阅读下列各小题，根据汉语及句末括号内的动词提示，用相关的动词短语完成句子(每空最多不超过四个词)。

阅读理解

                                                                                         Enjoying Dinner with Friends in America

        Americans, like many people else where in the world, like to invite friends to their homes for an evening of food, drink and conversation. Formal dinners, in fine homes and hotels in the United States are much the same as formal dinners anywhere in the world. But as most people in the United States have no servants (佣人) , their dinner parties at home are mostly informal. Guests may sit down at a table, or, as many new small homes have no separate (单独的) dining room or very small dining space, guests can also serve themselves and eat in the living room, holding their plates on their knees.

        A more enjoyable form of entertainment is the picnic. Americans are great picnickers, and almost every family has a picnic basket. Summer invitations are often for a picnic at a park or in the open countryside. Unless hamburgers or hot dogs are cooked over a fire, picnic food is usually cold sandwiches, salads, potato chips, etc. Watermelon is a favourite dessert, followed very often by baseball for the young and active, and naps (打盹) for the weary (疲惫的； 困乏的).

“硝酸”是高中化学将要学习的物质，浓硝酸见光会发生化学变化，其化学方程式为：

4HNO₃4NO₂↑+2H₂O+　X　↑．

（1）X的化学式为____ ．

（2）请在硝酸的化学式中标出氮元素的化合价：____ ．

（3）硝酸具有酸的一些共性，是因为它在水溶液中也能解离出____ （用化学符号）．