You know very well , many incident happens in our daily life.
➤ food get digested in human body.
➤we respire.
➤food is cooked.
➤grapes get fermented.
And many other incident occurs in nature .
In all the above situations, the nature
and the identity of the initial substance
have some what changed. We have already
learnt about physical and chemical changes of matter in our previous classes.
When ever a chemical change occurs, we
can say that a chemical reaction has
taken place.
How do we come to know that
a chemical reaction has taken place?
Let us perform some activities to find
the answer to these questions...
Activity -
→What do you observe?
Chemical equations can be made more concise and useful if we use chemical formulae instead of words. A chemical equation represents a chemical reaction. If you recall formulae of magnesium, oxygen and magnesium oxide, the above word-equation
can be written as –
Mg + O2 → MgO ---(1)
Count and compare the number of atoms of each element on the LHS and RHS of the arrow. Is the number of atoms of each element the
same on both the sides? If not, then the equation is unbalanced because the mass is not the same on both sides of the equation. Such a chemical equation is a skeletal chemical equation for a reaction. Equation (1) is a skeletal chemical equation for the burning of
magnesium in air.
➤ food get digested in human body.
➤we respire.
➤food is cooked.
➤grapes get fermented.
And many other incident occurs in nature .
In all the above situations, the nature
and the identity of the initial substance
have some what changed. We have already
learnt about physical and chemical changes of matter in our previous classes.
When ever a chemical change occurs, we
can say that a chemical reaction has
taken place.
How do we come to know that
a chemical reaction has taken place?
Let us perform some activities to find
the answer to these questions...
Activity -
CAUTION:- This Activity needs the teacher’s assistance. It would be better if . Students Wear eye protection.
→Clean a magnesium ribbon about 2 cm
→Clean a magnesium ribbon about 2 cm
long by rubbingit with sandpaper.
→Hold it with a pair of tongs.
→Burn it using a spirit lamp orburner and collect the ash soformed in a watchglass
→Hold it with a pair of tongs.
→Burn it using a spirit lamp orburner and collect the ash soformed in a watchglass
as show in Figure. Burn the magnesium
ribbon keeping itas far as possible from youreyes
→What do you observe?
You must have observed that magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.
After discuss about above activity , we can say that on the base of following observation chemical reaction has taken place.
➤ Change in state
➤ Change in colour
➤ Evolution of gas
➤ Change in temperature
WRITING A CHEMICAL EQUATIONS
can be written as –
Mg + O2 → MgO ---(1)
Count and compare the number of atoms of each element on the LHS and RHS of the arrow. Is the number of atoms of each element the
same on both the sides? If not, then the equation is unbalanced because the mass is not the same on both sides of the equation. Such a chemical equation is a skeletal chemical equation for a reaction. Equation (1) is a skeletal chemical equation for the burning of
magnesium in air.
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