### Why? What? How? - reasoning questions

Q.1. Give an example for dispersion of light?
Ans: formation of rainbow

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Q.2. Why do we have to keep newly constructed walls wet by watering them for a few days?

Ans: Curing is the process of controlling the rate and extent of moisture loss from concrete to ensure an uninterrupted hydration of portland cement after concrete has been placed and finished in its final position.  Watering helps in strengthening and increasing durability.  To avoid thermal shrinkage crack, the uniform temperature should be maintained. Watering helps in decreasing heat of hydration.

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Q.3. Why do the household gas lighters  give blue color spark when we press the lighter?

Ans: The color of the spark depends on the gas present in metal points of the lighter.  If the gas is helium, the spark is in red and if it is sodium gas then the spark will be in yellowish-orange color.  House hold lighters consist only surrounding air, and moreover nitrogen percentage is more, hence the color is in blue.  The principle behind it is piezo-electric effect, when we press the hammer, then hits the  piezo-electric crystal resulting in high voltage, and which causes to give spark when reacts with gas present in metal points.

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Q.4. Why ice floats on water explain with suitable example?

Ans: To understand this concept, first of all, we must know about the physical quantity, called 'density'. Density is the ratio of mass to volume.  Density indicates the weight of the object. Density depends on the mass and volume of the object. When mass remains constant then the density plays important role.  Hence, when the volume occupied by the object is more, then the density of the object is less.

Using this property, when water gets converted into ice, then the volume occupied by the ice is greater than that of water.  When the volume is increased then the density is decreased.  Hence, the ice floats on water due to lesser density.

For example, take 1 litre of glass water bottle and fill it with water completely without leaving any space i.e. there is no air present in it.  Then keep the bottle in freezer of refrigerator.  Wait for some hours, then we observe that the glass bottle breaks due to expansion of the volume of the ice.  In this way, we can understand that the density of ice is lesser than the water of same mass.

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