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IV.The boiling point is the temperature at which a substance begins to change from the liquid into the gaseous state.
II. Density
Activity 1:
Put a nail, the piece of wood, some oil & an ice cube in a beaker of water.
Ø Observation :
a. The nail sinks in the water.
b. The ice , the piece of wood & oil float on the surface of water.
Ø Conclusion:
a. The nail is denser than water, therefore it sinks.
b. The ice , candle & oil are less dense than water & therefore they float.
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Density is calculated by the rule
Density =
| Mass | Volume | Density | |
| Unit | gm | cm3 | g/ cm3 |
A solved example:
An object’s mass is measured in the lab. & found to be 50g.
The volume was also measured & found to be 25 cm3.
Can you find through calculations if this object floats or sinks in water.
Solution:
Density =
= = 2 g/ cm3
The density of this object is bigger than water 1 g/ Cm3 therefore it sinks.
Explain why:
1. Balloons filled with helium rise upwards. That’s because the density of helium is less than that of air.
2. Burning oil & flaming petroleum oil aren’t extinguished with water.
The density of oil is lower than water, therefore oil floats on top of water & keeps burning.
III. Melting point:
a. Melting is the change of the state of an object from solid to liquid.
b. Melting needs heating to occur.
c. The temperature at which melting of a substance begins is called melting point.
d. Metals such as iron, copper & aluminium have high melting points.
e. Ice, wax & butter have low melting points.
f. Metals have high melting points.
Explain why:
1. Metals are molten to make machines from them.
Bec. Molten metals are easily shaped or mixed into alloys.
2. Cooking pots are made of stainless steel alloy.
Bec. Both aluminum & stainless steel are good conductor of heat and
have high melting points.Stainless steel also doesn’t rust when water is heated inside it.
An alloy is a mixture of 2 metals or more, metals are mixed to improve their characteristics.
Examples are :
a. Gold & copper alloy used in making jewellery.
b. 
Nickel chrome alloy used in making the heating coils of electric heaters & boilers.
IV.The boiling point is the temperature at which a substance begins to change from the liquid into the gaseous state.
Different substances have different boiling points.
Date: 2015-12-24; view: 1352
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