Physical and Chemical Changes
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Class 7 Science: Physical and Chemical Changes
Madhya Pradesh Board (MPBSE)
1. Introduction (परिचय)
Everything in our surroundings undergoes changes. These changes are broadly classified into two main types based on whether new substances are formed or not.
2. Physical Changes (भौतिक परिवर्तन)
A change in which a substance undergoes a change in its physical properties (like shape, size, color, or state) is called a physical change.
- Key Characteristics (मुख्य विशेषताएँ):
- No new substance is formed (कोई नया पदार्थ नहीं बनता).
- It is generally reversible (यह सामान्यतः उत्क्रमणीय होता है).
- Very little or no energy (heat/light) is absorbed or released.
- Examples (उदाहरण):
- Melting of ice (बर्फ का पिघलना).
- Dissolving sugar in water (पानी में चीनी घोलना).
- Boiling of water into steam (पानी का वाष्प बनना).
- Breaking of a glass tumbler (कांच के गिलास का टूटना).
- Stretching a rubber band (रबर बैंड को खींचना).
3. Chemical Changes (रासायनिक परिवर्तन)
A change in which one or more new substances are formed is called a chemical change. It is also known as a chemical reaction.
- Key Characteristics (मुख्य विशेषताएँ):
- New substances with entirely different properties are formed (नए पदार्थों का निर्माण होता है).
- It is generally irreversible (यह सामान्यतः अनुत्क्रमणीय होता है).
- Heat, light, or sound may be produced or absorbed (ऊष्मा, प्रकाश या ध्वनि उत्पन्न या अवशोषित हो सकती है).
- Change in smell or color may take place (गंग या रंग में परिवर्तन हो सकता है).
- A gas may be evolved (गैस निकल सकती है).
- Examples (उदाहरण):
- Digestion of food in our body (भोजन का पचना).
- Ripening of fruits (फलों का पकना).
- Burning of paper or wood (कागज या लकड़ी का जलना).
- Formation of curd from milk (दूध से दही बनना).
- Rusting of iron (लोहे में जंग लगना).
4. Important Chemical Reactions in the Chapter (महत्वपूर्ण रासायनिक अभिक्रियाएँ)
A. Rusting of Iron (लोहे में जंग लगना)
When iron is exposed to oxygen and moisture (water vapor) in the air, a new brown substance called rust forms on its surface.
- Word Equation (शब्द समीकरण):
Iron (लोहा) + Oxygen (ऑक्सीเจน, O2) + Water (जल, H2O) -> Rust [Iron oxide] (जंग / आयरन ऑक्साइड) - Condition for Rusting: Presence of both Oxygen and Water (or moisture).
B. Reaction of Baking Soda and Vinegar (बेकिंग सोडा और सिरके की अभिक्रिया)
When baking soda (sodium hydrogen carbonate) is mixed with vinegar (acetic acid), carbon dioxide gas is released with a fizzing sound, along with other substances.
- Word Equation (शब्द समीकरण):
Vinegar (सिरका) + Baking Soda (बेकिंग सोडा) -> Carbon Dioxide (कार्बन डाइऑक्साइड) + Other substances (अन्य पदार्थ) - Special Test: When Carbon Dioxide (
CO2) gas is passed through freshly prepared lime water (calcium hydroxide), the lime water turns milky due to the formation of calcium carbonate.Lime Water (चूने का पानी) + CO2 -> Turns Milky (दूधिया हो जाता है)
5. Methods to Prevent Rusting (जंग से बचाने के उपाय)
Rusting destroys iron objects. To prevent it, we need to cut off the contact of iron with oxygen and moisture.
- Painting or Greasing (पेंट करना या ग्रीस लगाना): Applying a coat of paint or grease regularly.
- Galvanization (यסטיकरण / गैल्वेनीकरण): Depositing a layer of a metal like zinc or chromium on iron. (Note: Stainless steel is made by mixing iron with carbon, chromium, nickel, and manganese, which does not rust).
6. Crystallization (क्रिस्टलीकरण / रवेदार बनाना)
The process of obtaining large, pure crystals of a substance from its saturated solution is called crystallization.
- It is an example of a physical change because no new substance is formed; only the pure form of the substance separates out in crystal form.
- Example: Obtaining pure crystals of Copper Sulphate (नीला थोथा) from its hot solution.