Types of chemical reactions — displacement and combustion

Zinc is placed into copper sulfate solution. From the reactivity series, zinc is more reactive than copper, so zinc can displace copper from the compound. This is a displacement reaction.

$\text{zinc}+\text{copper sulfate}\to \text{zinc sulfate}+\text{copper}$ Copper metal is released as a solid, and zinc sulfate solution is formed.

Copper sulfate solution is blue because of the copper ions present. As copper is displaced and zinc sulfate forms instead, the blue colour fades. A reddish-brown solid (copper metal) deposits on the zinc surface.

The correct observation is that the solution turns colourless and a reddish-brown solid forms on the zinc. This matches the displacement of copper by zinc.

We need to find the observation that confirms a displacement reaction when zinc is added to copper sulfate solution.

If copper were being added to the solution rather than removed, the blue colour might intensify. But displacement removes copper ions from solution, so the blue colour should fade, not darken. Eliminate this option.

Gas production is not a characteristic of this displacement reaction. Bubbles might suggest a reaction with acid, but copper sulfate is not an acid. Eliminate this option.

Producing a visible flame (heat and light) is the hallmark of combustion, not displacement. Displacement reactions may release some heat but do not produce a flame. Eliminate this option.

The remaining option — solution turns colourless and a reddish-brown solid forms — correctly describes displacement: copper is deposited as a solid and the blue copper sulfate solution fades as zinc sulfate (colourless) forms.

For a displacement reaction to occur, the reacting element must be higher in the reactivity series than the element in the compound it is being added to.

The reactivity series (from high to low) includes: Zinc → Iron → Copper → Silver. Silver is less reactive than copper, so silver cannot displace copper from copper nitrate solution.

Magnesium is above zinc (reaction occurs ✓). Zinc is above copper (reaction occurs ✓). Iron is above copper (reaction occurs ✓). Only silver below copper means no displacement.

Silver placed into copper nitrate solution will not produce a displacement reaction because silver is less reactive than copper.

We need to find the reaction that will not occur — meaning the reacting element is less reactive than the element in the compound.

Magnesium is near the top of the reactivity series, well above zinc. Magnesium can displace zinc. This reaction occurs — eliminate this option.

Zinc is above copper in the reactivity series. Zinc can displace copper. This reaction occurs — eliminate this option.

Iron is above copper in the reactivity series. Iron can displace copper. This reaction occurs — eliminate this option.

Only silver placed into copper nitrate remains. Silver is below copper in the reactivity series, so it cannot displace copper. This reaction will not occur.