5 6 7 8 9 10 11 12 13 14 ph of some common substances

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5678910111213140pH of some common substances is shown in table 8.5.Table 8.5: pH of some common acids and basesCommon AcidspHCommon BasespHHCl (4%)0Blood plasma7.4Stomach acid1Egg white8Lemon juice2Sea water8Vinegar3Baking soda9Oranges3.5Antacids10Soda, grapes4Ammonia water11Sour milk4.5Lime water12Fresh milk5Drain cleaner13Human saliva6-8Caustic soda 4% (NaOH)14Pure water78.4.3 Determination of pHpH of a solution can be determined by using proper indicator or with the help ofa pH meter. The latter is a device which gives accurate value of pH. You will studymore about it in higher classes. We shall discuss here the use of indicators for findingout the pH of a solution.Universal Indicator/pH paper.It is a mixture of a number of indicators. It shows a specific colour at a given pH.A colour guides is provided with the bottle of the indicator or the strips of paperimpregnated with it which are called pH paper strips. The test solution is tested witha drop of the universal indicator, or a drop of the test solution is put on pH paper.The colour of the solution on the pH paper is compared with the colour chart/guardand pH is read from it. The pH values thus obtained are only approximate values.Fig. 8.7:Colour chart/guide of universal indicator/pH paper.8.4.2 Importance of pH in everyday lifepH plays a very important role in our everyday life. Some such examples aredescribed here.(a) pH in humans and animalsMost of the biochemical reactions taking place in our body are in a narrow pH rangeof 7.0 to 7.8. Even a small change in pH disturbs these processes.
Acids, Bases and SaltsSCIENCE AND TECHNOLOGY176NotesMODULE - 2Matter in our Surroundings(b) Acid RainWhen the pH of rain water falls below 5.6, it is called acid rain. When acid rainflows into rivers, the pH of the river water also falls and it become acidic. As a result,the survival of aquatic life become difficult.(c) pH in plantsPlants have a healthy growth only when the soil has a specific pH range which shouldbe neither highly alkaline nor highly acidic.(d) In digestive systemOur stomach produce hydrochloric acid which helps in digestion of food. Whenwe eat spicy food, stomach produces too much of acid which causes ‘acidity’ i.e.irritation and sometimes pain too. To get rid of this we use ‘antacids’ which are baseslike ‘milk of magnesia’ (suspension of magnesium hydroxide in water).(e) Self defence of animals and plantsBee sting causes severe pain and burning sensation.It is due to the presence of methanoic acid in it. Useof a mild base like baking soda can provides relieffrom pain.Some plants like ‘nettle plant’ have fine stinging hairwhich inject methanoic acid into the body of anyanimal or human being that comes in its contact. Thiscauses severe pain and buring sensation. The leavesof dock plant that grows near the nettle plant whenrubbed on the affected area provides relief.

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