Question

A group of students designed three separate experiments to investigate different physical and chemical processes. In each design, a specific uncontrolled variable or a systematic source of error was introduced.

* Experiment 1: To study how the concentration of reactant $A$ affects the rate of a chemical reaction, students combined reactant $A$ with reactant $B$ in three separate test tubes. They used $1.0\text{ M}$, $2.0\text{ M}$, and $3.0\text{ M}$ solutions of reactant $A$. However, they used test tubes of different diameters ($15\text{ mm}$, $20\text{ mm}$, and $25\text{ mm}$) for each concentration, measuring the time it took for the mixture to change color.
* Experiment 2: To study the effect of temperature on the rate of gas diffusion, students placed a gas canister at $20^\circ\text{C}$, $40^\circ\text{C}$, and $60^\circ\text{C}$ at one end of a closed horizontal tube and measured the time required for the gas to travel to the other end. Because the trials were performed on different days, the relative humidity in the room fluctuated between $30\%$ and $75\%$ during testing.
* Experiment 3: To compare the density of three different liquid samples ($X$, $Y$, and $Z$), students used a graduated cylinder to measure $50\text{ mL}$ of each liquid and recorded their masses using a digital balance. However, the balance was not zeroed (tared) before measuring Liquid $Z$, so the balance registered an initial reading of $+1.2\text{ g}$ before any mass was added.

Match each experiment with its primary source of error or confounding variable.