Chapter 18 Chemical Changes

When substances undergo a chemical change, ______________________.

• new substances are produced

• the temperature of the substance increases

• light is given out

• acids and/or alkalis are formed

Identify the reactants and products in the following chemical change.

• A

• B

• C

• D

Sam measured the mass of a piece of wood before and after it was burnt. He noticed that there was some residue left after the wood was completely burnt and that the total mass of the residue is less than the mass of the original piece of wood. This is because ________________________.

• the total mass of the reactants always decreases in a combustion reaction

• most of the wood have been converted into carbon dioxide gas

• only the impurities in the wood were burnt because wood is incombustible

• most of the mass have been lost as heat and light

When ice is strongly heated by an intense flame, the ice melts and the water eventually boils. This is ________________ because _______________________.

• a chemical change; steam is formed

• a chemical change; heat is involved

• not a chemical change; no new substances are formed

• not a chemical change; no light is given out

Which of the following is not a chemical change?

• Photosynthesis

• Respiration

• Boiling

• Combustion

An unknown white solid substance was heated strongly over a flame. After several minutes, the substance turned yellowish-brown and some vapour was released. This chemical change is known as ___________________.

• thermal decomposition

• combustion

• respiration

• electrolysis

Electrolysis is a chemical change process that uses ______________.

• electric current

• oxygen

• photosynthesis

• heat

Sam mixes two solutions. One has a pH value of 4 and the other has a pH value of 9. A chemical reaction occurs. Which of the following chemical changes is most likely to occur?

• Neutralisation

• Oxidation

• Electrolysis

• Decomposition

Which of the following processes best describes the following chemical change?

Carbon dioxide + Water → Glucose + Oxygen

• Oxidation

• Photosynthesis

• Glycolysis

• Neutralisation

Ginny noticed that when her Science teacher spilled some hydrochloric acid on the teacher’s bench in the Science laboratory, she sprinkled some white powder on the acid before wiping it off. There was a chemical reaction and some bubbles were observed. What chemical change has occurred?

• Effervescence

• Respiration

• Oxidation

• Neutralisation