Q. Which of the following statements are true, and which are false?
1. Burning of candle wax is a physical change.
2. Hydrogen gas is a combustible gas.
3. A chemical reaction is always accompanied by rise in temperature.
Answers
Answer:
1. True
2. True
3. False
Explanation:
1. Physical Changes: On heating, candle wax gets melted. Since it again turns into solid wax on cooling. So, the melting of wax and vapourisation of melted wax are physical changes. Chemical Changes : The wax near flame burns and gives new substances like carbon dioxide, carbon soot, water vapour, heat and light.
2. Hydrogen possesses the NFPA 704's highest rating of 4 on the flammability scale because it is flammable when mixed even in small amounts with ordinary air; ignition can occur at a volumetric ratio of hydrogen to air as low as 4% due to the oxygen in the air and the simplicity and chemical properties of the reaction.
3. If two substances react and the temperature of the mixture increases, the reaction is exothermic. All chemical reactions are not accompanied by intake of energy.
Answer:
1. Burning of a candle shows both physical and chemical change.
Explanation:
Physical change: When a candle is lit, the heat of the flame melts the solid wax to liquid wax. This signifies a physical change from solid state to liquid state. As the wax melts, the size of the candle decreases. On cooling, it solidifies and may take different shapes.
Chemical change: The wax acts as a fuel when the candle is lit. The fuel is majorly carbon. The carbon combines with oxygen to form another chemical substance called carbon dioxide. The unburnt carbon is deposited as a black substance called soot. This signifies a chemical change.
3. Chemical changes are always accompanied by some form of heat energy: either heat is evolved( exothermic) or heat is absorbed ( endothermic).