C.
State one differ
6. Define the following.
a. Irreversible changes
b. Fast changes
c. Chemical changes
7. Give reasons.
a. Digestion is a chemical change as well as a physical change.
b. Metals are used for making tools.
8. Answer the following questions in detail.
a. How is the melting of wax different from the burning of wax?
b. How does mixing bring about a change? Explain with the help of an example,
C. Why are gaps left between railway trac
Answers
Answer:
a.A change is called irreversible if it cannot be changed back again.
b.The changes that are completed in short or very short duration of time are called fast changes.
c.Chemical changes occur when a substance combined with another to form a new substance,is called chemical changes.
7.a.During the process of digestion,the food is broken down into smaller molecules.The salivary glands presents in our mouth.In this process,there is a formation of a new substance.Therefore it is considered to be a chemical change.
b.Metals are used to cook food to make cooking utensils because metals are good conductor of heat and electricity.Since,metal is a good conductor of heat then we can cook food faster whereas non metals are bad conductors so they are not used to make utensils.
8.a.Melting to wax is temporary and wax can settle again on cooling and turns into a solid.Hence,the change is only physical and the state of matter changes.Burning of the candle is permanent because once it is burnt it cannot be converted into the candle.
b.When substances are mixed together, it can result either in a physical change or a chemical change. Example: If iron and sulphur are mixed, no chemical change occurs. But when the mixture is heated, it glows after some time and a black substance is formed which is different from iron and sulphur.
c.Small gaps which function as expansion joints are deliberately left between the rail ends to allow for expansion of the rails in hot weather. ... Because of these small gaps, when trains pass over jointed tracks they make a "clickety-clack" sound.