NCERT Solutions for Class 7 Science Chapter 5: Physical and Chemical Reactions

1. Physical and Chemical Changes This chapter physical and chemical changes dives into the fascinating world of changes happening around us, categorizing them into two main types: physical and chemical changes. Physical Changes: ● Definition: Changes in the physical properties (size, shape, state) of a substance without altering its chemical composition. No new substances are formed. ● Examples: ○ Crushing a paper ball (changes shape) ○ Boiling water (changes state from liquid to gas) ○ Dissolving sugar in water (changes size of sugar particles) ● Reversibility: Many physical changes are reversible. For example, crumpled paper can be flattened, and boiling water can condense back into liquid. Chemical Changes: ● Definition: Changes that result in the formation of new substances with entirely different properties from the original ones. The chemical composition of the starting materials changes. ● Examples: ○ Burning wood (forms ash and smoke) ○ Rusting of iron (iron combines with oxygen) ○ Cooking food (ingredients undergo chemical reactions to form new compounds) ● Indicators: Some changes, like the release of heat, gas, or a color change, can signal a chemical reaction is happening. Understanding these changes is crucial for various reasons: ● Material Science: Knowing how materials undergo physical and chemical changes helps us develop new materials with desired properties.

2. ● Everyday Life: Many processes involve physical and chemical changes, from cooking to cleaning. ● Safety: Chemical changes can be either beneficial (cooking) or harmful (pollution). Recognizing them helps us handle materials safely. Exercise 1.Classify the changes involved in the following processes as physical or chemical changes: (a) Photosynthesis (b) Dissolving sugar in water (c) Burning of coal (d) Melting of wax (e) Beating aluminium to make aluminium foil (f) Digestion of food Ans : (a) Photosynthesis: Chemical change (b) Dissolving sugar in water: Physical change – (c) Burning of coal: Chemical change (d) Melting of wax: Physical change (e) Beating aluminium to make aluminium foil: Physical change (f) Digestion of food: Chemical change 2. State the following sentence True or False (a) Cutting a log of wood into pieces is a chemical change. (True/ False) (b) Formation of manure from leaves is a physical change. (True/ False)

3. (c)Iron pipes coated with zinc do not get rusted easily. (True/ False) (d)Iron and rust are the same substances. (True/ False) (e)Condensation of steam is not a chemical change. (True/ False) Ans : (a) False (b) False (c) True (d) False (e)True 3.Fill in the blanks in the following statements: (a) When carbon dioxide is passed through lime water, it turns milky due to the formation of . (b) The chemical name of baking soda is . (c) Two methods by which rusting of iron can be prevented are __________ and __________ (d) Changes in which only ____________ properties of a substance change are called physical changes. (e) Changes in which new substances are formed are called _____________ changes.. Ans :

4. (a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate. (b) The chemical name of baking soda is sodium bicarbonate. (c) Two methods by which rusting of iron can be prevented are painting and galvanization. (d) Changes in which only the physical properties of a substance change are called physical changes. (e) Changes in which new substances are formed are called chemical changes. 4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain. Ans : This is a chemical change. ● New Substance Formed: When baking soda (sodium bicarbonate) reacts with lemon juice (citric acid), a new substance called sodium citrate is formed along with water and carbon dioxide gas (the bubbles). ● Chemical Composition Change: The original ingredients, baking soda and lemon juice, have different chemical compositions compared to the products formed (sodium citrate, water, and carbon dioxide). 5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place. Ans : When a candle burns, it undergoes both physical and chemical changes: Physical Changes:

5. ● Melting: The wax near the flame melts from a solid to a liquid state due to the heat. ● Evaporation: The melted wax vaporizes and rises as a gas. Chemical Changes: ● Combustion: The wax vapor reacts with oxygen in the air (a chemical reaction). This releases heat, light, and carbon dioxide gas. Another Example: Cooking food: ● Physical Changes: Chopping vegetables changes their size and shape. Ingredients like butter or cheese might melt due to heat. ● Chemical Changes: The heat triggers complex chemical reactions within the food. Proteins may denature (unfold), starches break down into sugars, and new flavor compounds are formed. These reactions lead to the cooked texture and taste of the food. 6. How would you show that setting of a curd is a chemical change? Ans : Curd proves a chemical change. 1.New Stuff: Milk’s sugar (lactose) changes to lactic acid. This acid “curdles” milk proteins, forming a new solid – the curd! 2.Big Differences: Milk is sweet and liquidy, while curd is sour and solid. They even digest differently! 3.No Going Back: Once curdled, you can’t turn it back to milk. 7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes. ~ Ans : Burning wood and chopping it are different because: ● Burning: Creates new stuff (ash, gas) – like a chemical makeover!

6. ● Chopping: Just changes the shape and size – the wood is still wood, just smaller pieces. 8. Describe how crystals of copper sulphate are prepared. Ans : 1.Make a super-copper-soup: Dissolve copper sulfate powder in hot water until no more dissolves. 2.Filter (optional): Get rid of any cloudiness with filter paper (like straining soup). 3.Crystal Time! Let the solution cool in a quiet spot. Crystals will slowly appear. 4.Grow big crystals (optional): Tie a perfect crystal seed (like a tiny crystal friend) to a string and dip it in. 5.Harvest your crystals! After some time, crystals will form. Pour out the leftover liquid and let them dry completely. 9. Explain how painting of an iron gate prevents it from rusting? Ans : Painting an iron gate is like putting on a raincoat! The paint blocks air and moisture, the rust-causing culprits, from getting to the iron. No air, no moisture, means no rust! 10. Explain why rusting of iron objects is faster in coastal areas than in deserts. Ans : Rusty surprise at the beach but not the desert ● Coast: Lots of moisture in the air (like a wet sponge) means rust forms faster on iron objects. ● Desert: Dry air with little moisture (like a dry sponge) makes rusting much slower. 11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change- A) then it bums (Change-B). The following statements pertain to these changes. Choose the correct one.

7. (i) Process-A is a chemical change. (ii) Process-B is a chemical change. (iii) Both processes A and B are chemical changes. (iv) None of these processes is a chemical change. Ans : (iv) None of these processes is a chemical change. 12.Anaerobic bacteria digest animal waste and produce biogas (Change-A). The biogas is then burnt as fuel (Change-B). The following statements pertain to these changes. Choose the correct one. (i) Process-A is a chemical change. (ii) Process-B is a chemical change. (iii) Both processes A and B are chemical changes. (iv) None of these processes is a chemical change. Ans : (iii) Both processes A and B are chemical changes.