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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.

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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.

                           
 
 
   
       
 
 
   
1 cm3 Oil Mass = 0.82g
   
1 cm3 Iron Mass = 7.87g
 
1 cm3 Water Mass = 1g
 

 


Density is calculated by the rule

Density =

Unit Mass Volume Density
  g 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.

Explain why:

1. Metals are molten to make machines from them.

Molten metals are easily shaped or mixed into alloys.

 

2. Cooking pots are made of stainless steel alloy.

Both aluminum & stainless steel 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.

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Читайте в этой же книге: Part 3. Water Pollution | Disastrous Consequences | A molecule consists of atoms. | The atomic number is the number of protons in an atom. | The atom is the basic unit of matter which goes into in a chemical reaction | VOCABULARY EXERCISES |
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