2018年8月18日雅思大作文机经预测：话题2

　　参考题目2

　　Eating sweet foods produces acid in the mouth, which can cause tooth decay. (High acid levels are measured by low pH values) Descr...

　　Eating sweet foods produces acid in the mouth, which can cause tooth decay. (High acid levels are measured by low pH values) Describe the information below and discuss the implications for dental health. You should write at least 150 words. Allow yourself 20 minutes for this task.

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　　解题思路

　　【首段】总述过多的糖会伤害牙齿，因为糖会使得pH值下降 【二段】甜食让pH值下降到5.5以下，时间越长越容易蛀牙 【三段】分述蔗糖最易引起蛀牙 【四段】分述果糖和蜂蜜对pH值的影响 【尾段】总述果糖和蜂蜜要比蔗糖好

　　范文

　　Anyone who has visited a dentist has been told that eating excessive amounts of sweets risks harming the teeth. This is because sweets lower pH levels in the mouth to dangerous levels. When the pH level in the mouth is kept above 5.5, acidity is such that teeth are unlikely to be in danger of decay. Sweet foods, however, cause pH in the mouth to drop for a time, and the longer pH levels remain below 5.5, the greater the opportunity for decay to occur. By comparing fruit sugar, cane sugar and honey, which are all common ingredients of sweet foods, we find that cane sugar lowers pH levels for the longest period, thus producing the greatest risk of the three. Approximately five minutes after consuming cane sugar, pH levels drop to as little as pH 3.5. They then begin to rise slowly, but do not rise above pH 5.5 until at least 30 minutes have elapsed. By contrast, fruit sugar, which causes the mouth's acidity to fall to just above pH 4, poses a danger for a shorter period: tooth decay is unlikely 20 minutes after consumption. Honey appears an even less risky substance. Though acidity falls to about pH 4.75 within five minutes of consumption, it returns to above pH 5.5 in under fifteen minutes. The implications, then, are that people who insist on eating sweet foods should be aware of the ingredients, and that fruit sugar or honey appear preferable to cane sugar.