Which of the following are chemical changes?
(a) Growth of a plant
(b) Rusting of iron
(c) Mixing of iron filings and sand
(d) Cooking of food
(e) Digestion of food
(f) Freezing of water
(g) Burning of a candle.
Answer: Mixing of iron and sand; freezing of water are examples of physical changes; while others are examples of chemical changes.
a) Growth of a plant: Growth of the plant include the formation of new compounds. These are chemical processes.
b) Rusting of iron: In rusting, iron reacts with oxygen and forms its oxide. During this process, water acts as a catalyst. So rusting is a chemical change.
c) Mixing of iron filings and sand: Here mixing does not lead to any chemical reaction. So, this is an example of only physical change.
d) The cooking of food: In cooking, many complex food particles are broken into smaller particles. Frying of foods in oil is the chemical change that change the taste of the food.
e) Digestion of food: Digestion of food involves the chemical breakdown of food into simple carbohydrates, fats, and amino acids.
f) Freezing of water: Water freezes at 0°C. The freeing of water involves only the physical change of states.
g) Burning of a candle: Here wax reacts with oxygen and forms carbon dioxide with the release of heat as energy.
See also: Question 10: Classify the following into elements, compounds, and mixtures.