A student investigated the effect of the size of marble chips on the rate of the reaction between marble chips and hydrochloric acid.
This is the method used.
1.
Add 10.0 g of marble chips into the flask.
2. Add 50 cm3 of hydrochloric acid and start a timer.
3. Record the mass lost from the flask every 10 seconds.
4. Repeat steps 1 to 3 with different sizes of marble chips.

Figure 1 shows the apparatus.
Which is the control variable?