Question

1. grind salt into a powder.
2. heat a small beaker of water.
3. completely dissolve the salt in the water while stirring.
4. add silver nitrate solution to form a white solid.

during which step of this procedure does a chemical change occur?
1
2
3
4

Explanation:

Step1: Analyze step 1

Grinding salt into powder is a physical change as it only changes the particle - size, not the chemical composition of salt.

Step2: Analyze step 2

Heating water is a physical change. It just changes the temperature and possibly the state (if it boils), but no new substances are formed.

Step3: Analyze step 3

Dissolving salt in water is a physical change. The salt dissociates into ions in water, but it can be recovered by evaporating the water, so no new chemical substances are formed.

Step4: Analyze step 4

When silver nitrate solution is added to the salt - water solution, a chemical reaction occurs. Sodium chloride (from salt) reacts with silver nitrate to form silver chloride (a white solid) and sodium nitrate. The chemical equation is $NaCl(aq)+AgNO_3(aq)=AgCl(s)+NaNO_3(aq)$. A new substance (silver chloride) is formed, which is a sign of a chemical change.

Answer:

4