1.Write two properties of pure substance.
2 A burning candle shows both physical and chemical changes.Explain
3.What do you observe when quick lime is dissolved in water.
4.How is distillation more advantageous than evaporation?
5.What is variable valency?Give two examples of elements showing variable valency.
Answers
Answer:
Burning of candle melts the wax and hence physical state of wax has changed from solid to liquid. Again the wax combines with the atmosphere oxygen and changes to carbon dioxide, heat and light.
Answer:
1) Pure substances are mostly homogeneous in nature containing only one type of atoms or molecules. These substances mainly have a constant or uniform composition throughout. The substances have fixed boiling and melting points. A pure substance usually participates in a chemical reaction to form predictable products.
2) A burning candle produces wax vapours which solidify and form wax again is physical change. At the same time wax on burning produces water vapours and carbondioxide which escape into the atmosphere which are new substances with new properties. ... Hence burning of candle shows both physical and chemcial changes.
3) When quicklime is added to water, it forms slaked lime along with the evolution of heat. There will be a rise in the temperature of the bucket. Calcium oxide reacts with water to form calcium hydroxide, also called slaked lime.
4) In distillation, the liquids with boiling points close to each other can be separated whereas in evaporation ,the difference in boiling points has to be large. This is the advantage distillation has over evaporation.
5) Certain elements have more than one valency. They exhibit variable valency. For example, copper, tin, iron and mercury exhibit variable valency. Copper combines with oxygen to form cuprous oxide (Cu2O) and cupric oxide (CuO).