(i) independent variable: the depth from which the water sample was obtained ✔

dependent variable: the electric current recorded by the ammeter, «a measure of the conductivity of the sample» ✔

(ii) Any two of:

the potential difference (voltage) of the DC power source ✔

OR

the distance between the two graphite electrodes «and the depth to which they are immersed» ✔

OR

the volume of water sample used / the surface area of electrode in contact with the sample ✔

(iii) $\dfrac{0.01}{2.65} =$ 0.004 «to 1 significant figure» ✔

(iv) conclusion: as the depth increases, the electric current, «and therefore the conductivity of the sample,» increases ✔

explanation: «the denser, more concentrated brine sinks and collects towards the bottom of the lake, so» deeper samples contain a higher concentration of dissolved ions «and so conduct electricity more strongly for the same applied voltage» ✔

(v) cathode: $\ce{2H2O(l) + 2e- -> H2(g) + 2OH-(aq)}$

anode: $\ce{2Br-(aq) -> Br2(aq) + 2e-}$

(i) [1] each for a correctly identified independent and dependent variable; the dependent variable must reference the current or its link to conductivity, not just ‘conductivity’ alone. (ii) [1] each for two genuinely different controllable variables; ‘the water sample’ alone is too vague to credit. (iii) [1] for 0.004, only if given to 1 significant figure (matching the 1 significant figure of the absolute uncertainty 0.01 A); 0.0038 or an equivalent percentage (0.4%) without simplification to 1 sf does not earn the mark. (iv) M1: qualitative conclusion consistent with the data (current/conductivity increases with depth); M2: an explanation based on ion concentration that is consistent with the direction of that trend; ECF applies if the stated trend is reversed but the explanation is internally consistent with it. (v) [1] for each correct half-equation; state symbols are not required for the mark. Using section 19, $\ce{Br-}$ is oxidized in preference to $\ce{Cl-}$