Which of the following is always a chemical reaction?

• A. Burning
• B. Dissolving sugar in water
• C. Freezing water
• D. Evaporation of water

Answer: (A)

Understanding the difference between chemical changes and physical changes helps here. A chemical reaction is a process that makes new substances with different properties from the starting materials, often releasing or absorbing energy in the process.

Burning fits this idea perfectly. When something burns, it reacts with oxygen to form new substances such as carbon dioxide and water, and energy is released as heat and light. The original material no longer exists in its original form, which is the hallmark of a chemical change.

Dissolving sugar in water is typically a physical change. The sugar crystals disperse, and the dissolved substance is still the same chemical compound (though spread out in the solution); you can recover the sugar by evaporating the water without changing the sugar itself. There’s no new substance formed in this step.

Freezing water is another physical change. The water molecules are still H2O, just arranged in a different, solid structure. There’s no chemical transformation involved.

Evaporation of water is also a physical change, where water molecules escape into the air as vapor, but the chemical composition remains unchanged.

So, burning is the process that is always a chemical reaction because it creates new substances through a chemical change.