or a layer of oil on top.” — It is tiny droplets spread evenly through the whole liquid.

6. NK1 · Final check

Model answer. A DNA molecule is two strands coiled together to form a double helix. Each strand contains chemicals called bases. Bases on the two strands form pairs, and bonds between the bases of each pair hold the strands together. The bases always pair in the same way: A with T, and C with G.

A full answer includes

• Two strands coiled together to form a double helix.
• Each strand contains bases.
• Bonds between pairs of bases hold the strands together.
• A always pairs with T, and C with G.

Also accept

• two strands, double helix; bases; bonds between base pairs; A–T and C–G

Watch for

• “Any base can pair with any other base.” — Your description needs the pairing rule as its last point: A always pairs with T, and C always pairs with G.

7. NK2 · Final check

Model answer. Every C is paired with a G, so there are 18 G bases. C and G together make 36 bases, so A and T make 100 − 36 = 64. A and T are equal, so A = 64 ÷ 2 = 32.

Checked automatically

• Number of A bases: 32 (within ±0.000000001)
  • Common wrong answer 25 : The four bases are not all equal: only A = T and C = G.
  • Common wrong answer 18 : C pairs with G, not with A.
  • Common wrong answer 64 : That is A and T together; A is half of it.
  • Common wrong answer 82 : Take away both C and G (18 + 18), then halve what is left.

A full answer includes

• G = 18 (it pairs with C).
• A + T = 100 − 36 = 64.
• A = T, so A = 32 (a whole number of bases).

Also accept

• 32

Watch for

• “All four bases come in equal numbers, so A = 25.” — Only partners are equal: A = T and C = G.