Question

investigations were carried out in a science lab to explore the topic of chemical and physical changes.
investigation a
step 1. add 5 tsp. salt to 100 ml warm water and stir until most or all of the salt is no longer visible.
step 2. heat the salt solution on a burner until only a white solid remains.
investigation b
step 1. mix 10 tsp. white sugar into 100 ml water and stir until most or all of the sugar is no longer visible.
step 2. heat the sugar solution on a burner until the solution thickens and turns brown.
in which step(s) did a chemical change most likely occur?
○ investigation a, step 1 and investigation b, step 1
○ investigation a, step 2 and investigation b, step 2
○ investigation a, step 2 only
○ investigation b, step 2 only

Explanation:

• Investigation A, Step 1: Dissolving salt in water is a physical change. The salt can be recovered by evaporation (as shown in Step 2 of Investigation A). There is no new substance formed at a molecular level.
• Investigation A, Step 2: Evaporating water from the salt - water solution is a physical change. It is just a separation of the salt (which was already present) from water.
• Investigation B, Step 1: Dissolving sugar in water is a physical change. The sugar can potentially be recovered (if we just evaporate water gently, but in Step 2, a chemical change occurs).
• Investigation B, Step 2: When the sugar solution is heated until it thickens and turns brown, a chemical change occurs. Sugar (sucrose, \(C_{12}H_{22}O_{11}\)) undergoes a chemical reaction (such as caramelization). New substances are formed (complex mixtures of polymers and other compounds), and the process is not easily reversible to get back the original sugar.

Answer:

Investigation B, Step 2 only