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Practice each question, then open the answer or worked solution.
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Solution: Fresh milk is turned to curd due to the production of lactic acid. Lactic acid reduces the pH of the milk.
A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Solution: (a) He shifted the pH of the fresh milk from 6 to slightly alkaline to prevent milk from getting sour due to the production of lactic acid.
(b) This milk takes a long time to set into curd because the lactic acid produced here first neutralises the pH, and then the pH is reduced to turn the milk into curd.
Plaster of Paris should be stored in a moisture-proof container. Explain why.
Solution: Plaster of Paris should be stored in a moisture-proof container because moisture can affect the Plaster of Paris by slowing down the setting of the plaster because of hydration. This will turn plaster useless.
What is a neutralisation reaction? Give two examples.
Solution: The reaction of the acid + base gives a product of salt + water, which is considered a neutralisation reaction.
Examples:
NaOH + HCl → NaCl + H2O
Mg(OH)2 + H2CO3 → MgCO3 + 2H2O
Give two important uses of washing soda and baking soda.
| Washing soda | Baking soda |
| 1. It is used as an electrolyte | 1. It can be used to test the garden soil for acidity. If bubbles are developed, then the soil is too acidic |
| 2. It can be used domestically as a water softener for laundry. | 2. If used on washing the car, then it will remove dead bug bodies without damaging the colour or the paint on the car. |