Maharashtra Board Class 10 Science Part 1 Chapter 3 Chemical Reactions and Equations PDF Download

Chemical Reactions And Equations

In this chapter, we will learn about chemical reactions and equations. We will study how to write chemical equations, balance them, and understand different types of chemical reactions.

Can You Recall?

1. What are the types of molecules of elements and compounds?

2. What is meant by valency of elements?

3. What is the requirement for writing molecular formulae of different compounds? How are the molecular formulae of the compounds written?

In earlier standards we have seen how compounds are formed by chemical combination of elements. We have also learnt that the driving force behind formation of a chemical bond is to attain an electronic configuration with a complete octet. The atoms attain a complete octet by giving, taking or sharing of electrons with each other.

Chemical Reaction

Some of the scientists of the 18th and 19th century carried out fundamental experiments on chemical reactions. They proved from their experiments that during chemical reactions composition of the matter changes and that change remains permanent. On the contrary during physical change only the state of matter changes and this change is often temporary in nature.

Identify Physical And Chemical Changes From The Phenomena Given In The Following Table

3.1 Some Common Phenomenon

Teacher's Note

Physical changes are temporary and reversible. For example, when ice melts into water, we can freeze it back. But when we cook food, we cannot get the raw food back. This is a chemical change.

Exam Trick

Remember: If you can undo it easily, it is physical change. If a new substance forms and you cannot undo it, it is chemical change. Just like you can melt and freeze ice again, but you cannot uncooked a cooked egg.

Points To Remember

Physical change is temporary and reversible.
Chemical change is permanent and irreversible.
During physical change, only the state of matter changes, not the substance.
During chemical change, a completely new substance forms.
In chemical change, the composition of matter changes.

Try This

Note: Do the following activities in group a of friends. Take help of your teacher wherever necessary.

Apparatus: Thermometer, evaporating dish, tripod stand, funnel, Bunsen burner, etc.

Chemicals: Lime stone powder, copper sulphate, calcium chloride, potassium chromate, zinc dust, sodium carbonate, phthalic anhydride, etc.

Procedure: Carry out the activities 1 to 5 as given below. Read and record the temperatures in the activities 2 to 4.

A physical change takes place due to change in the parameters such as temperature, pressure. Often a physical change in reversible. The composition of matter remains the same in a physical change. For example, ice is transformed into water on heating and water is transformed into ice on cooling. On the contrary, if the composition of matter changes during a process then it is called a chemical change. When we call a particular process or phenomenon as a chemical change, some chemical reactions are taking place in the concerned matter.

A chemical reaction is a process in which some substances undergo bond breaking and are transformed into new substances by formation of new bonds. The substances taking part in chemical reaction are called reactants, whereas the substances formed as a result of a chemical reaction by formation of new bonds are called products. For example, formation of carbon dioxide gas by combustion of coal in air is a chemical reaction. In this reaction coal (carbon) and oxygen (from air) are the reactants while carbon dioxide is the product. A chemical reaction is represented by writing a chemical equation.

3.3 Observation table

1. Take a spoonful of lime stone powder in an evaporating dish. Heat it strongly on a high blue flame.

2. Add zinc (Zn) dust into the copper sulphate (CuSO₄) solution.

3. Add potassium chromate (K₂CrO₄) solution to barium sulphate (BaSO₄) solution.

4. Add sodium carbonate (Na₂CO₃) solution to the calcium chloride (CaCl₂) solution.

5. Take phthalic anhydride in the evaporating dish. Close the end of the stem of a funnel with a cotton plug. Keep this funnel inverted on the evaporating dish. Heat the evaporating dish on a tripod stand slowly on a low flame. What did you observe in the funnel during heating?

Record the observation of all the activities. What did you find? Complete the following observation table with reference to the activities 1 to 5.

Teacher's Note

In our daily life, we see many chemical changes. For example, when we make tea, the tea leaves change colour and taste. This is a chemical change because new substances form.

Exam Trick

Remember: Look for three signs of chemical change - colour change, gas release, or temperature change. If any of these happen, it is likely a chemical change.

Points To Remember

Reactants are substances that start a reaction.
Products are new substances formed after the reaction.
Chemical equations show what happens in a reaction.
Energy may be released or absorbed during chemical reactions.
We can observe chemical reactions through colour, gas, or temperature changes.

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