The Mishnah (11:1) teaches that if left a tzluchit(container) of mei chatat uncovered and then subsequently found it covered, the water is invalid. The Bartenura explains that this is because we assume that a person came and covered the tzluchit and most people are not tahor for the purposes of mei chatat.
The Gemara (Eiruvin 9b) cites a Beraita that sounds very similar to our Mishnah. One critical difference is that the Beraita states that the water is tameh (impure) unlike our Mishnah that states it is pasul (invalid). For Rashi, the change in wording is very important. He understands that Beraita must be deal with water that has been drawn for mei chatat but has not yet had kiddush. The reason is that if we were dealing with mei chatat, stating that the water is tameh would be unnecessary since we mei chatat in general is metameh. If it were discussing mei chatat it should have stated that is was pasul (like our Mishnah).
The Tosfot (s.v. temeah) however justify the usage of the term tameh even for mei chatat. The Beraita is teaching us that the mei chatat is metame the tzluchit – which was clearly not the case prior to the discovery. If that is the case, why did the Gemara cite the Beraita and not our Mishnah?
The Tosfot (s.v. Oh Yarad) poses this question and suggests that the Beraita was preferred since it contains that reason why the mei chatat is tameh – “we say that a tameh person found it and uncovered it”.
The Tosfot however continues explaining that the fact that the Beraita states that the water is tameh as opposed to pasul has further significance. Firstly, there is an opinion (Zevachim 93a) that mei chatat that become tameh can still be used to purify someone from tumat met. Had the Beraita used the word pasul it would have implicitly rejected that opinion.
Secondly, the choice of the word tameh is important to differentiate between this case and the latter one in both the Mishnah and Beraita. In that case, if one left the mei chatat covered and then found it uncovered, the water is pasul out of concern that either an animal drank some or dew fell into it. The concern there is not tumah, but the mixing in of other liquids. Furthermore, since the nature of the latter case is not tumah, the tzluchit remains tahor unlike our case. This difference between whether the water is pasul or tameh is amongst others (see Parah 9:8). Consequently, the choice of words is necessary to highlight the legal differences between these two sections of the Mishnah and Beraita.
The Shita Mekubetzet however understand that the text brought in the Gemara as proof is not a Beraita but our Mishnah. He explains that it is common for the Gemara to either abridge or elaborate Mishnayot from Zerayim or Tahorot and sometimes alter the wording as needed.
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