Chemical Reactions (Introduction) MCQs Quiz | Class 10
Class: X | Subject: Science (Code 086) | Unit: Unit I: Chemical Substances – Nature and Behaviour. This quiz covers the Meaning of chemical reactions and observation indicators like evolution of gas, formation of precipitate, change in colour, and change in temperature. Attempt these 10 MCQs to test your understanding, then submit to check your score and download the answer key PDF.
Overview of Chemical Reactions
A chemical reaction is a process where one or more substances (reactants) undergo a chemical change to form new substances (products) with entirely new properties. In Class 10 Science, understanding the nature of these reactions and how to identify them is the foundation of chemistry.
1. Indicators of a Chemical Reaction
We can determine if a chemical reaction has taken place by observing changes in the system. The four main indicators are:
- Change in State: A substance may change from solid to liquid, liquid to gas, etc. Example: Burning of candle wax involves conversion of solid wax to gaseous carbon dioxide and water vapour.
- Change in Colour: Formation of new substances often leads to a color change. Example: When iron nails are dipped in blue copper sulphate solution, the solution turns green due to the formation of ferrous sulphate.
- Evolution of Gas: Many reactions release gases. Example: Zinc granules reacting with dilute sulphuric acid release hydrogen gas bubbles.
- Change in Temperature: Reactions can either absorb heat (endothermic) or release heat (exothermic). Example: Adding water to quicklime (calcium oxide) releases a large amount of heat.
2. Formation of Precipitate
A precipitate is an insoluble solid that emerges from a liquid solution. Example: When lead nitrate solution mixes with potassium iodide solution, a yellow precipitate of lead iodide is formed.
Quick Revision Summary
| Reaction Example | Main Observation |
|---|---|
| Burning of Magnesium Ribbon | Dazzling white flame, white powder (MgO) formed |
| Zinc + Dilute Sulphuric Acid | Evolution of Hydrogen gas |
| Lead Nitrate + Potassium Iodide | Yellow Precipitate (Lead Iodide) |
| Quicklime + Water | Rise in temperature (Heat evolved) |
5 Extra Practice Questions
- What is the chemical formula of the white powder formed when magnesium burns in air? (Ans: MgO)
- Which gas burns with a popping sound? (Ans: Hydrogen)
- Is the digestion of food a physical or chemical change? (Ans: Chemical Change)
- Name the gas evolved when calcium carbonate is heated. (Ans: Carbon Dioxide)
- What colour does litmus paper turn in the presence of an acid? (Ans: Blue to Red)
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