为什么苯甲酸及苯甲酸钠在酸性溶液中（pH=4以下）防腐作用强，而碱性条件下（pH升高）防腐作用减弱？
参考答案：
苯甲酸及苯甲酸钠在溶液中存在分子型与离子型的解离平衡，只有分子型才能透过微生物的细胞壁而发挥防腐（抑菌）作用，而离子型无（防腐）抑菌作用。当溶液中pH值升高时苯甲酸及苯甲酸钠离子型比例增多，防腐作用减弱，pH=4以下，分子型比例增多，防腐作用增强。
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When you are invited to a meal in Thailand, the words of the invitation __1__ "come and  eat rice". In fact, __2__ all Thai dishes are eaten with rice. The food that is __3__ is always cut into pieces, so there is no need to use knives and forks but, instead, special __4__ and forks are used. The Thais used to eat with their hands and now there are __5__ some people who eat this way. There is a special w  ay of __6__ it. First they wash their right hands in a bowl of water - they only eat with their right hands. They are __7__ in order not to let the food touch the palms (手掌) of their hands. After the meal, the hands are again carefully __8__. The me  al usually has several different __9__. They are all hot. They are served in bowls. As Thailand has a long coastline (海岸线), it is not _10__ that fish and shellfish (贝类食物) play animportant part in Thai cooking. |