A student made bread dough by mixing flour, water, sugar and yeast, then pushed a ball of the dough into the bottom of a measuring cylinder, as shown in Fig...

This question is about anaerobic respiration in yeast, reading a scale, completing a results table and improving accuracy.

(a) Reading the height of the dough against the scale drawn in Fig. 3.1, a value of about 6.5 cm is expected (accept the reading you take from the printed scale within the marker's tolerance) [1]. Line the reading up with the top of the dough and read the nearest scale mark.

(b) The gas that made the dough rise is carbon dioxide [1]. When yeast respires anaerobically it converts sugar into ethanol and carbon dioxide.

(c) The increase in height after 20 minutes is the final height minus the starting height: \( 9.0 - 4.0 = 5.0\ \text{cm} \) [1]. The completed table is:

(d) The dough rose because the respiring yeast produces carbon dioxide gas [1], and this gas is trapped as bubbles inside the sticky dough, which pushes the dough upwards so its height increases [1]. The gas cannot escape freely, so it inflates the dough like tiny balloons.

(e) The cylinder was kept in a warm place because warmth speeds up respiration / raises the activity of the yeast enzymes, so more carbon dioxide is made per minute and the dough rises faster [1].

(f) If different temperatures were tested, the independent variable (the one deliberately changed) would be the temperature [1].

(g) One way to make the height readings more accurate is to read the scale at eye level, level with the top of the dough, to avoid parallax error [1].