Calcium reacts with dilute hydrochloric acid:

(a)

• (i) Calcium reacts with dilute hydrochloric acid: $$\text{Ca(s)} + \text{2HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$
• (ii) Magnesium reacts with dilute hydrochloric acid: $$\text{Mg(s)} + \text{2HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$
• (iii) Zinc reacts with dilute hydrochloric acid: $$\text{Zn(s)} + \text{2HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$

(b)

• (i) Circuit symbols:
  • (α) Capacitor: $$\text{—| |—}$$
  • (β) Diode: $$\text{—}\blacktriangleright\text{|—}$$
  • (γ) Resistor: \text{—}\text{/\/\/\}\text{—}
• (ii) To reverse bias the p-n junction diode, the positive terminal of the battery must be connected to the cathode (the line side) and the negative terminal to the anode (the triangle side). $$\text{—[ ]—}\text{|}\blacktriangleleft\text{—}$$ (The square represents the battery, with the longer line being the positive terminal. The diode is reversed compared to the original image, or the battery polarity is reversed.)

(c)
Two functions of potassium in crops are:

• It regulates water balance within the plant, influencing stomatal opening and closing.
• It activates enzymes essential for protein synthesis, sugar transport, and overall plant metabolism.