The examples in the question are neither in Genitive case, nor in Accusative case. They are in a lesser know Partitive case. Please see this excellent answer: What are the lesser known Russian cases?
Most nouns in Partitive case have the same forms as in Genitive case, but sometimes there is a different form for Partitive.
Here are some examples:
Чай / tea — Genitive: Нет чая; Accusative: Я люблю чай; Partitive: Налейте стакан чаю.
Сахар / sugar — Genitive: Нет сахара; Accusative: Я люблю сахар; Partitive: Насыпьте ложку сахару.
Воздух / air — Genitive: Без воздуха; Accusative: Мне нужен воздух; Partitive: Хочу хоть глоток воздуха.
Вода/ water — Genitive: Из воды; Accusative: Включите воду; Partitive: Плесните немного воды.
Горчица / mustard — Genitive: Нет горчицы; Accusative: Купите горчицу; Partitive: Возьмите ложку горчицы.
In such examples where in English one would say: "I want some water/some tea/some sugar/some peace/etc.", in Russian one would use Partitive case.
UPDATE
Here are the words which have a form in Partitive case that is different from Genitive: Русские существительные с разделительным падежом
