Which of the following are two indicators that a chemical reaction is taking place?

• A. Dissolving of salt and change in size
• B. Formation of a gas and change in color
• C. Evaporation of water and increase in temperature
• D. Change in shape and melting of a solid

Answer: (B)

Verified step by step guidance

1. Understand that a chemical reaction involves the transformation of substances into new substances with different properties, unlike physical changes which only affect the form or state of a substance.
2. Identify indicators of chemical reactions, which typically include the formation of a gas (bubbling or fizzing), a change in color, formation of a precipitate, or a change in temperature due to energy being absorbed or released.
3. Analyze each option to determine if it represents a chemical change or a physical change: dissolving salt and change in size are physical changes; evaporation of water and increase in temperature involve physical change and possibly a physical process; change in shape and melting of a solid are physical changes.
4. Recognize that 'formation of a gas' and 'change in color' are classic signs of a chemical reaction because they indicate new substances are being formed.
5. Conclude that the correct indicators of a chemical reaction from the given options are 'formation of a gas' and 'change in color' because these reflect chemical changes rather than physical changes.